The Role of the Transmembrane and of the Intraviral Domain of Glycoproteins in Membrane Fusion of Enveloped Viruses

Fusion of enveloped viruses with their target membrane is mediated by viral integral glycoproteins. A conformational change of their ectodomain triggers membrane fusion. Several studies suggest that an extended, triple-stranded rod-shaped -helical coiled coil resembles a common structural and functional motif of the ectodomain of fusion proteins. From that, it is believed that essential features of the fusion process are conserved among the various enveloped viruses. However, this has not been established so far for the highly conserved transmembrane and intraviral sequences of fusion proteins. The article will focus on the role of both sequences in the fusion process. Recent studies from various enveloped viruses strongly imply that a transmembrane domain with a minimum length is required for later steps of membrane fusion, i.e., the formation and enlargement of the aqueous fusion pore. Although no specific sequence of the TM is necessary for pore formation, distinct properties and motifs of the domain may be obligatory to ascertain full fusion activity. However, with some exceptions, the intraviral domain seems to be not required for fusion activity of viral fusion proteins.     

Type I Interferon Signaling Is Decoupled from Specific Receptor Orientation through Lenient Requirements of the Transmembrane Domain

Type I interferons serve as the first line of defense against pathogen invasion. Binding of IFNs to its receptors, IFNAR1 and IFNAR2, is leading to activation of the IFN response. To determine whether structural perturbations observed during binding are propagated to the cytoplasmic domain, multiple mutations were introduced into the transmembrane helix and its surroundings. Insertion of one to five alanine residues near either the N or C terminus of the transmembrane domain (TMD) likely promotes a rotation of 100° and a translation of 1.5 Å per added residue. Surprisingly, the added alanines had little effect on the binding affinity of IFN to the cell surface receptors, STAT phosphorylation, or gene induction. Similarly, substitution of the juxtamembrane residues of the TMD with alanines, or replacement of the TMD of IFNAR1 with that of IFNAR2, did not affect IFN binding or activity. Finally, only the addition of 10 serine residues (but not 2 or 4) between the extracellular domain of IFNAR1 and the TMD had some effect on signaling. Bioinformatic analysis shows a correlation between high sequence conservation of TMDs of cytokine receptors and the ability to transmit structural signals. Sequence conservation near the TMD of IFNAR1 is low, suggesting limited functional importance for this region. Our results suggest that IFN binding to the extracellular domains of IFNAR1 and IFNAR2 promotes proximity between the intracellular domains and that differential signaling is a function of duration of activation and affinity of binding rather than specific conformational changes transmitted from the outside to the inside of the cell.     

Studies of Receptor Tyrosine Kinase Transmembrane Domain Interactions: The EmEx-FRET Method

The energetics of transmembrane (TM) helix dimerization in membranes and the thermodynamic principles behind receptor tyrosine kinase (RTK) TM domain interactions during signal transduction can be studied using Förster resonance energy transfer (FRET). For instance, FRET studies have yielded the stabilities of wild-type fibroblast growth factor receptor 3 (FGFR3) TM domains and two FGFR3 pathogenic mutants, Ala391Glu and Gly380Arg, in the native bilayer environment. To further our understanding of the molecular mechanisms of deregulated FGFR3 signaling underlying different pathologies, we determined the effect of the Gly382Asp FGFR3 mutation, identified in a multiple myeloma cell line, on the energetics of FGFR3 TM domain dimerization. We measured dimerization energetics using a novel FRET acquisition and processing method, termed “emission-excitation FRET (EmEx-FRET),” which improves the precision of thermodynamic measurements of TM helix association. The EmEx-FRET method, verified here by analyzing previously published data for wild-type FGFR3 TM domain, should have broad utility in studies of protein interactions, particularly in cases when the concentrations of fluorophore-tagged molecules cannot be controlled.     

The human glucagon receptor encoding gene: structure, cDNA sequence and chromosomal localization

Characterization of the human glucagon-receptor-encoding gene (GGR) should provide a greater understanding of blood glucose regulation and may reveal a genetic basis for the pathogenesis of diabetes. A cDNA encoding a complete functional human glucagon receptor (GGR) was isolated from a liver cDNA library by a combination of polymerase chain reaction and colony hybridization. The cDNA encodes a receptor protein with 80% identity to rat GGR that binds [¹²⁵I] glucagon and transduces a signal leading to increases in the concentration of intracellular cyclic adenosine 3′,5′-monophosphate. Southern blot analysis of human DNA reveals a hybridization pattern consistent with a single GGR locus. In situ hybridization to metaphase chromosome preparations maps the GGR locus to chromosome 17q25. Analysis of the genomic sequence shows that the coding region spans over 5.5 kb and is interrupted by 12 introns.     

Deletion of Phenylalanine 508 in the First Nucleotide-binding Domain of the Cystic Fibrosis Transmembrane Conductance Regulator Increases Conformational Exchange and Inhibits Dimerization

Deletion of Phe-508 (F508del) in the first nucleotide-binding domain (NBD1) of the cystic fibrosis transmembrane conductance regulator (CFTR) results in destabilization of the domain, intramolecular interactions involving the domain, and the entire channel. The destabilization caused by F508del manifests itself in defective channel processing and channel gating defects. Here, we present NMR studies of the effect of F508del and the I539T stabilizing mutation on NBD1 dynamics, with a view to understanding these changes in stability. Qualitatively, F508del NMR spectra exhibit significantly more peak broadening than WT spectra due to the enhanced intermediate time scale (millisecond to microsecond) motions in the mutant. Unexpectedly, studies of fast (nanosecond to picosecond) motions revealed that F508del NBD1 tumbles more rapidly in solution than WT NBD1. Whereas F508del tumbles at a rate nearly consistent with the monomeric state, the WT protein tumbles significantly more slowly. Paramagnetic relaxation enhancement experiments confirm that NBD1 homodimerizes in solution in the expected head-to-tail orientation. NMR spectra of WT NBD1 reveal significant concentration-dependent chemical shift perturbations consistent with NBD1 dimerization. Chemical shift analysis suggests that the more rapid tumbling of F508del is the result of an impaired ability to dimerize. Based on previously published crystal structures and NMR spectra of various NBD1 mutants, we propose that deletion of Phe-508 affects Q-loop conformational sampling in a manner that inhibits dimerization. These results provide a potential mechanism for inhibition of channel opening by F508del and support the dimer interface as a target for cystic fibrosis therapeutics.     

FGF Pyramus Has a Transmembrane Domain and Cell-Autonomous Function in Polarity

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Transmembrane helix heterodimerization in lipid bilayers: probing the energetics behind autosomal dominant growth disorders

Here, we show that the energetics of transmembrane helix heterodimer formation can be characterized in liposomes using F?rster resonance energy transfer (FRET). We present the theory and the protocol for measuring the free energy of heterodimerization, and the total (hetero and homo-dimeric) dimer fraction. We use the presented methodology to determine the propensity for heterodimer formation between wild-type fibroblast growth factor receptor 3 (FGFR3) transmembrane domain and the Ala391Glu mutant, linked to Crouzon syndrome with acanthosis nigricans.     

Synthesis of a Double Transmembrane Domain Fragment of Ste2p by Native Chemical Ligation

Native chemical ligation (NCL) approaches have been applied extensively to soluble proteins. Fewer successes have been achieved with membrane peptides. In this report, the synthesis and semisynthesis by NCL of peptides corresponding to 1.7 transmembrane domains of the α-factor receptor from Saccharomyces cerevisiae is described. Synthesis was achieved when the ligation point was approximately in the middle of the loop joining the two transmembrane regions. In contrast, little to no ligation was observed when the ligation point was at the putative membrane interface of the sixth transmembrane domain (TM6) and the third extracellular loop (EL3). Ligations of a chemically synthesized 22-residue thioester with a synthetic 29-residue N-Cys peptide and a biosynthetic 73-residue N-Cys peptide were successfully achieved in both trifluoroethanol/guanidinium hydrochloride (TFE/GnHCl) and sodium dodecyl sulfate (SDS) media when mercaptoethanesulfonic acid (MESNA) was used as a catalyst. The resulting 51-residue and 95-residue ligation products were purified by reversed phase HPLC and recovered on a mg scale. Both peptides were >95% pure as determined by HPLC and had the expected molecular weight as judged by mass spectrometry. Segmental labeling of the 95-residue fragment, in which the N-Cys portion was [¹⁵N] labeled, resulted in a peptide that gave an NMR spectrum which was comparable to that of the unligated 73-residue peptide alone. R B Merrifield personified the finest qualities of a human being. He was an outstanding individual who influenced the way research is conducted by tens of thousands of scientists. At the same time he was a warm, humble, sincere man who was extremely kind and generous. I (FN) personally saw his generosity during a seminar he invited me to give at Rockefeller University. He was already a Nobel laureate but he treated me as a colleague and the encouragement he offered concerning my research program was very important for my future in academia. It is an honor to be among the participants in a volume honoring his contributions to peptide science.     

蛋白质可逆磷酸化调节植物细胞离子跨膜运动研究进展

蛋白激酶和蛋白磷酸酶催化的可逆磷酸化是植物细胞中多种信号转导途径中重要的组成因子.本文对蛋白质可逆磷酸化通过调节多种离子跨膜运动而参与植物细胞激发子信号途径、毒性物质诱导的钙离子内流、盐胁迫适应、气孔运动以及蛋白质可逆磷酸化参与胞外与胞内之间Ca2 状况信息传递,调节花粉管顶端Ca2 离子通道活性进行综述,以揭示蛋白质可逆磷酸化在植物细胞离子跨膜运动中的调控作用,为蛋白质可逆磷酸化调节植物生长发育、响应逆境胁迫等机理的研究提供参考.     

The Transmembrane Domains of β and IX Subunits Mediate the Localization of the Platelet Glycoprotein Ib-IX Complex to the Glycosphingolipid-enriched Membrane Domain

We have previously reported that the structural elements of the GP Ib-IX complex required for its localization to glycosphingolipid-enriched membranes (GEMs) reside in the Ibβ and IX subunits. To identify them, we generated a series of cell lines expressing mutant GP Ibβ and GP IX where 1) the cytoplasmic tails (CTs) of either or both GP Ibβ and IX are truncated, and 2) the transmembrane domains (TMDs) of GP Ibβ and GP IX were swapped with the TMD of a non-GEMs associating molecule, human transferrin receptor. Sucrose density fractionation analysis showed that the removal of either or both of the CTs from GP Ibβ and GP IX does not alter GP Ibα-GEMs association when compared with the wild type. In contrast, swapping of the TMDs of either GP Ibβ or GP IX with that of transferrin receptor results in a significant loss (∼50%) of GP Ibα from the low density GEMs fractions, with the largest effect seen in the dual TMD-replaced cells (>80% loss) when compared with the wild type cells (100% of GP Ibα present in the GEMs fractions). Under high shear flow, the TMD-swapped cells adhere poorly to a von Willebrand factor-immobilized surface to a much lesser extent than the previously reported disulfide linkage dysfunctional GP Ibα-expressing cells. Thus, our data demonstrate that the bundle of GP Ibβ and GP IX TMDs instead of their individual CTs is the structural element that mediates the β/IX complex localization to the membrane GEMs, which through the α/β disulfide linkage brings GP Ibα into the GEMs.     

A Unique Phenylalanine in the Transmembrane Domain Strengthens Homodimerization of the Syndecan-2 Transmembrane Domain and Functionally Regulates Syndecan-2

The syndecans are a type of cell surface adhesion receptor that initiates intracellular signaling events through receptor clustering mediated by their highly conserved transmembrane domains (TMDs). However, the exact function of the syndecan TMD is not yet fully understood. Here, we investigated the specific regulatory role of the syndecan-2 TMD. We found that syndecan-2 mutants in which the TMD had been replaced with that of syndecan-4 were defective in syndecan-2-mediated functions, suggesting that the TMD of syndecan-2 plays one or more specific roles. Interestingly, syndecan-2 has a stronger tendency to form sodium dodecyl sulfate (SDS)-resistant homodimers than syndecan-4. Our structural studies showed that a unique phenylalanine residue (Phe¹⁶⁷) enables an additional molecular interaction between the TMDs of the syndecan-2 homodimer. The presence of Phe¹⁶⁷ was correlated with a higher tendency toward oligomerization, and its replacement with isoleucine significantly reduced the SDS-resistant dimer formation and cellular functions of syndecan-2 (e.g. cell migration). Conversely, replacement of isoleucine with phenylalanine at this position in the syndecan-4 TMD rescued the defects observed in a mutant syndecan-2 harboring the syndecan-4 TMD. Taken together, these data suggest that Phe¹⁶⁷ in the TMD of syndecan-2 endows the protein with specific functions. Our work offers new insights into the signaling mediated by the TMD of syndecan family members.     

"Frozen" dynamic dimer model for transmembrane signaling in bacterial chemotaxis receptors.

The crystal structures of the ligand binding domain of a bacterial aspartate receptor suggest a simple mechanism for transmembrane signaling by the dimer of the receptor. On ligand binding, one domain rotates with respect to the other, and this rotational motion is proposed to be transmitted through the membrane to the cytoplasmic domains of the receptor.     

FGFR3 Transmembrane Domain Interactions Persist in the Presence of Its Extracellular Domain

Isolated receptor tyrosine kinase transmembrane (TM) domains have been shown to form sequence-specific dimers in membranes. Yet, it is not clear whether studies of isolated TM domains yield knowledge that is relevant to full-length receptors or whether the large glycosylated extracellular domains alter the interactions between the TM helices. Here, we address this question by quantifying the effect of the pathogenic A391E TM domain mutation on the stability of the fibroblast growth factor receptor 3 dimer in the presence of the extracellular domain and comparing these results to the case of the isolated TM fibroblast growth factor receptor 3 domains. We perform the measurements in plasma membrane-derived vesicles using a Förster-resonance-energy-transfer-based method. The effect of the mutation on dimer stability in both cases is the same (∼−1.5 kcal/mol), suggesting that the interactions observed in simple TM-peptide model systems are relevant in a biological context.     

Structure and functions of arrestins.

Transmembrane signal transductions in a variety of cell types that mediate signals as diverse as those carried by neurotransmitters, hormones, and sensory signals share basic biochemical mechanisms that include: (1) an extracellular perturbation (neurotransmitter, hormone, odor, light); (2) specific receptors; (3) coupling proteins, such as G proteins; and (4) effector enzymes or ion channels. Parallel to these amplification reactions, receptors are precisely inactivated by mechanisms that involve protein kinases and regulatory proteins called arrestins. The structure and functions of arrestins are the focus of this review.     

Oligomeric potential of the M2 muscarinic cholinergic receptor

G protein-coupled receptors are known to exist as oligomers. Although such aggregates often are referred to as dimers, there is little direct evidence regarding their oligomeric size. In the present investigation, c-Myc-, FLAG-, and influenza hemagglutinin (HA)-tagged forms of the M2 muscarinic receptor have been coexpressed in Sf9 cells to probe for aggregates larger than a dimer. Immunochromatography, immunoprecipitation, and immunoblotting were carried out with various combinations of antibodies directed against the different epitopes to demonstrate that all three tagged forms of the receptor can be immunopurified within a single complex. Extracts of the M2 muscarinic receptor from Sf9 cells therefore contain aggregates that are at least trimeric, and the levels detected point to the existence of larger complexes. The data also suggest that the oligomers coexist with a sizeable population of monomers.     

Mutational Analysis of the Histidine-containing Phosphotransfer (HPt) Signaling Domain of the ArcB Sensor in Escherichia coli

The Escherichia coli ArcB sensor is involved in anaerobic phosphotransfer signal transduction. ArcB is a hybrid sensor that contains three types of phosphotransfer signaling domains in its primary amino acid sequence, namely, transmitter (or His-kinase), receiver, and histidine-containing phosphotransfer (HPt) domains. However, examination of the function of the newly-discovered HPt domain (named ArcB^c) is still at a very early stage. To gain a general insight into the structure and function of the widespread HPt domains, on the basis of its three-dimensional crystal structure, in this study we constructed a certain set of mutants each having a single amino acid substitution in the HPt domain of ArcB. These ArcB^c mutants were characterized and evaluated, based on the in vivo ability to signal the OmpR receiver via trans-phosphorylation.     

Intermonomer Hydrogen Bonds Enhance GxxxG-Driven Dimerization of the BNIP3 Transmembrane Domain: Roles for Sequence Context in Helix-Helix Association in Membranes

We determined the sequence dependence of human BNIP3 transmembrane domain dimerization using the biological assay TOXCAT. Mutants in which intermonomer hydrogen bonds between Ser172 and His173 are abolished show moderate interaction, indicating that side-chain hydrogen bonds contribute to dimer stability but are not essential to dimerization. Mutants in which a GxxxG motif composed of Gly180 and Gly184 has been abolished show little or no interaction, demonstrating the critical nature of the GxxxG motif to BNIP3 dimerization. These findings show that side-chain hydrogen bonds can enhance the intrinsic dimerization of a GxxxG motif and that sequence context can control how hydrogen bonds influence helix-helix interactions in membranes. The dimer interface mapped by TOXCAT mutagenesis agrees closely with the interfaces observed in the NMR structure and inferred from mutational analysis of dimerization on SDS-PAGE, showing that the native dimer structure is retained in detergents. We show that TOXCAT and SDS-PAGE give complementary and consistent information about BNIP3 transmembrane domain dimerization: TOXCAT is insensitive to mutations that have modest effects on self-association in detergents but readily discriminates among mutations that completely disrupt detergent-resistant dimerization. The close agreement between conclusions reached from TOXCAT and SDS-PAGE data for BNIP3 suggests that accurate estimates of the relative effects of mutations on native-state protein-protein interactions can be obtained even when the detergent environment is strongly disruptive.     

Complex Patterns of Histidine, Hydroxylated Amino Acids and the GxxxG Motif Mediate High-affinity Transmembrane Domain Interactions

Specific interactions of transmembrane helices play a pivotal role in the folding and oligomerization of integral membrane proteins. The helix-helix interfaces frequently depend on specific amino acid patterns. In this study, a heptad repeat pattern was randomized with all naturally occurring amino acids to uncover novel sequence motifs promoting transmembrane domain interactions. Self-interacting transmembrane domains were selected from the resulting combinatorial library by means of the ToxR/POSSYCCAT system. A comparison of the amino acid composition of high-and low-affinity sequences revealed that high-affinity transmembrane domains exhibit position-specific enrichment of histidine. Further, sequences containing His preferentially display Gly, Ser, and/or Thr residues at flanking positions and frequently contain a C-terminal GxxxG motif. Mutational analysis of selected sequences confirmed the importance of these residues in homotypic interaction. Probing heterotypic interaction indicated that His interacts in trans with hydroxylated residues. Reconstruction of minimal interaction motifs within the context of an oligo-Leu sequence confirmed that His is part of a hydrogen bonded cluster that is brought into register by the GxxxG motif. Notably, a similar motif contributes to self-interaction of the BNIP3 transmembrane domain.