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The resistance of methicillin-resistant Staphylococcus aureus (MRSA) to all β-lactam classes limits treatment options for serious infections involving this organism. Our goal is to discover new agents that restore the activity of β-lactams against MRSA, an approach that has led to the discovery of two classes of natural product antibiotics, a cyclic depsipeptide (krisynomycin) and a lipoglycopeptide (actinocarbasin), which potentiate the activity of imipenem against MRSA strain COL. We report here that these imipenem synergists are inhibitors of the bacterial type I signal peptidase SpsB, a serine protease that is required for the secretion of proteins that are exported through the Sec and Tat systems. A synthetic derivative of actinocarbasin, M131, synergized with imipenem both in vitro and in vivo with potent efficacy. The in vitro activity of M131 extends to clinical isolates of MRSA but not to a methicillin-sensitive strain. Synergy is restricted to β-lactam antibiotics and is not observed with other antibiotic classes. We propose that the SpsB inhibitors synergize with β-lactams by preventing the signal peptidase-mediated secretion of proteins required for β-lactam resistance. Combinations of SpsB inhibitors and β-lactams may expand the utility of these widely prescribed antibiotics to treat MRSA infections, analogous to β-lactamase inhibitors which restored the utility of this antibiotic class for the treatment of resistant Gram-negative infections.

A novel series of EP(4) ligands, based on a benzyl indoline scaffold, has been discovered. It was found that agonism and antagonism in this series can be easily modulated by minor modifications on the benzyl group. The pharmacokinetic, metabolic and pharmacological profiles of these compounds was explored. It was found that these compounds show good pharmacokinetics in rat and are efficacious in pre-clinical models of pain and inflammation.

Polar residues comprise about 15% of the transmembrane (TM) domains of proteins, where they can stabilize structure via native side chain-side chain interhelical hydrogen bonds between TM helices. However, non-native H-bonds may be implicated in disease states, through limiting protein dynamics during transport and/or misfolding the protein by inducing non-native rotational positions about TM helical axes. Here we have undertaken an investigation of the presence and strength of H-bond interactions within a series of helix-loop-helix ("hairpin") constructs derived from TM helices 3 and 4 (italic) of the cystic fibrosis transmembrane conductance regulator (CFTR) (prototypic sequence G(194)LALAHFVWIAPLQ(207)VALLMGLIWELLQASAFAGLGFLIV(232)LALFQ(237)AGLG(241)) in which wild-type Q207 in TM3 forms an interhelical H-bond with CF-phenotypic mutant V232D in TM4 [Therien, A. G., Grant, F. E., and Deber, C. M. (2001) Nat. Struct. Biol 8, 597-601]. In the present work, a library of 21 TM3/4 constructs was prepared, where Asp residues were placed individually at TM4 positions 221-241. Using gel shift assays-in which H-bond-linked hairpins (closed conformation) migrate faster than the elongated forms (open conformation)-we found that Q207 in TM3 is able to "capture" all 21 TM4 D mutations into measurable populations of interhelical H-bonds. A similar library of TM4 D mutants-but also containing Q207L-reverted to wild-type migration rates, confirming Q207 as the polar partner for TM4 D residues. In view of the broad capture range of Q207, these results emphasize the potential consequences to folding and dynamics of introducing polar mutations into the TM domains of membrane proteins in the vicinity of a native polar TM residue.

Previous experiments on the cystic fibrosis transmembrane conductance regulator suggested that non-native polar residues within membrane domains can compromise protein structure/function. However, depending on context, replacement of a native residue by a non-native residue can result either in genetic disease or in benign effects (e.g., polymorphisms). Knowledge of missense mutations that frequently cause protein malfunction and subsequent disease can accordingly reveal information as to the impact of these residues in local protein environments. We exploited this concept by performing a statistical comparison of disease-causing mutations in protein membrane-spanning domains versus soluble domains. Using the Human Gene Mutation Database of 240 proteins (including 80 membrane proteins) associated with human disease, we compared the relative phenotypic propensity to cause disease of the 20 naturally occurring amino acids when removed from-or inserted into-native protein sequences. We found that in transmembrane domains (TMDs), mutations involving polar residues, and ionizable residues in particular (notably arginine), are more often associated with protein malfunction than soluble proteins. To further test the hypothesis that interhelical cross-links formed by membrane-embedded polar residues stabilize TMDs, we compared the occurrence of such residues in the TMDs of mesophilic and thermophilic prokaryotes. Results showed a significantly higher proportion of ionizable residues in thermophilic organisms, reinforcing the notion that membrane-embedded electrostatic interactions play critical roles in TMD stability.

The enzymatic activity of the Na,K-ATPase, or sodium pump, is modulated by members of the so-called FXYD family of transmembrane proteins. The best characterized member, FXYD2, also referred to as the gamma subunit, has been shown to decrease the apparent Na+ affinity and increase the apparent ATP affinity of the pump. The effect on ATP affinity had been ascribed to the cytoplasmic C-terminal end of the protein, whereas recent observations suggest that the transmembrane (TM) segment of gamma mediates the Na+ affinity effect. Here we use a novel approach involving synthetic transmembrane mimetic peptides to demonstrate unequivocally that the TM domain of gamma effects the shift in apparent Na+ affinity. Specifically, we show that incubation of these peptides with membranes containing alphabeta pumps modulates Na+ affinity in a manner similar to transfected full-length gamma subunit. Using mutated gamma peptides and transfected proteins, we also show that a specific glycine residue, Gly-41, which is associated with a form of familial renal hypomagnesemia when mutated to Arg, is important for this kinetic effect, whereas Gly-35, located on an alternate face of the transmembrane helix, is not. The peptide approach allows for the analysis of mutants that fail to be expressed in a transfected system.

Using a helix-loop-helix construct consisting of the adjacent transmembrane segments 3 and 4 of the cystic fibrosis transmembrane conductance regulator (CFTR) labeled with pyrene at both N and C termini, we describe a system for the study of intramolecular helix-helix interactions within a polytopic membrane protein. Through measurement of pyrene excimer band intensity as a determinant of helix-helix proximity, we show that the helices retain tertiary contacts in detergent micelles. Notably, the nature of the micellar detergent can alter the stability of these contacts, with perfluorooctanoate highly supportive, lysophosphatidylcholine and lysophosphatidylglycerol somewhat less tolerant, and SDS largely intolerant of such interactions. This construct is further employed to study the role of the acyl chain length of micellar detergents in modulating interhelical packing; detergents having acyl chains of 9 carbons display the greatest extent of helical packing. These results provide important information regarding the role of lipids on membrane protein folding and conformation as well as demonstrate the usefulness of a pyrene-based system in studying the forces that govern interhelical packing.

Transmembrane (TM) alpha-helices are surrounded by the hydrocarbon chains of the lipid bilayer. The low dielectric constant of this environment makes it extremely unfavorable for a residue with a polar side chain to exist in a non-H-bonded state. Therefore, in combination with a wild-type polar residue partner, a polar TM mutant could generate, in some cases, a non-native H-bond that could impair native protein structure/function-and possibly lead to a disease state. We have examined protein mutation databases and have found many examples of TM-based apolar to polar mutations that are, in fact, a cause of human disease. Here we review the various molecular defects that such mutations can produce, including impeding protein dynamics by side-chain-side-chain interhelical H-bond cross-links; alteration of helical packing through steric hindrance; and disruption of a protein active site. We further note that the reverse case--membrane-embedded polar to apolar mutations--can similarly cause human disease, implying that native interhelical H-bonds can also play pivotal roles in stabilizing native TM domains. As a specific example, we show that the Gly to Arg mutation occurs statistically more frequently in TM domains as compared to its occurrence in soluble domains, suggesting that TM-based G-to-R mutations have a high "phenotypic propensity" for disease. A more complete understanding of how mutations involving polar residues in TM domains of proteins translate into compromised function may aid in the development of novel therapeutics.

This article reviews our studies of the γ subunit of the sodium pump. γ is a member of the FXYD family of small, single transmembrane proteins and is expressed predominantly in the kidney tubule. There are two major variants of γ which function similarly to bring about two distinct effects, one on K′ATP and the other, on KK, the affinity of the pump for K+ acting as a competitor of cytoplasmic Na+. In this way, γ is believed to provide a self-regulatory mechanism for maintaining the steady-state activity of the pump in the kidney. Our studies also suggest that K+ antagonism of cytoplasmic Na+ activation of the pump is relevant not only to the presence of γ in the kidney, but probably some hitherto undefined factor(s) in other tissues, most notably heart. The interesting possibility that not only γ but other members of the FXYD family regulate ion transport in a tissue-specific manner is discussed.

The Na+-K+-ATPase, or sodium pump, is the membrane-bound enzyme that maintains the Na+and K+gradients across the plasma membrane of animal cells. Because of its importance in many basic and specialized cellular functions, this enzyme must be able to adapt to changing cellular and physiological stimuli. This review presents an overview of the many mechanisms in place to regulate sodium pump activity in a tissue-specific manner. These mechanisms include regulation by substrates, membrane-associated components such as cytoskeletal elements and the γ-subunit, and circulating endogenous inhibitors as well as a variety of hormones, including corticosteroids, peptide hormones, and catecholamines. In addition, the review considers the effects of a range of specific intracellular signaling pathways involved in the regulation of pump activity and subcellular distribution, with particular consideration given to the effects of protein kinases and phosphatases.

Tissue-distinct interactions of the Na+-K+-ATPase with Na+and K+, independent of isoform-specific properties, were reported previously (A. G. Therien, N. B. Nestor, W. J. Ball, and R. Blostein. J. Biol. Chem. 271: 7104–7112, 1996). In this paper, we describe a detailed analysis of tissue-specific kinetics particularly relevant to regulation of pump activity by intracellular K+, namely K+inhibition at cytoplasmic Na+sites. Our results show that the order of susceptibilities of α1pumps of various rat tissues to K+/Na+antagonism, represented by the ratio of the apparent affinity for Na+binding at cytoplasmic activation sites in the absence of K+to the affinity constant for K+as a competitive inhibitor of Na+binding at cytoplasmic sites, is red blood cell < axolemma ≈ rat α1-transfected HeLa cells < small intestine < kidney < heart. In addition, we have carried out an extensive analysis of the kinetics of K+binding and occlusion to the cytoplasmic cation binding site and find that, for most tissues, there is a relationship between the rate of K+binding/occlusion and the apparent affinity for K+as a competitive inhibitor of Na+activation, the order for both parameters being heart ≥ kidney > small intestine ≈ rat α1-transfected HeLa cells. The notion that modulations in cytoplasmic K+/Na+antagonism are a potential mode of pump regulation is underscored by evidence of its reversibility. Thus the relatively high K+/Na+antagonism characteristic of kidney pumps was reduced when rat kidney microsomal membranes were fused into the dog red blood cell.

Two new series of EP 4 antagonists based on naphthalene/quinoline scaffolds have been identified as part of our on-going efforts to develop treatments for inflammatory pain. One series contains an acidic sulfonylurea pharmacophore, whereas the other is a neutral amide. Both series show subnanomolar intrinsic binding potency towards the EP 4 receptor, and excellent selectivity towards other prostanoid receptors. While the amide series generally displays poor pharmacokinetic parameters, the sulfonylureas exhibit greatly improved profile. MF - 592 , the optimal compound from the sulfonylurea series, has a desirable overall preclinical profile that suggests it is suitable for further development.

The discovery of highly potent and selective second generation EP(4) antagonist MK-2894 (34d) is discussed. This compound exhibits favorable pharmacokinetic profile in a number of preclinical species and potent anti-inflammatory activity in several animal models of pain/inflammation. It also shows favorable GI tolerability profile in rats when compared to traditional NSAID indomethacin.

A new series of EP4 antagonists based on a quinoline acylsulfonamide scaffold have been identified as part of our on-going efforts to develop treatments for chronic inflammation. These compounds show subnanomolar intrinsic binding potency towards the EP4 receptor, and excellent selectivity towards other prostanoid receptors. Acceptable pharmacokinetic profiles have also been demonstrated across a series of preclinical species.

The Na,K-ATPase, or sodium pump, is a ubiquitously expressed membrane-bound enzyme that controls the transmembrane (TM) gradients of sodium and potassium ions in animal cells. The enzyme comprises two subunits, alpha and beta, and in the kidney, is also associated with a small single-spanning membrane protein, the gamma subunit. This 65 amino acid residues protein has been linked to a form of dominant renal hypomagnesaemia resulting from substitution of a highly conserved glycine residue (Gly41) to arginine residue. In order to characterize the quaternary structure of the gamma subunit, and effects of the G41R mutation thereupon, we synthesized a series of peptides (wild-type and mutant) that span the gamma subunit TM region. Using circular dichroism spectroscopy, we show that the 32-amino acid residue peptides are random coils in aqueous buffer but spontaneously adopt an alpha-helical conformation in the presence of detergent micelles (sodium dodecyl sulfate, SDS, and perfluorooctanoate, PFO). Furthermore, fluorescence resonance energy transfer experiments, combined with polyacrylamide gel electrophoresis, demonstrate that while gamma-TM does not self-associate in SDS, it forms oligomers in PFO, a detergent that tolerates relatively weak associations between membrane proteins. Importantly, oligomerization of gamma-TM is abrogated in a peptide that contains either the disease-causing mutation G41R, or the more conservative mutation G41L. On the other hand, a peptide that contains a Gly-to-Arg substitution on a different face of the helix, at position 35, retains its ability to oligomerize. Our results provide evidence for a link between renal hypomagnesaemia and gamma subunit oligomerization.

The structural analysis of membrane proteins has long been hampered by the hydrophobic nature of these proteins. Our approach to the problem has involved the use of small double-spanning membrane peptides inserted into detergent micelles. Specifically, we have focused on a peptide, TM 3–4, derived from CFTR, the membrane channel associated with cystic fibrosis (CF). The 88 amino acid peptide contains residues 194–241 (GLALAHFYWIAPLQVALLMGLIWELLQASAFAGLGFLIVLALFQAGLG), including the third and fourth transmembrane segments (TMs), of CFTR, and contains a His-Tag and an S-tag for purification purposes. In a previous study, we have shown that TM 3–4 spontaneously inserts into detergent micelles and adopts an a-helical conformation [1]. However, it has remained unclear whether its tertiary structure is that of a helical-hairpin loop, as would be predicted from structural models of full length CFTR. In this report, we address this question and test the effects of various detergents on interhelical packing in this construct. Specifically, we have used pyrene as a probe of the distance between the two TMs in TM 3–4. Pyrene behaves atypically for a fluorophore in that although it absorbs light at 345 nm and emits it at ~385 and ~405 nm when in its monomelic form, it can form excited dimers (excimers) which emit light at ~485 nm [2]. Therefore, the ratio of excimer fluorescence (at 485 nm) to monomer fluorescence (at 385 nm) is a measure of the extent to which pyrene molecules are able to dimerize in a given system.

The functional role of the gamma subunit of the Na,K-ATPase was studied using rat gamma cDNA-transfected HEK-293 cells and an antiserum (gammaC33) specific for gamma. Although the sequence for gamma was verified and shown to be larger (7237 Da) than first reported, it still comprises a single initiator methionine despite the expression of a gammaC33-reactive doublet on immunoblots. Kinetic analysis of the enzyme of transfected compared with control cells and of gammaC33-treated kidney pumps shows that gamma regulates the apparent affinity for ATP. Thus, gamma-transfected cells have a decreased K'ATP as shown in measurements of (i) K'ATP of Na,K-ATPase activity and (ii) K+ inhibition of Na-ATPase at 1 microM ATP. Consistent with the behavior of gamma-transfected cells, gammaC33 pretreatment increases K'ATP of the kidney enzyme and K+ inhibition (1 microM ATP) of both kidney and gamma-transfected cells. These results are consistent with previous findings that an antiserum raised against the pig gamma subunit stabilizes the E2(K) form of the enzyme (Therien, A. G., Goldshleger, R., Karlish, S. J., and Blostein, R. (1997) J. Biol. Chem. 272, 32628-32634). Overall, our data demonstrate that gamma is a tissue (kidney)-specific regulator of the Na,K-ATPase that can increase the apparent affinity of the enzyme for ATP in a manner that is reversible by anti-gamma antiserum.

The Na,K-ATPase comprises a catalytic alpha subunit and a glycosylated beta subunit. Another membrane polypeptide, gamma, first described by Forbush et al. (Forbush, B., III, Kaplan, J. H., and Hoffman, J. F. (1978) Biochemistry 17, 3667-3676) associates with alpha and beta in purified kidney enzyme preparations. In this study, we have used a polyclonal anti-gamma antiserum to define the tissue specificity and topology of gamma and to address the question of whether gamma has a functional role. The trypsin sensitivity of the amino terminus of the gamma subunit in intact right-side-out pig kidney microsomes has confirmed that it is a type I membrane protein with an extracellular amino terminus. Western blot analysis shows that gamma subunit protein is present only in membranes from kidney tubules (rat, dog, pig) and not those from axolemma, heart, red blood cells, kidney glomeruli, cultured glomerular cells, alpha1-transfected HeLa cells, all derived from the same (rat) species, nor from three cultured cell lines derived from tubules of the kidney, namely NRK-52E (rat), LLC-PK (pig), or MDCK (dog). To gain insight into gamma function, the effects of the anti-gamma serum on the kinetic behavior of rat kidney sodium pumps was examined. The following evidence suggests that gamma stabilizes E1 conformation(s) of the enzyme and that anti-gamma counteracts this effect: (i) anti-gamma inhibits Na,K-ATPase, and the inhibition increases at acidic pH under which condition the E2(K) --E1 phase of the reaction sequence becomes more rate-limiting, (ii) the oligomycin-stimulated increase in the level of phosphoenzyme was greater in the presence of anti-gamma indicating that the antibody shifts the E1 left and right arrow left and right arrow E2P equilibria toward E2P, and (iii) when the Na+-ATPase reaction is assayed with the Na+ concentration reduced to levels (/=2 mM) which limit the rate of the E1 ----E2P transition, anti-gamma is stimulatory. These observations taken together with evidence that the pig gamma subunit, which migrates as a doublet on polyacrylamide gels, is sensitive to digestion by trypsin, and that Rb+ ions partially protect it against this effect, indicate that the gamma subunit is a tissue-specific regulator which shifts the steady-state equilibria toward E1. Accordingly, binding of anti-gamma disrupts alphabeta-gamma interactions and counteracts these modulatory effects of the gamma subunit.

There has been a recent surge of interest in the literature for the anaphylatoxins C3a and C5a and their role in inflammatory diseases. While knockout studies have provided important information regarding the role of C3a in allergic diseases such as asthma ([1][1], [2][2]), the publication of a