Fusion with wild-type SNARE domains is controlled by juxtamembrane domains,transmembrane anchors,and Sec17

Membrane fusion requires tethers, SNAREs of R, Qa, Qb, and Qc families, and chaperones of the SM, Sec17/SNAP, and Sec18/NSF families. SNAREs have N-domains, SNARE domains that zipper into 4-helical RQaQbQc coiled coils, a short juxtamembrane (Jx) domain, and (often) a C-terminal transmembrane anchor. We reconstitute fusion with purified components from yeast vacuoles, where the HOPS protein combines tethering and SM functions. The vacuolar Rab, lipids, and R-SNARE activate HOPS to bind Q-SNAREs and catalyze trans-SNARE associations. With SNAREs initially disassembled, as they are on the organelle, we now report that R- and Qa-SNAREs require their physiological juxtamembrane (Jx) regions for fusion. Swap of the Jx domain between the R- and Qa-SNAREs blocks fusion after SNARE association in trans. This block is bypassed by either Sec17, which drives fusion without requiring complete SNARE zippering, or transmembrane-anchored Qb-SNARE in complex with Qa. The abundance of the trans-SNARE complex is not the sole fusion determinant, as it is unaltered by Sec17, Jx swap, or the Qb-transmembrane anchor. The sensitivity of fusion to Jx swap in the absence of a Qb transmembrane anchor is inherent to the SNAREs, because it remains when a synthetic tether replaces HOPS.     

Active conformation of the erythropoietin receptor: random and cysteine-scanning mutagenesis of the extracellular juxtamembrane and transmembrane domains

In the absence of erythropoietin (Epo) cell surface Epo receptors (EpoR) are dimeric; dimerization is mediated mainly by the transmembrane domain. Binding of Epo changes the orientation of the two receptor subunits. This conformational change is transmitted through the juxtamembrane and transmembrane domains, leading to activation of JAK2 kinase and induction of proliferation and survival signals. To define the active EpoR conformation(s) we screened libraries of EpoRs with random mutations in the transmembrane domain and identified several point mutations that activate the EpoR in the absence of ligand, including changes of either of the first two transmembrane domain residues (Leu(226) and Ile(227)) to cysteine. Following this discovery, we performed cysteine-scanning mutagenesis in the EpoR juxtamembrane and transmembrane domains. Many mutants formed disulfide-linked receptor dimers, but only EpoR dimers linked by cysteines at positions 223, 226, or 227 activated EpoR signal transduction pathways and supported proliferation of Ba/F3 cells in the absence of cytokines. These data suggest that activation of dimeric EpoR by Epo binding is achieved by reorienting the EpoR transmembrane and the connected cytosolic domains and that certain disulfide-bonded dimers represent the activated dimeric conformation of the EpoR, constitutively activating downstream signaling. Based on our data and the previously determined structure of Epo bound to a dimer of the EpoR extracellular domain, we present a model of the active and inactive conformations of the Epo receptor.     

Mutations in the amino terminus of the cystic fibrosis transmembrane conductance regulator enhance endocytosis

Efficient endocytosis of the cystic fibrosis transmembrane conductance regulator (CFTR) is mediated by a tyrosine-based internalization signal in the CFTR carboxyl-terminal tail 1424YDSI1427. In the present studies, two naturally occurring cystic fibrosis mutations in the amino terminus of CFTR, R31C, and R31L were examined. To determine the defect that these mutations cause, the Arg-31 mutants were expressed in COS-7 cells and their biogenesis and trafficking to the cell surface tested in metabolic pulse-chase and surface biotinylation assays, respectively. The results indicated that both Arg-31 mutants were processed to band C at approximately 50% the efficiency of the wild-type protein. However, once processed and delivered to the cell surface, their half-lives were the same as wild-type protein. Interestingly, indirect immunofluorescence and cell surface biotinylation indicated that the surface pool was much smaller than could be accounted for based on the biogenesis defect alone. Therefore, the Arg-31 mutants were tested in internalization assays and found to be internalized at 2x the rate of the wild-type protein. Patch clamp and 6-methoxy-N-(3-sulfopropyl)quinolinium analysis confirmed reduced amounts of functional Arg-31 channels at the cell surface. Together, the results suggest that both R31C and R31L mutations compromise biogenesis and enhance internalization of CFTR. These two additive effects contribute to the loss of surface expression and the associated defect in chloride conductance that is consistent with a disease phenotype.     

NMR structural studies of interactions of a small, nonpeptidyl Tpo mimic with the thrombopoietin receptor extracellular juxtamembrane and transmembrane domains

Thrombopoietin (Tpo) is a glycoprotein growth factor that supports hematopoietic stem cell survival and expansion and is the principal regulator of megakaryocyte growth and differentiation. Several small, nonpeptidyl molecules have been identified as selective human Tpo receptor (hTpoR) agonists. To understand how the small molecule Tpo mimic SB394725 interacts and activates hTpoR, we performed receptor domain swap and mutagenesis studies. The results suggest that SB394725 interacts specifically with the extracellular juxtamembrane region (JMR) and the transmembrane (TM) domain of hTpoR. Solution and solid-state NMR structural studies using a peptide containing the JMR-TM sequences showed that this region of hTpoR, unexpectedly, consists of two alpha-helices separated by a few nonhelical residues. SB394725 interacts specifically with His-499 in the TM domain and a few distinct residues in the JMR-TM region and affects several specific C-terminal TM domain residues. The unique structural information provided by these studies both sheds light on the distinctive mechanism of action of SB394725 and provides valuable insight into the mechanism of ligand-induced cytokine receptor activation.     

Membrane and juxtamembrane targeting by PH and PTB domains

Modular pleckstrin homology (PH) and phospho-tyrosine binding (PTB) domains are present in a remarkably large number of proteins from yeast to humans. With a common core fold, these domain families have evolved to recognize membrane embedded phospholipids, in particular phosphoinositides, peripheral membrane proteins, and peptide motifs in juxtamembrane regions of integral membrane proteins. As the result of intensive investigation using biochemical, biophysical, and structural approaches, common ligand recognition principles have emerged along with insights into the structural variations that account for the diversity of ligand specificities. Analyses of membrane targeting in cells have revealed additional determinants beyond the primary ligand binding sites. In this review, we highlight unifying recognition principles and further illustrate with examples how divergent mechanisms contribute to membrane and juxtamembrane targeting by PH and PTB domains.     

Mutations of immunoglobulin transmembrane and cytoplasmic domains: effects on intracellular signaling and antigen presentation

The membrane-bound form of immunoglobulin serves as an antigen-specific receptor for B cells mediating signal transduction and antigen presentation. We have developed an assay that reconstitutes both these physiologic responses with respect to the antigen phosphorylcholine. By introducing specific mutations in the human Ig mu chain gene, we have shown that certain transmembrane residues and the short cytoplasmic domain are crucial for these two activities. Moreover, elimination of a single transmembrane hydroxyl group severely inhibits antigen presentation without affecting signal transduction, suggesting that these two functions are mediated by different protein interactions.     

Mutations in transmembrane domains 5 and 7 of the human excitatory amino acid transporter 1 affect the substrate-activated anion channel

L-Glutamate is the predominant excitatory neurotransmitter in the brain, and its extracellular concentration is tightly controlled by the excitatory amino acid transporters (EAATs). The transport of 1 glutamate molecule is coupled to the cotransport of 3 Na+ and 1 H+ and the countertransport of 1 K+. In addition to substrate transport, the binding of glutamate and Na+ activates an anion current which is thermodynamically uncoupled from the transport process. We have identified three amino acid residues in EAAT1 (D272 in TM5, K384 and R385 in TM7) that influence the amplitude of the anion channel current relative to the transport current. Transporters containing the mutations R268A, D272A, D272K, K384A, K384D, R385A, and R385D were expressed in Xenopus laevis oocytes and their transport and anion channel functions measured using the two-electrode voltage clamp techniques. The D272, K384, and R385 mutant transporters showed no change in transport properties but have increased levels of anion channel activity compared to wild-type transporters. These results identify additional residues of the EAAT1 transporter that may contribute to the gating mechanism of the anion channel of glutamate transporters and also provide hints as to how substrate binding leads to channel activation.     

Critical role of the pleckstrin homology and cysteine-rich domains in Vav signaling and transforming activity

Vav family proteins are members of the Dbl family of guanine nucleotide exchange factors and activators of Rho family small GTPases. In addition to the Dbl homology (DH) domain important for guanine nucleotide exchange factor catalytic function, all Dbl family proteins contain an adjacent pleckstrin homology (PH) domain that serves to regulate DH domain activity. Although the role of the PH domain in Vav function has been evaluated extensively, its precise role and whether it serves a distinct role in different Vav proteins remain unresolved. Additionally, the precise role of an adjacent cysteine-rich domain (CRD) in regulating DH domain function is also unclear. In this study, we evaluated the contribution of these putative protein-protein or protein-lipid interaction domains to Vav signaling and transforming activity. In contrast to previous observations, we found that the PH domain is critical for Vav transforming activity. Similarly, the CRD was also essential and served a function distinct from that of the PH domain. Although mutation of either domain reduced Vav membrane association, addition of plasma membrane targeting sequences to either the CRD or PH domain mutant proteins did not restore Vav transforming activity. This result contrasts with other Dbl family proteins, where a membrane targeting sequence alone was sufficient to restore the loss of function caused by mutation of the PH domain. Furthermore, green fluorescent protein fusion proteins containing the PH domain or CRD, or both, failed to target to the plasma membrane, suggesting that these two domains also serve regulatory functions independent of promoting membrane localization. Finally, we found that phosphatidylinositol 3-kinase activation may promote Vav membrane association via phosphatidylinositol 3,4,5-triphosphate binding to the PH domain.     

Intramolecular interactions between the juxtamembrane domain and phosphatase domains of receptor protein-tyrosine phosphatase RPTPmu. Regulation of catalytic activity

RPTPmu is a receptor-like protein-tyrosine phosphatase (RPTP) whose ectodomain mediates homotypic cell-cell interactions. The intracellular part of RPTPmu contains a relatively long juxtamembrane domain (158 amino acids; aa) and two conserved phosphatase domains (C1 and C2). The membrane-proximal C1 domain is responsible for the catalytic activity of RPTPmu, whereas the membrane-distal C2 domain serves an unknown function. The regulation of RPTP activity remains poorly understood, although dimerization has been proposed as a general mechanism of inactivation. Using the yeast two-hybrid system, we find that the C1 domain binds to an N-terminal noncatalytic region in RPTPmu, termed JM (aa 803-955), consisting of a large part of the juxtamembrane domain (120 aa) and a small part of the C1 domain (33 aa). When co-expressed in COS cells, the JM polypeptide binds to both the C1 and the C2 domain. Strikingly, the isolated JM polypeptide fails to interact with either full-length RPTPmu or with truncated versions of RPTPmu that contain the JM region, consistent with the JM-C1 and JM-C2 interactions being intramolecular rather than intermolecular. Furthermore, we find that large part of the juxtamembrane domain (aa 814-922) is essential for C1 to be catalytically active. Our findings suggest a model in which RPTPmu activity is regulated by the juxtamembrane domain undergoing intramolecular interactions with both the C1 and C2 domain.     

Identification of amino acids in the transmembrane and juxtamembrane domains of the platelet-derived growth factor receptor required for productive interaction with the bovine papillomavirus E5 protein.

The bovine papillomavirus E5 protein forms a stable complex with the cellular platelet-derived growth factor (PDGF) beta receptor, resulting in receptor activation and cell transformation. Amino acids in both the putative transmembrane domain and extracytoplasmic carboxyl-terminal domain of the E5 protein appear important for PDGF receptor binding and activation. Previous analysis indicated that the transmembrane domain of the receptor was also required for complex formation and receptor activation. Here we analyzed receptor chimeras and point mutants to identify specific amino acids in the PDGF beta receptor required for productive interaction with the E5 protein. These receptor mutants were analyzed in murine Ba/F3 cells, which do not express endogenous receptor. Our results confirmed the importance of the transmembrane domain of the receptor for complex formation, receptor tyrosine phosphorylation, and mitogenic signaling in response to the E5 protein and established that the threonine residue in this domain is required for these activities. In addition, a positive charge in the extracellular juxtamembrane domain of the receptor was required for E5 interaction and signaling, whereas replacement of the wild-type lysine with either a neutral or acidic amino acid inhibited E5-induced receptor activation and transformation. All of the receptor mutants defective for activation by the E5 protein responded to acute treatment with PDGF and to stable expression of v-Sis, a form of PDGF. The required juxtamembrane lysine and transmembrane threonine are predicted to align precisely on the same face of an alpha helix packed in a left-handed coiled-coil geometry. These results establish that the E5 protein and v-Sis recognize distinct binding sites on the PDGF beta receptor and further clarify the nature of the interaction between the viral transforming protein and its cellular target.     

Lactisole interacts with the transmembrane domains of human T1R3 to inhibit sweet taste

The detection of sweet-tasting compounds is mediated in large part by a heterodimeric receptor comprised of T1R2+T1R3. Lactisole, a broad-acting sweet antagonist, suppresses the sweet taste of sugars, protein sweeteners, and artificial sweeteners. Lactisole's inhibitory effect is specific to humans and other primates; lactisole does not affect responses to sweet compounds in rodents. By heterologously expressing interspecies combinations of T1R2+T1R3, we have determined that the target for lactisole's action is human T1R3. From studies with mouse/human chimeras of T1R3, we determined that the molecular basis for sensitivity to lactisole depends on only a few residues within the transmembrane region of human T1R3. Alanine substitution of residues in the transmembrane region of human T1R3 revealed 4 key residues required for sensitivity to lactisole. In our model of T1R3's seven transmembrane helices, lactisole is predicted to dock to a binding pocket within the transmembrane region that includes these 4 key residues.     

Mutations in the heparin-binding domains of human basic fibroblast growth factor alter its biological activity

Eleven structural analogues of human basic fibroblast growth factor (bFGF) have been prepared by site-directed mutagenesis of a synthetic bFGF gene to examine the effect of amino acid substitutions in the three putative heparin-binding domains on FGF's biological activity. After expression in Escherichia coli, the mutant proteins were purified to homogeneity by use of heparin-Sepharose chromatography and analyzed for their ability to stimulate DNA synthesis in human foreskin fibroblasts. Recombinant human bFGF 1-146 and [Ala69,Ser87]bFGF, an analogue where two of the four cysteines had been replaced by alanine and serine, were equipotent to standard bovine basic fibroblast growth factor. Substitution of aspartic acid-19 by arginine in the first heparin-binding domain yielded a molecule that stimulated a higher total mitogenic response in fibroblasts as compared to bFGF. In addition, replacement of either arginine-107 in the second domain or glutamine-123 in the third domain with glutamic acid resulted in compounds that were 2 and 4 times more potent than bFGF. In contrast, substitution of arginine-107 with isoleucine reduced the activity of the molecule by 100-fold. Combination of domain substitutions to generate the [Glu107,123]bFGF and [Arg19,Lys123,126]bFGF mutants did not show any additivity of the mutations on biological activity. Alterations in the biological activity of the analogues was dependent on both the site of and the type of modification. Increased positive charge in the first domain and increased negative charge in the second and third domains enhanced biological potency. The altered activities of the derivatives appear to be due in part to changes in the affinity of the analogues for heparin. We conclude that changes in all three of the putative heparin-binding domains result in altered mitogenic activity and heparin interaction of basic fibroblast growth factor.     

Association of IL1R polymorphism with HLA-B27 positive in Iranian patients with ankylosing spondylitis

Ankylosing spondylitis (AS) is one of the most common causes of inflammatory arthritis, with an estimated prevalence of 0.1-0.9%. Genetic factors have been strongly implicated in its aetiology, and heritability as assessed by twin studies has been estimated to be >90%. HLA- B27 is almost essential for inheritance of AS; it is not merely sufficient for explaining the pattern of familial recurrence of the disease. This study's purpose is to investigate the association of ankylosing spondylitis with single-nucleotide polymorphisms (SNPs) in the IL-1 family: IL-1a (-889C/T) rs1800587, IL-1b (-511C/T) rs16944, IL-1b (+3962C/T) rs1143634, IL-1R (Pst-1 1970C/T) rs2234650 and IL-1RA (Mspa-1 11100C/T) rs315952. 99 unrelated Iranian AS patients and 217 healthy control subjects were selected. Cytokine typing was performed by the polymerase chain reaction with sequence-specific primers assay. The allele and genotype frequencies of the polymorphisms were determined: The IL1α rs1800587, IL1β rs16944 and IL1β rs1143634 were not significantly associated with AS. Genotype frequencies at IL1R rs2234650 differed between cases and controls (χ(2)=8.85; p=0.01); the IL1R rs2234650 C/T and T/T genotypes were less common in AS patients than controls. The IL1R rs2234650 C/T genotype was inversely associated with AS comparing with the IL1R rs2234650 C/C genotype (OR=0.48; p=0.005). IL1R rs2234650 C/T genotype was less common in patients than controls (OR=0.37; p=0.02).Furthermore IL1R rs2234650 T allele was strongly associated with HLA-B2702 patients rather than HLA-B2705 but was not associated with HLA-B27 negative patients (OR=0.33; p=0.01). Polymorphisms of IL1α rs1800587, IL1β rs16944 and IL1β rs1143634 were not significantly associated with ankylosing spondylitis but inversely in this study IL1R rs2234650 was significantly associated and carriage of T allele in IL1R rs2234650 seems to be protective, while carriage of C allele result in two fold higher risk of developing AS.     

IL12B and IL23R gene SNPs in Japanese psoriasis

Psoriasis is a common human skin disease whereby abnormal production of inflammatory mediators is believed to play an important role in its pathogenesis. The IL12B gene, which encodes the shared IL-12p40 subunit in two cytokines, IL-12 and IL-23, and the IL23R gene, which encodes a subunit of the receptor for IL-23, were identified as psoriasis-susceptibility genetic factors in recent candidate gene and genome-wide association studies of Chinese and Europeans. Since there are significant differences in the incidence of psoriasis between Europeans and Japanese suggesting a genetic ethnic effect, we examined the association of IL12B and IL23R gene polymorphisms with psoriasis in a cohort of Japanese. In this study, we genotyped two SNPs (rs3212227 and rs6887695) in the IL12B gene and one SNP (rs11209026) in the IL23R gene using 560 Japanese psoriasis cases and 560 controls and compared our results with those previously published for Europeans and Asians. Our study showed significant associations between psoriasis and both IL12B gene SNPs, rs3212227 (odds ratio (OR)?=?1.35, P?=?4.94E-04) and rs6887695 (OR?=?1.32, P?=?2.00E-03), but no significant association between psoriasis and the IL23R SNP, rs11209026. Furthermore, a significant haplotype association was found for the IL12B gene protective haplotype C-C (OR?=?0.71, P?=?1.84E-04) in Japanese, as previously elucidated in the studies of European ancestry.     

L-selectin transmembrane and cytoplasmic domains are monomeric in membranes

A recombinant protein termed CLS, which corresponds to the C-terminal portion of human L-selectin and contains its entire transmembrane and cytoplasmic domains (residues Ser473-Arg542), has been produced and its oligomeric state in detergents characterized. CLS migrates in the SDS polyacrylamide gel at a pace that is typically expected from a complex twice of its molecular weight. Additional studies revealed, however, that this is due to residues in the cytoplasmic domain, as mutations in this region or its deletion significantly increased the electrophoretic rate of CLS. Analytical ultracentrifugation and fluorescence resonance energy transfer studies indicated that CLS reconstituted in dodecylphosphocholine detergent micelles is monomeric. When the transmembrane domain of L-selectin is inserted into the inner membrane of Escherichia coli as a part of a chimeric protein in the TOXCAT assay, little oligomerization of the chimeric protein is observed. Overall, these results suggest that transmembrane and cytoplasmic domains of L-selectin lack the propensity to self-associate in membranes, in contrast to the previously documented dimerization of the transmembrane domain of closely related P-selectin. This study will provide constraints for future investigations on the interaction of L-selectin and its associating proteins.     

Single-spanning transmembrane domains in cell growth and cell-cell interactions

As a whole, integral membrane proteins represent about one third of sequenced genomes, and more than 50% of currently available drugs target membrane proteins, often cell surface receptors. Some membrane protein classes, with a defined number of transmembrane (TM) helices, are receiving much attention because of their great functional and pharmacological importance, such as G protein-coupled receptors possessing 7 TM segments. Although they represent roughly half of all membrane proteins, bitopic proteins (with only 1 TM helix) have so far been less well characterized. Though they include many essential families of receptors, such as adhesion molecules and receptor tyrosine kinases, many of which are excellent targets for biopharmaceuticals (peptides, antibodies, et al.). A growing body of evidence suggests a major role for interactions between TM domains of these receptors in signaling, through homo and heteromeric associations, conformational changes, assembly of signaling platforms, etc. Significantly, mutations within single domains are frequent in human disease, such as cancer or developmental disorders. This review attempts to give an overview of current knowledge about these interactions, from structural data to therapeutic perspectives, focusing on bitopic proteins involved in cell signaling.     

Mutations in transmembrane domains 1, 4 and 9 of the Plasmodium falciparum chloroquine resistance transporter alter susceptibility to chloroquine, quinine and quinidine

Mutations in the Plasmodium falciparum chloroquine (CQ) resistance transporter (PfCRT) can result in verapamil-reversible CQ resistance and altered susceptibility to other antimalarials. PfCRT contains 10 membrane-spanning domains and is found in the digestive vacuole (DV) membrane of intraerythrocytic parasites. The mechanism by which PfCRT mediates CQ resistance is unclear although it is associated with decreased accumulation of drug within the DV. On the permissive background of the P. falciparum 106/1(K76) parasite line, we used single-step drug selection to generate isogenic clones containing unique pfcrt point mutations that resulted in amino acid changes in PfCRT transmembrane domains 1 (C72R, K76N, K76I and K76T) and 9 (Q352K, Q352R). The resulting changes of charge and hydropathy affected quantitative CQ susceptibility and accumulation as well as the stereospecific responses to quinine and quinidine. These results, together with a previously described S163R mutation in transmembrane domain 4, indicate that transmembrane segments 1, 4 and 9 of PfCRT provide important structural components of a substrate recognition and translocation domain. Charge-affecting mutations within these segments may affect the ability of PfCRT to bind different quinoline drugs and determine their net accumulation in the DV.