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Plasma cells daily secrete their own mass in antibodies, which fold and assemble in the endoplasmic reticulum (ER). To reach these levels, cells require pERp1, a novel lymphocyte-specific small ER-resident protein, which attains expression levels as high as BiP when B cells differentiate into plasma cells. Although pERp1 has no homology with known ER proteins, it does contain a CXXC motif typical for oxidoreductases. In steady state, the CXXC cysteines are locked by two parallel disulfide bonds with a downstream C(X) 6 C motif, and pERp1 displays only modest oxidoreductase activity. pERp1 emerged as a dedicated folding factor for IgM, associating with both heavy and light chains and promoting assembly and secretion of mature IgM.

Background: Endoplasmic reticulum (ER) stress has been implicated in causing hepatic insulin resistance. Results: Fructose-fed XBP1 knock-out mice were protected from hepatic insulin resistance despite increased hepatic ER stress and JNK activation. Conclusion: ER stress and hepatic JNK activation can be disassociated from hepatic insulin resistance. Significance: Hepatic ER stress is not a direct causal factor in hepatic insulin resistance., Hepatic insulin resistance has been attributed to both increased endoplasmic reticulum (ER) stress and accumulation of intracellular lipids, specifically diacylglycerol (DAG). The ER stress response protein, X-box-binding protein-1 (XBP1), was recently shown to regulate hepatic lipogenesis, suggesting that hepatic insulin resistance in models of ER stress may result from defective lipid storage, as opposed to ER-specific stress signals. Studies were designed to dissociate liver lipid accumulation and activation of ER stress signaling pathways, which would allow us to delineate the individual contributions of ER stress and hepatic lipid content to the pathogenesis of hepatic insulin resistance. Conditional XBP1 knock-out (XBP1Δ) and control mice were fed fructose chow for 1 week. Determinants of whole-body energy balance, weight, and composition were determined. Hepatic lipids including triglyceride, DAGs, and ceramide were measured, alongside markers of ER stress. Whole-body and tissue-specific insulin sensitivity were determined by hyperinsulinemic-euglycemic clamp studies. Hepatic ER stress signaling was increased in fructose chow-fed XBP1Δ mice as reflected by increased phosphorylated eIF2α, HSPA5 mRNA, and a 2-fold increase in hepatic JNK activity. Despite JNK activation, XBP1Δ displayed increased hepatic insulin sensitivity during hyperinsulinemic-euglycemic clamp studies, which was associated with increased insulin-stimulated IRS2 tyrosine phosphorylation, reduced hepatic DAG content, and reduced PKCϵ activity. These studies demonstrate that ER stress and IRE1α-mediated JNK activation can be disassociated from hepatic insulin resistance and support the hypothesis that hepatic insulin resistance in models of ER stress may be secondary to ER stress modulation of hepatic lipogenesis.

et al., Correct endoplasmic reticulum (ER) function is critical for the health of secretory cells, such as the pancreatic β-cell, and ER stress is often a contributory factor to β-cell death in type 2 diabetes.Wehave used an insulin-secreting cell line with inducible expression of dominant negative (DN) HNF1α, a transcription factor vital for correct β-cell development and function, to show that HNF1α is required for Xbp1 transcription and maintenance of the normal ER stress response. DN HNF1α expression sensitizes the β-cell to ER stress by directly downregulating Xbp1 transcription, whereas Atf6 is unaffected. Furthermore, DN HNF1α alters calcium homeostasis, resulting in elevated cytoplasmic calcium and increased store-operated calcium entry, whereas mitochondrial calcium uptake is normal. Loss of function of XBP1 is toxic to the β-cell and decreases production of the ER chaperone BiP, even in the absence of ER stress. DN HNF1α-induced sensitivity to cyclopiazonic acid can be partially rescued with the chemical chaperone tauroursode-oxycholate. Rat insulin 2 promoter-DN HNF1α mouse islets express lower levels of BiP mRNA, synthesize less insulin, and are sensitized to ER stress relative to matched control mouse islets, suggesting that this mechanism is also operating in vivo., This work was supported by an European Foundation for the Study of Diabetes/Lilly grant, Swiss National Science Foundation Grant 310000-116750/1, and EuroDia 6th Framework Programme Grant LSHM-CT-2006-518153.

Jamming Protein Translocation Antibiotics are tremendously important drugs in modern medicine, yet we are still learning precisely how they work. SecY is a bacterial membrane protein that is part of a complex that allows protein secretion across the membrane. Van Stelten et al. (p. 753 ; see the Perspective by Breukink ) found in Escherichia coli cells that if the protein translocator complex becomes jammed with a protein that cannot pass through, the SecY protein is degraded by the protease FtsH, leading to cell death. Cells could be protected by increasing amounts of an inhibitor of FtsH, the YccA protein. Antibiotics that block protein translation also caused jamming of the SecY machinery and destruction of SecY, thus contributing to cell death.

Transcription activation of some Saccharomyces cerevisiae genes is paralleled by their repositioning to the nuclear periphery, but the mechanism underlying gene anchoring is poorly defined. We show that the nuclear pore complex-associated Mlp1p and the shuttling mRNA export receptor Mex67p contribute to the stable association of the activated GAL10 and HSP104 genes with the nuclear periphery. However, we find no obligatory link between gene positioning and gene expression. Furthermore, gene anchoring correlates with the cotranscriptional recruitment of Mex67p to transcribing genes. Notably, the association of Mex67p with chromatin is not mediated by RNA. Interestingly, a mutant GAL2 gene lacking the coding region is still able to recruit Mex67p upon transcriptional activation and to relocate to the nuclear periphery. Together these data suggest that, at least for GAL2, nascent messenger ribonucleoprotein does not play a major role in gene anchoring and that the early recruitment of Mex67p contributes to gene repositioning by virtue of an RNA-independent process.

The highly abundant molecular chaperone Hsp90 functions with assistance from auxiliary factors, collectively referred to as Hsp90 cochaperones, and the Hsp70 system. Hsp104, a molecular chaperone required for stress tolerance and for maintenance of [psi(+)] prions in the budding yeast Saccharomyces cerevisiae, appears to collaborate only with the Hsp70 system. We now report that several cochaperones previously thought to be dedicated to Hsp90 are shared with Hsp104. We show that the Hsp90 cochaperones Sti1, Cpr7, and Cns1, which utilize tetratricopeptide repeat (TPR) domains to interact with a common surface on Hsp90, form complexes with Hsp104 in vivo and that Sti1 and Cpr7 interact with Hsp104 directly in vitro. The interaction is Hsp90 independent, as further emphasized by the fact that two distinct TPR domains of Sti1 are required for binding Hsp90 and Hsp104. In a striking parallel to the sequence requirements of Hsp90 for binding TPR proteins, binding of Sti1 to Hsp104 requires a related acidic sequence at the C-terminal tail of Hsp104. While Hsp90 efficiently sequesters the cochaperones during fermentative growth, respiratory conditions induce the interaction of a fraction of Hsp90 cochaperones with Hsp104. This suggests that cochaperone sharing may favor adaptation to altered metabolic conditions.

The protein kinase Gcn2 stimulates translation of the yeast transcription factor Gcn4 upon amino acid starvation. Using genetic and biochemical approaches, we show that Gcn2 is regulated by the molecular chaperone Hsp90 in budding yeast Saccharomyces cerevisiae. Specifically, we found that (i) several Hsp90 mutant strains exhibit constitutive expression of a GCN4-lacZ reporter plasmid; (ii) Gcn2 and Hsp90 form a complex in vitro as well as in vivo; (iii) the specific inhibitors of Hsp90, geldanamycin and macbecin I, enhance the association of Gcn2 with Hsp90 and inhibit its kinase activity in vitro; (iv) in vivo, macbecin I strongly reduces the levels of Gcn2; (v) in a strain expressing the temperature-sensitive Hsp90 mutant G170D, both the accumulation and activity of Gcn2 are abolished at the restrictive temperature; and (vi) the Hsp90 cochaperones Cdc37, Sti1, and Sba1 are required for the response to amino acid starvation. Taken together, these data identify Gcn2 as a novel target for Hsp90, which plays a crucial role for the maturation and regulation of Gcn2.

BACKGROUND: Glucose-regulated protein 78 (GRP78) is highly expressed in first trimester cytrophoblastic cells (CTBs), especially in syncytiotrophoblast (STB). However, the role of GRP78 in these cells has never been investigated. METHODOLOGY/PRINCIPAL FINDINGS: In this study, we have examined the role of GRP78 in trophoblast fusion using the Bewo choriocarcinoma cell line as a model of cytotrophoblast fusion. Down regulation of GRP78 by siRNA or chemical inhibitors and use of antibodies against GRP78 in culture medium significantly decreased forskolin-induced fusion capacity of Bewo cells suggesting the involvement of membrane GRP78 in trophoblast fusion. GRP78 expression was also studied in preeclamptic (PE) CTBs which are known to have lower fusion capacity compared to control CTBs. Interestingly, despite the increase of GRP78 mRNA in PE CTBs, membrane GRP78 is significantly decreased in PE CTBs compared to control CTBs, suggesting that relocation of GRP78 from the endoplasmic reticulum to cell surface is probably altered in PE CTBs. CONCLUSIONS: Our results imply that membrane GRP78 could play an important role in syncytialisation. They also suggest that deregulation of GRP78 expression or relocation at cell surface might be involved in pregnancy complication associated with defective syncytialisation, such as preeclampsia.