Search

The serine palmitoyltransferase (SPT) complex catalyzes the first and rate-limiting step in sphingolipid biosynthesis in all eukaryotes. ORM/ORMDL proteins are negative regulators of SPT that respond to cellular sphingolipid levels. However, the molecular basis underlying ORM/ORMDL-dependent homeostatic regulation of SPT is not well understood. We determined the cryo–electron microscopy structure of Arabidopsis SPT-ORM1 complex, composed of LCB1, LCB2a, SPTssa, and ORM1, in an inhibited state. A ceramide molecule is sandwiched between ORM1 and LCB2a in the cytosolic membrane leaflet. Ceramide binding is critical for the ORM1-dependent SPT repression, and dihydroceramides and phytoceramides differentially affect this repression. A hybrid β sheet, formed by the amino termini of ORM1 and LCB2a and induced by ceramide binding, stabilizes the amino terminus of ORM1 in an inhibitory conformation. Our findings provide mechanistic insights into sphingolipid homeostatic regulation via the binding of ceramide to the SPT-ORM/ORMDL complex that may have implications for plant-specific processes such as the hypersensitive response for microbial pathogen resistance.

SPTLC1-related disorder is a late onset sensory-autonomic neuropathy associated with perturbed sphingolipid homeostasis which can be improved by supplementation with the serine palmitoyl-CoA transferase (SPT) substrate, l-serine. Recently, a juvenile form of motor neuron disease has been linked to SPTLC1 variants. Variants affecting the p.S331 residue of SPTLC1 cause a distinct phenotype, whose pathogenic basis has not been established. This study aims to define the neuropathological and biochemical consequences of the SPTLC1 p.S331 variant, and test response to l-serine in this specific genotype.We report clinical and neurophysiological characterisation of two unrelated children carrying distinct p.S331 SPTLC1 variants. The neuropathology was investigated by analysis of sural nerve and skin innervation. To clarify the biochemical consequences of the p.S331 variant, we performed sphingolipidomic profiling of serum and skin fibroblasts. We also tested the effect of l-serine supplementation in skin fibroblasts of patients with p.S331 mutations.In both patients, we recognised an early onset phenotype with prevalent progressive motor neuron disease. Neuropathology showed severe damage to the sensory and autonomic systems. Sphingolipidomic analysis showed the coexistence of neurotoxic deoxy-sphingolipids with an excess of canonical products of the SPT enzyme. l-serine supplementation in patient fibroblasts reduced production of toxic 1-deoxysphingolipids but further increased the overproduction of sphingolipids.Our findings suggest that p.S331 SPTLC1 variants lead to an overlap phenotype combining features of sensory and motor neuropathies, thus proposing a continuum in the spectrum of SPTLC1-related disorders. l-serine supplementation in these patients may be detrimental.

Sphingolipids are essential lipids in eukaryotic membranes. In humans, the first and rate-limiting step of sphingolipid synthesis is catalyzed by the serine palmitoyltransferase holocomplex, which consists of catalytic components (SPTLC1 and SPTLC2) and regulatory components (ssSPTa and ORMDL3). However, the assembly, substrate processing and regulation of the complex are unclear. Here, we present 8 cryo-electron microscopy structures of the human serine palmitoyltransferase holocomplex in various functional states at resolutions of 2.6–3.4 A. The structures reveal not only how catalytic components recognize the substrate, but also how regulatory components modulate the substrate-binding tunnel to control enzyme activity: ssSPTa engages SPTLC2 and shapes the tunnel to determine substrate specificity. ORMDL3 blocks the tunnel and competes with substrate binding through its amino terminus. These findings provide mechanistic insights into sphingolipid biogenesis governed by the serine palmitoyltransferase complex. Cryo-EM structures of the enzyme complexes catalyzing the rate-limiting step in sphingolipid synthesis reveal mechanisms of substrate recognition and modulation by regulatory subunits.

Amyotrophic lateral sclerosis (ALS) is a progressive, neurodegenerative disease of the lower and upper motor neurons with sporadic or hereditary occurrence. Age of onset, pattern of motor neuron degeneration and disease progression vary widely among individuals with ALS. Various cellular processes may drive ALS pathomechanisms, but a monogenic direct metabolic disturbance has not been causally linked to ALS. Here we show SPTLC1 variants that result in unrestrained sphingoid base synthesis cause a monogenic form of ALS. We identified four specific, dominantly acting SPTLC1 variants in seven families manifesting as childhood-onset ALS. These variants disrupt the normal homeostatic regulation of serine palmitoyltransferase (SPT) by ORMDL proteins, resulting in unregulated SPT activity and elevated levels of canonical SPT products. Notably, this is in contrast with SPTLC1 variants that shift SPT amino acid usage from serine to alanine, result in elevated levels of deoxysphingolipids and manifest with the alternate phenotype of hereditary sensory and autonomic neuropathy. We custom designed small interfering RNAs that selectively target the SPTLC1 ALS allele for degradation, leave the normal allele intact and normalize sphingolipid levels in vitro. The role of primary metabolic disturbances in ALS has been elusive; this study defines excess sphingolipid biosynthesis as a fundamental metabolic mechanism for motor neuron disease.

Serine palmitoyltransferase (SPT), an endoplasmic reticulum-localized membrane enzyme composed of a catalytic LCB1/LCB2 heterodimer and a small activating subunit (Tsc3 in yeast; ssSPTs in mammals), is negatively regulated by the evolutionarily conserved family of proteins known as the ORMs. In yeast, SPT, the ORMs, and the PI4P phosphatase Sac1, copurify in the “SPOTs” complex. However, neither the mechanism of ORM inhibition of SPT nor details of the interactions of the ORMs and Sac1 with SPT are known. Here we report that the first transmembrane domain (TMD1) of Lcb1 is required for ORM binding to SPT. Loss of binding is not due to altered membrane topology of Lcb1 since replacing TMD1 with a heterologous TMD restores membrane topology but not ORM binding. TMD1 deletion also eliminates ORM-dependent formation of SPT oligomers as assessed by co-immunoprecipitation assays and in vivo imaging. Expression of ORMs lacking derepressive phosphorylation sites results in constitutive SPT oligomerization, while phosphomimetic ORMs fail to induce oligomerization under any conditions. Significantly, when LCB1–RFP and LCB1ΔTMD1–GFP were coexpressed, more LCB1ΔTMD1–GFP was in the peripheral ER, suggesting ORM regulation is partially accomplished by SPT redistribution. Tsc3 deletion does not abolish ORM inhibition of SPT, indicating the ORMs do not simply prevent activation by Tsc3. Binding of Sac1 to SPT requires Tsc3, but not the ORMs, and Sac1 does not influence ORM-mediated oligomerization of SPT. Finally, yeast mutants lacking ORM regulation of SPT require the LCB-P lyase Dpl1 to maintain long-chain bases at sublethal levels.

Sphingolipids typically have an 18-carbon (C18) sphingoid long chain base (LCB) backbone. Although sphingolipids with LCBs of other chain lengths have been identified, the functional significance of these low-abundance sphingolipids is unknown. The LCB chain length is determined by serine palmitoyltransferase (SPT) isoenzymes, which are trimeric proteins composed of two large subunits (SPTLC1 and SPTLC2 or SPTLC3) and a small subunit (SPTssa or SPTssb). Here we report the identification of an Sptssb mutation, Stellar (Stl), which increased the SPT affinity toward the C18 fatty acyl-CoA substrate by twofold and significantly elevated 20-carbon (C20) LCB production in the mutant mouse brain and eye, resulting in surprising neurodegenerative effects including aberrant membrane structures, accumulation of ubiquitinated proteins on membranes, and axon degeneration. Our work demonstrates that SPT small subunits play a major role in controlling SPT activity and substrate affinity, and in specifying sphingolipid LCB chain length in vivo. Moreover, our studies also suggest that excessive C20 LCBs or C20 LCB-containing sphingolipids impair protein homeostasis and neural functions.

Sphingolipid levels are tightly regulated to maintain cellular homeostasis. During pathologic conditions such as in aging, inflammation, and metabolic and neurodegenerative diseases, levels of some sphingolipids, including the bioactive metabolite ceramide, are elevated. Sphingolipid metabolism has been linked to autophagy, a critical catabolic process in both normal cell function and disease; however, the in vivo relevance of the interaction is not well-understood. Here, we show that blocking autophagy in the liver by deletion of the Atg7 gene, which is essential for autophagosome formation, causes an increase in sphingolipid metabolites including ceramide. We also show that overexpression of serine palmitoyltransferase to elevate de novo sphingolipid biosynthesis induces autophagy in the liver. The results reveal autophagy as a process that limits excessive ceramide levels and that is induced by excessive elevation of de novo sphingolipid synthesis in the liver. Dysfunctional autophagy may be an underlying mechanism causing elevations in ceramide that may contribute to pathogenesis in diseases.

Sphingolipid synthesis is initiated by condensation of Ser with palmitoyl-CoA producing 3-ketodihydrosphinganine (3-KDS), which is reduced by a 3-KDS reductase to dihydrosphinganine. Ser palmitoyltransferase is essential for plant viability. Arabidopsis thaliana contains two genes (At3g06060/TSC10A and At5g19200/TSC10B) encoding proteins with significant similarity to the yeast 3-KDS reductase, Tsc10p. Heterologous expression in yeast of either Arabidopsis gene restored 3-KDS reductase activity to the yeast tsc10Δ mutant, confirming both as bona fide 3-KDS reductase genes. Consistent with sphingolipids having essential functions in plants, double mutant progeny lacking both genes were not recovered from crosses of single tsc10A and tsc10B mutants. Although the 3-KDS reductase genes are functionally redundant and ubiquitously expressed in Arabidopsis, 3-KDS reductase activity was reduced to 10% of wild-type levels in the loss-of-function tsc10a mutant, leading to an altered sphingolipid profile. This perturbation of sphingolipid biosynthesis in the Arabidopsis tsc10a mutant leads an altered leaf ionome, including increases in Na, K, and Rb and decreases in Mg, Ca, Fe, and Mo. Reciprocal grafting revealed that these changes in the leaf ionome are driven by the root and are associated with increases in root suberin and alterations in Fe homeostasis.

In yeast, Tsc10p catalyzes reduction of 3-ketosphinganine to dihydrosphingosine. In mammals, it has been proposed that this reaction is catalyzed by FVT1, which despite limited homology and a different predicted topology, can replace Tsc10p in yeast. Silencing of FVT1 revealed a direct correlation between FVT1 levels and reductase activity, showing that FVT1 is the principal 3-ketosphinganine reductase in mammalian cells. Localization and topology studies identified an N-terminal membrane-spanning domain in FVT1 (absent in Tsc10p) oriented to place it in the endoplasmic reticulum (ER) lumen. In contrast, protease digestion studies showed that the N terminus of Tsc10p is cytoplasmic. Fusion of the N-terminal domain of FVT1 to green fluorescent protein directed the fusion protein to the ER, demonstrating that it is sufficient for targeting. Although both proteins have two predicted transmembrane domains C-terminal to a cytoplasmic catalytic domain, neither had an identifiable lumenal loop. Nevertheless, both Tsc10p and the residual fragment of FVT1 produced by removal of the N-terminal domain with factor Xa protease behave as integral membrane proteins. In addition to their topological differences, mutation of conserved catalytic residues had different effects on the activities of the two enzymes. Thus, while FVT1 can replace Tsc10p in yeast, there are substantial differences between the two enzymes that may be important for regulation of sphingolipid biosynthesis in higher eukaryotes.

Serine palmitoyltransferase (SPT) catalyzes the first committed step in sphingolipid biosynthesis. In yeast, SPT is composed of a heterodimer of 2 highly-related subunits, Lcb1p and Lcb2p, and a third subunit, Tsc3p, which increases enzyme activity markedly and is required for growth at elevated temperatures. Higher eukaryotic orthologs of Lcb1p and Lcb2p have been identified, but SPT activity is not highly correlated with coexpression of these subunits and no ortholog of Tsc3p has been identified. Here, we report the discovery of 2 proteins, ssSPTa and ssSPTb, which despite sharing no homology with Tsc3p, each substantially enhance the activity of mammalian SPT expressed in either yeast or mammalian cells and therefore define an evolutionarily conserved family of low molecular weight proteins that confer full enzyme activity. The 2 ssSPT isoforms share a conserved hydrophobic central domain predicted to reside in the membrane, and each interacts with both hLCB1 and hLCB2 as assessed by positive split ubiquitin 2-hybrid analysis. The presence of these small subunits, along with 2 hLCB2 isofoms, suggests that there are 4 distinct human SPT isozymes. When each SPT isozyme was expressed in either yeast or CHO LyB cells lacking endogenous SPT activity, characterization of their in vitro enzymatic activities, and long-chain base (LCB) profiling revealed differences in acyl-CoA preference that offer a potential explanation for the observed diversity of LCB seen in mammalian cells.

The structural organization and topology of the Lcb1p subunit of yeast and mammalian serine palmitoyltransferases (SPT) were investigated. In the yeast protein, three membrane-spanning domains were identified by insertion of glycosylation and factor Xa cleavage sites at various positions. The first domain of the yeast protein, located between residues 50 and 84, was not required for the stability, membrane association, interaction with Lcb2p, or enzymatic activity. Deletion of the comparable domain of the mammalian protein SPTLC1 also had little effect on its function, demonstrating that this region is not required for membrane localization or heterodimerization with SPTLC2. The second and third membrane-spanning domains of yeast Lcb1p, located between residues 342 and 371 and residues 425 and 457, respectively, create a luminal loop of approximately 60 residues. In contrast to the first membrane-spanning domain, the second and third membrane-spanning domains were both required for Lcb1p stability. In addition, mutations in the luminal loop destabilized the SPT heterodimer indicating that this region of the protein is important for SPT structure and function. Mutations in the extreme carboxyl-terminal region of Lcb1p also disrupted heterodimer formation. Taken together, these data suggest that in contrast to other members of the alpha-oxoamine synthases that are soluble homodimers, the Lcb1p and Lcb2p subunits of the SPT heterodimer may interact in the cytosol, as well as within the membrane and/or the lumen of the endoplasmic reticulum.

Lowering the growth temperature of HepG2 cells from 37 degrees C to 20 degrees C results in a 73% reduction in human squalene synthase (HSS) protein, a 76% reduction in HSS mRNA, and a 96% reduction in promoter activity of a secreted alkaline phosphatase-HSS reporter gene. A similar decrease in either mRNA or protein levels is observed for 3-hydroxy-3-methylglutaryl CoA reductase, farnesyl diphosphate synthase, the LDL receptor, and fatty acid synthase. All these proteins and mRNAs show either a decrease or a complete loss of sterol-dependent regulation in cells grown at 20 degrees C. In contrast, sterol regulatory element binding proteins (SREBPs)-1 and -2 exhibit a 2- to 3-fold increase in mRNA levels at 20 degrees C. The membrane-bound form of the SREBPs is dramatically increased, but the proteolytic processing to the nuclear (N-SREBP) form is inhibited under these conditions. Overexpression of the N-SREBP or SREBP cleavage-activating protein (SCAP), but not site-1 or site-2 proteases, restores the activation of the HSS promoter at 20 degrees C, most likely by liberating the SCAP-SREBP complex so that it can move to the Golgi for processing. These results indicate that the cholesterol synthesizing machinery is down-regulated at low temperatures, and points to the transport of the SCAP-SREBP complex to the Golgi as the specific down-regulated step.

The relationship between serine palmitoyltransferase (SPT) activity and ORMDL regulation of sphingolipid biosynthesis was investigated in mammalian HEK293 cells. Each of the three human ORMDLs reduced the increase in long-chain base synthesis seen after overexpression of wild-type SPT or SPT containing the C133W mutation in hLCB1, which produces the non-catabolizable sphingoid base, 1-deoxySa. ORMDL-dependent repression of sphingoid base synthesis occurred whether SPT was expressed as individual subunits or as a heterotrimeric single-chain SPT fusion protein. Overexpression of the single-chain SPT fusion protein under the control of a tetracycline-inducible promoter in stably transfected cells resulted in increased endogenous ORMDL expression. This increase was not transcriptional; there was no significant increase in any of the ORMDL mRNAs. Increased ORMDL protein expression required SPT activity since overexpression of a catalytically inactive SPT with a mutation in hLCB2a had little effect. Significantly, increased ORMDL expression was also blocked by myriocin inhibition of SPT as well as fumonisin inhibition of the ceramide synthases, suggesting that increased expression is a response to a metabolic signal. Moreover, blocking ORMDL induction with fumonisin treatment resulted in significantly greater increases in in vivo SPT activity than was seen when ORMDLs were allowed to increase, demonstrating the physiological significance of this response.

Farnesyl diphosphate synthase (FPPS: EC2.5.1.10), a key enzyme in isoprenoid metabolic pathways, catalyzes the synthesis of farnesyl diphosphate (FPP) an intermediate in the biosynthesis of both sterol and non-sterol isoprenoid end products. The localization of FPPS to peroxisomes has been reported ( Krisans, S. K., J. Ericsson, P. A. Edwards, and G. A. Keller. 1994. J. Biol. Chem. 269: 14165-14169). Using indirect immunofluorescence and immunoelectron microscopic tech- niques we show here that FPPS is localized predominantly in the peroxisomes of rat hepatoma H35 cells. However, the par- tial release of 60-70% of cellular FPPS activity is observed by selective permeabilization of these cells with digitonin. Under these conditions, lactate dehydrogenase, a cytosolic enzyme, is completely released whereas catalase, a known peroxisomal enzyme, is fully retained. Digitonin treatment of H35 cells dif- ferentially affects the release of other peroxisomal enzymes involved in isoprenoid metabolism. For instance, mevalonate kinase and phosphomevalonate kinase are almost totally re- leased (95% and 91%, respectively), whereas 3-hydroxy-3- methylglutaryl-CoA reductase is fully retained. Indirect immu- noflourescence studies indicate that FPPS is localized in peroxisomes of Chinese hamster ovary (CHO)-K1 cells but is dispersed in the cytosol of ZR-82 cells, a mutant that lacks per- oxisomes. Unlike in H35 cells, FPPS is completely released upon digitonin permeabilization of CHO-K1 and ZR-82 cells. In contrast, under the same permeabilization conditions, cata- lase is fully retained in CHO-K1 cells but completely released from ZR-82 cells. These studies indicate that FPPS and other enzymes in the isoprenoid biosynthetic pathways, in- volved in the formation of FPP, are differentially associated with peroxisomes and may easily diffuse to the cytosol. Based on these observations, the significance and a possible regula- tory model in the formation of isoprenoid end-products are discussed. —Gupta, S. D., R. S. Mehan, T. R. Tansey, H-T. Chen, G. Goping, I. Goldberg, and I. Shechter. Differential binding of proteins to peroxisomes in rat hepatoma cells: unique association of enzymes involved in isoprenoid metabo- lism. J. Lipid Res. 1999. 40: 1572-1584. Supplementary key words peroxisomescholesterolisoprenoids • immunofluorescencecellular permeabilizationfarnesyl diphosphate synthasecatalase

The topological and functional organization of the two isoforms of the small subunits of human serine palmitoyltransferase (hssSPTs) that activate the catalytic hLCB1/hLCB2 heterodimer was investigated. A variety of experimental approaches placed the N termini of the ssSPTs in the cytosol, their C termini in the lumen, and showed that they contain a single transmembrane domain. Deletion analysis revealed that the ability to activate the heterodimer is contained in a conserved 33-amino acid core domain that has the same membrane topology as the full-length protein. In combination with analysis of isoform chimera and site-directed mutagenesis, a single amino acid residue in this core (Met(25) in ssSPTa and Val(25) in ssSPTb) was identified which confers specificity for palmitoyl- or stearoyl-CoA, respectively, in both yeast and mammalian cells. This same residue also determines which isoform is a better activator of a mutant heterodimer, hLCB1(S331F)/hLCB2a, which has increased basal SPT activity and decreased amino acid substrate selectivity. This suggests that the role of the ssSPTs is to increase SPT activity without compromising substrate specificity. In addition, the observation that the C-terminal domains of ssSPTa and ssSPTb, which are highly conserved within each subfamily but are the most divergent regions between isoform subfamilies, are not required for activation of the heterodimer or for acyl-CoA selectivity suggests that the ssSPTs have additional roles that remain to be discovered.

It has been suggested previously that copper transport in Escherichia coli is mediated by the products of at least six genes, cutA, cutB, cutC, cutD, cutE, and cutF. A mutation in one or more of these genes results in an increased copper sensitivity (D. Rouch, J. Camakaris, and B. T. O. Lee, p. 469-477, in D. H. Hamer and D. R. Winge, ed., Metal Ion Homeostasis: Molecular Biology and Chemistry, 1989). Copper-sensitive cutC and cutF mutants were transformed with a genomic library of E. coli, and copper-tolerant transformants were selected. Two distinct clones were identified, each of which partially restores copper tolerance in both the cutC and cutF mutants of E. coli. Subcloning, physical mapping, and sequence analysis have revealed that the cutC gene is located at 42.15 min on the E. coli genome and encodes a cytoplasmic protein of 146 amino acids and that the cutF gene is located at 4.77 min on the E. coli genome and is allelic to the nlpE gene independently identified by Silhavy and coworkers (W. B. Snyder, L. J. B. Davis, P. N. Danese, C. L. Cosma, and T. J. Silhavy, J. Bacteriol. 177:4216-4223, 1995). Results from the genetic mapping of the copper-sensitive mutations in the cutF mutant and sequencing of the cutC and cutF (nlpE) alleles from both cutC and cutF mutants indicate that both the cutC and cutF mutants are in fact double mutants altered in these two genes, and mutations in both the genes appear to be required for the copper-sensitive phenotype in each mutant.

A temperature-sensitive (ts) mutant of Salmonella typhimurium that accumulated unmodified murein prolipoprotein at 42 degrees C but not at 30 degrees C was identified. In vivo and in vitro studies of the biosynthesis of Braun's lipoprotein revealed that this mutant (SE5221) was defective in the glyceryl modification of prolipoprotein. The ts mutation was mapped to 60.6 min of the S. typhimurium chromosome and was linked to argA and cysH. A clone with a 1.4-kilobase S. typhimurium DNA insert that complemented the ts mutation and restored the prolipoprotein modification activity both in vivo and in vitro was isolated. DNA sequencing of the complementing region revealed an open reading frame encoding a protein with 291 amino acids lacking NH2-terminal signal sequence. This open reading frame is immediately 5' to the thyA gene and is allelic to umpA of Escherichia coli. Wild-type strains harboring the cloned gene exhibited elevated levels of prolipoprotein modification activity. At the non-permissive temperature, the mutation affected both growth and viability, and the mutant cells exhibited anomalous cell morphology. The ts phenotype was suppressed by the introduction of a lpp::Tn10 mutation. These results suggest that the cloned gene encodes prolipoprotein glyceryl transferase (lgt), and in the wild-type background, this prolipoprotein modification enzyme is essential for the growth and viability of S. typhimurium.

On screening 440 temperature-sensitive (ts) mutants of Salmonella typhimurium, a mutant strain SE5312 which accumulated apolipoprotein (ALP) at 42 degrees C was identified. In vitro assay of apolipoprotein N-acyltransferase activity indicated that the mutant cell envelope contained reduced activity as compared to the wild-type strain. Transduction with a Mud-P22 mapping set placed the ts mutation to 14-17 min region of the S. typhimurium chromosome. P22 transduction using transposon insertions in this region revealed a linkage of the ts mutation to cobD (6%), nag (8%), and corC68 (99%). The ts phenotype was complemented by a 2.3-kilobase EcoRI subclone derived from lambda-phage 170 of Kohara's bank of Escherichia coli. Restriction enzyme analysis of the cloned DNA revealed that this 2.3-kilobase EcoRI fragment included the copper transport (cutE) gene in E. coli. The mutant strain SE5312 was copper-sensitive at 30 degrees C, and the complementing clone conferred copper resistance and restored the ALP N-acyltransferase activity in the mutant cell. Wild-type strain of S. typhimurium harboring this clone exhibited elevated levels of ALP N-acyltransferase activity. These results suggest that the cloned gene encodes the ALP N-acyltransferase. Upon shift to the non-permissive temperature, the viability of the mutant cells decreased, and the mutant cells assumed anomalous morphology. Temperature-resistant revertants could be readily isolated, and a subset of tr revertants contained no detectable lipoprotein. A lpp::Tn10 derivative of the mutant SE5312 was also temperature-resistant. These observations suggest that ALP N-acyltransferase is essential for the growth and viability of S. typhimurium, and this requirement is decreased in the absence of major outer membrane lipoprotein.

The autosomal dominant peripheral sensory neuropathy HSAN1 results from mutations in the LCB1 subunit of serine palmitoyltransferase (SPT). Serum from patients and transgenic mice expressing a disease-causing mutation (C133W) contain elevated levels of 1-deoxysphinganine (1-deoxySa), which presumably arise from inappropriate condensation of alanine with palmitoyl-CoA. Mutant heterodimeric SPT is catalytically inactive. However, mutant heterotrimeric SPT has approximately 10-20% of wild-type activity and supports growth of yeast cells lacking endogenous SPT. In addition, long chain base profiling revealed the synthesis of significantly more 1-deoxySa in yeast and mammalian cells expressing the heterotrimeric mutant enzyme than in cells expressing wild-type enzyme. Wild-type and mutant enzymes had similar affinities for serine. Surprisingly, the enzymes also had similar affinities for alanine, indicating that the major affect of the C133W mutation is to enhance activation of alanine for condensation with the acyl-CoA substrate. In vivo synthesis of 1-deoxySa by the mutant enzyme was proportional to the ratio of alanine to serine in the growth media, suggesting that this ratio can be used to modulate the relative synthesis of sphinganine and 1-deoxySa. By expressing SPT as a single-chain fusion protein to ensure stoichiometric expression of all three subunits, we showed that GADD153, a marker for endoplasmic reticulum stress, was significantly elevated in cells expressing mutant heterotrimers. GADD153 was also elevated in cells treated with 1-deoxySa. Taken together, these data indicate that the HSAN1 mutations perturb the active site of SPT resulting in a gain of function that is responsible for the HSAN1 phenotype.

It has been postulated that the N-acyl fatty acid attached to the amino terminus of the major Escherichia coli lipoprotein is derived from the fatty acid at the 1-position of phosphatidylethanolamine (PtdEtn) (Jackowski, S., and Rock, C.O. (1986) J. Biol. Chem. 261, 11328-11333). To ascertain the role of PtdEtn in the conversion of apolipoprotein to the mature lipoprotein, the lipoprotein from E. coli strain AH930 (pss::kan) containing a null mutation in the phosphatidylserine synthase gene (pss) was studied. Pulse labeling with [35S]methionine for 30 s or 5 min revealed the formation of mature lipoprotein in both wild-type (W3110) and mutant (AH930) cells. [3H]Palmitate-labeled lipoproteins from both the mutant and wild-type cells were found to contain nearly identical amounts of alkali-resistant (amide-linked, 41-42%) and alkali-labile (ester-linked, 58-59%) fatty acids. Edman degradation and dansylation of the immuno-affinity-purified [35S]cysteine-labeled lipoprotein showed that the NH2 terminus of the lipoprotein in the mutant was blocked as in the wild type. In vitro assay of apolipoprotein N-acyltransferase using membranes either from the mutant or the wild-type strain as the source of both the enzyme and the acyl donor revealed that both membranes were equally active in the conversion of [35S]methionine-labeled apolipoprotein to lipoprotein. These data strongly suggest that PtdEtn is not essential for the N-acylation of apolipoprotein to form lipoprotein, and other major phospholipids such as phosphatidylglycerol and cardiolipin can serve as the donor of fatty acid in the N-acylation of apolipoprotein.

Apolipoprotein N-acyltransferase, the enzyme catalyzing the conversion of apolipoprotein to mature lipoprotein, was detected by an in vitro assay using [35S]methionine-labeled apolipoprotein as the substrate. Triton X-100 solubilized the enzyme, and was required for its activity. The enzyme showed a broad pH optimum (pH 6.5-7.5). N-Acylation of apolipoprotein with ethanol-washed membranes was dependent on exogenous phospholipids, with phosphatidylethanolamine, phosphatidylglycerol and cardiolipin all showing about 10- to 20-times enhancement of the enzyme activity in the delipidated membranes. Incubation of apolipoprotein with [3H]palmitate-labeled membranes resulted in the incorporation of [3H]palmitate into lipoprotein. The enzyme was found to be enriched in the inner membrane and in the inner membrane/outer membrane mixed fractions of the E. coli cell envelope.

Changes in farnesyl protein transferase (FPTase) activity and FPTase beta-subunit protein levels were determined in IgE-sensitized RBL-2H3 mast cells in response to polyvalent antigen administration. Ten minutes after the addition of DNP modified BSA to mast cells, whose high affinity receptor for IgE (FcvarepsilonRI) contained bound anti-DNP IgE, FPTase specific activity increased by 54 +/- 28%. Time course studies showed FPTase specific activity doubled during a 20- to 30-min period after antigen-induced cell aggregation. Also, an increase in FPTase beta-subunit protein during this time ( approximately 30%) was observed; this protein increase was not accompanied by a similar increase in FPTase beta-subunit m-RNA levels. The FcvarepsilonRI aggregation had no significant effect on the activities of other enzymes involved with farnesyl diphosphate (FPP) metabolism: FPP synthase, isopentenyl diphosphate isomerase, geranylgeranyl protein transferase, and squalene synthase. Specific inhibition of FPTase activity by manumycin was studied to determine what role FPTase plays in mast cell activation. Manumycin profoundly inhibited hexosaminidase release in activated cells, indicating FPTase is required for signal transduction involved with protein exocytosis from RBL-2H3 mast cells.

Three distinct clones from a Salmonella typhimurium genomic library were identified which suppressed the copper-sensitive (Cu(s)) phenotype of cutF mutants of Escherichia coli. One of these clones, pCUTFS2, also increased the copper tolerance of cutA, -C, and -E mutants, as well as that of a lipoprotein diacylglyceryl transferase (lgt) mutant of E. coli. Characterization of pCUTFS2 revealed that the genes responsible for suppression of copper sensitivity (scs) reside on a 4.36-kb DNA fragment located near 25.4 min on the S. typhimurium genome. Sequence analysis of this fragment revealed four open reading frames (ORF120, ORF627, ORF207, and ORF168) that were organized into two operons. One operon consisted of a single gene, scsA (ORF120), whereas the other operon contained the genes scsB (ORF627), scsC (ORF207), and scsD (ORF168). Comparison of the deduced amino acid sequences of the predicted gene products showed that ScsB, ScsC, and ScsD have significant homology to thiol-disulfide interchange proteins (CutA2, DipZ, CycZ, and DsbD) from E. coli and Haemophilus influenzae, to an outer membrane protein (Com1) from Coxiella burnetii, and to thioredoxin and thioredoxin-like proteins, respectively. The two operons were subcloned on compatible plasmids, and complementation analyses indicated that all four proteins are required for the increased copper tolerance of E. coli mutants. In addition, the scs locus also restored lipoprotein modification in lgt mutants of E. coli. Sequence analyses of the S. typhimurium scs genes and adjacent DNAs revealed that the scs locus is flanked by genes with high homology to the cbpA (predicted curved DNA-binding protein) and agp (acid glucose phosphatase) genes of E. coli located at 22.90 min (1,062.07 kb) and 22.95 min (1,064.8 kb) of the E. coli chromosome, respectively. However, examination of the E. coli chromosome revealed that these genes are absent at this locus and no evidence has thus been obtained for the occurrence of the scs locus elsewhere on the genome.

Publisher Summary This chapter summarizes current knowledge of the lipid modification enzymes in the pathway for the maturation of bacterial lipoproteins. Lipid modification of bacterial lipoproteins is catalyzed by three enzymes: diacylglyceryltransferase, signal peptidase II, and N -acyltransferase. Major phospholipids in the bacterial cell envelope serve as both the diacylglyceryl and acyl donors. Phosphatidylglycerol (PG) is the major diacylglyceryl donor for diacylglyceryltransferase, whereas N -acyltransferase does not exhibit a preference for any particular acyl donor. The ability of Tricine- sodium dodecyl sulfate polyacrylamide gel (SDS-PAGE) to separate the various intermediates in the biosynthesis of murein lipoprotein is exploited to develop assays for diacylglyceryl modification of prolipoprotein and for N -acylation of apolipoprotein. A simpler and quicker peptide-based assay for diacylglyceryltransferase is available and a similar one for N -acyltransferase is required for its purification. The availability of the genes for the enzymes should enable hyper expression and the development of simple purification protocols. The specificity toward PG in vivo is demonstrated in vitro by delipidating the inverted vesicles with aqueous acetone extraction followed by incubation of the delipidated enzyme preparations with individual phospholipid species.

The biosynthetic pathway of sulfoquinovosyldiacylglycerol (SQDG) was investigated using groundnut (Arachis hypogaea) leaf discs and 35S-labeled precursors. [35S]SO4(2-) was actively taken up by the leaf discs and rapidly incorporated into SQDG. After 2 h, 1.5% of the [35S]SO4(2-) added to the incubation medium was taken up, of which 28% was incorporated into SQDG. The methanol-water phases of the lipid extracts of the leaf discs were analyzed for the 35S-labeled intermediates. Up to 2 h of incubation, cysteic acid, 3-sulfopyruvate, 3-sulfolactate, 3-sulfolactaldehyde, and sulfoquinovose (SQ) which have been proposed as intermediates [Davies et al. (1966) Biochem. J. 98, 369-373] were not labeled. Only a negligible amount of radioactivity was observed in these compounds after incubation for 4 h and more. Addition of sodium molybdate inhibited the uptake of [35S]SO4(2-) as well as its incorporation into SQDG by the leaf discs, suggesting that 3'-phosphoadenosine-5'-phosphosulfate may be involved in the biosynthesis of SQDG. Addition of unlabeled cysteic acid to the incubation medium enhanced the uptake of [35S]SO4(2-) but did not affect its incorporation into SQDG. 35S-labeled cysteic acid was taken up by the leaf discs and metabolized to sulfoacetic acid but not incorporated into SQ or SQDG. These results show that cysteic acid is not an intermediate in SQDG biosynthesis. [35S]SQ was taken up by the leaf discs and incorporated into SQDG in a time-dependent manner. [35S]Sulfoquinovosylglycerol was also taken up by the leaf discs but not incorporated into SQDG. It is concluded that SQDG is not biosynthesized by the proposed sulfoglycolytic pathway in higher plants. Though [35S]SQ was converted to SQDG, the rates are much lower compared to [35S]SO4(2-) incorporation, which suggests that a more direct pathway involving sulfonation of a lipid precursor may exist in higher plants.

Metabolism of the plant sulfolipid—sulfoquinovosyldiacylglycerol (SQDG)—was studied in animal tissues. In vivo experiments with $[^{35}S]$SQDG in guinea pigs showed that this lipid is not absorbed intact in the gastrointestinal tract. In these experiments, 3 h after administration of $[^{35}S]$SQDG, the intestinal mucosa contained 1 to 5% of the radioactivity as SQDG, while the remainder was in a water-soluble form. Analysis of the water-soluble components showed that about 60% of the radioactivity was present as sulfoquinovosylglycerol (SQG) and the remainder was present as free $SO^{2−}_4$. In the blood, 99% of the radioactivity was present as $SO^2_4$, SQG was not observed. In liver, only very little radioactivity was observed and appeared to be mainly in the form of $SO^{2−}_4$. Experiments with everted intestinal sacs of guinea pigs confirmed the formation of SQG, $SO^{2−}_4$, and, in addition, sulfoquinovosylmonoacylglycerol (SQMG) in this tissue. In vitro experiments with saline extracts of acetone powders of pancreas and intestinal mucosa of guinea pig, sheep, and rat showed that $[^{35}S]$SQDG was deacylated to SQMG (sulfolipase A activity) and SQG (sulfolipase B activity). It is concluded that animal tissues deacylate SQDG in a stepwise manner to SQG. It is further metabolized to yield free $SO^{2−}_4$ by cleavage of the C-S bond which appears to be brought about by the intestinal microflora. Sheep pancreatic sulfolipases were characterized. Bile salts, sodium dodecyl sulfate, and Triton X-100 inhibited the pancreatic sulfolipases, while $CaCl_2$ activated them. Substrate competition experiments and investigations on substrate specificity with a partially purified preparation indicated that relatively specific sulfolipase(s) may exist in pancreas. Among the species tested, guinea pig tissues showed the highest sulfolipase A and B activities followed sheep and rat tissues. Pancreatic enzymes were 18 to 60 times more active than intestinal enzymes.