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.     

Ionic Interactions Promote Transmembrane Helix-Helix Association Depending on Sequence Context

Folding and oligomerization of integral membrane proteins frequently depend on specific interactions of transmembrane helices. Interacting amino acids of helix-helix interfaces may form complex motifs and exert different types of molecular forces. Here, a set of strongly self-interacting transmembrane domains (TMDs), as isolated from a combinatorial library, was found to contain basic and acidic residues, in combination with polar nonionizable amino acids and C-terminal GxxxG motifs. Mutational analyses of selected sequences and reconstruction of high-affinity interfaces confirmed the cooperation of these residues in homotypic interactions. Probing heterotypic interaction indicated the presence of interhelical charge-charge interactions. Furthermore, simple motifs of an ionizable residue and GxxxG are significantly overrepresented in natural TMDs, and a specific combination of these motifs exhibits high-affinity heterotypic interaction. We conclude that intramembrane charge-charge interactions depend on sequence context. Moreover, they appear important for homotypic and heterotypic interactions of numerous natural TMDs.     

Tryptophan supports interaction of transmembrane helices

Interactions of transmembrane helices play an important role in folding and oligomerization of integral membrane proteins. The interfacial residues of these helices frequently correspond to heptad repeat motifs. In order to uncover novel mechanisms underlying these interactions, we randomised a heptad repeat pattern with a complete set of amino acids. Those sequences that were capable of high-affinity self-interaction upon integration into bacterial inner membranes were selected by means of the POSSYCCAT system. A comparison between selected and non-selected sequences reveals that high-affinity sequences were strongly enriched in tryptophan residues that accumulated at specific positions of the heptad motif. Mutation of Trp in selected clones significantly reduced self-interaction of the transmembrane segments without affecting their efficiency of membrane integration. Conversely, grafting Trp onto artificial transmembrane segments strongly enhanced their interaction. We conclude that tryptophan supports interaction of transmembrane segments.     

The GxxxG motif: a framework for transmembrane helix-helix association

In order to identify strong transmembrane helix packing motifs, we have selected transmembrane domains exhibiting high-affinity homo-oligomerization from a randomized sequence library based on the right-handed dimerization motif of glycophorin A. Sequences were isolated using the TOXCAT system, which measures transmembrane helix-helix association in the Escherichia coli inner membrane. Strong selection was applied to a large range of sequences ( approximately 10(7) possibilities) and resulted in the identification of sequence patterns that mediate high-affinity helix-helix association. The most frequent motif isolated, GxxxG, occurs in over 80% of the isolates. Additional correlations suggest that flanking residues act in concert with the GxxxG motif, and that size complementarity is maintained at the interface, consistent with the idea that the identified sequence patterns represent packing motifs. The convergent identification of similar sequence patterns from an analysis of the transmembrane domains in the SwissProt sequence database suggests that these packing motifs are frequently utilized in naturally occurring helical membrane proteins.     

Statistical analysis of amino acid patterns in transmembrane helices: the GxxxG motif occurs frequently and in association with beta-branched residues at neighboring positions

To find motifs that mediate helix-helix interactions in membrane proteins, we have analyzed frequently occurring combinations of residues in a database of transmembrane domains. Our analysis was performed with a novel formalism, which we call TMSTAT, for exactly calculating the expectancies of all pairs and triplets of residues in individual sequences, taking into account differential sequence composition and the substantial effect of finite length in short segments. We found that the number of significantly over and under-represented pairs and triplets was much greater than the random expectation. Isoleucine, glycine and valine were the most common residues in these extreme cases. The main theme observed is patterns of small residues (Gly, Ala and Ser) at i and i+4 found in association with large aliphatic residues (Ile, Val and Leu) at neighboring positions (i.e. i+/-1 and i+/-2). The most over-represented pair is formed by two glycine residues at i and i+4 (GxxxG, 31.6 % above expectation, p<1x10(-33)) and it is strongly associated with the neighboring beta-branched residues Ile and Val. In fact, the GxxxG pair has been described as part of the strong interaction motif in the glycophorin A transmembrane dimer, in which the pair is associated with two Val residues (GVxxGV). GxxxG is also the major motif identified using TOXCAT, an in vivo selection system for transmembrane oligomerization motifs. In conjunction with these experimental observations, our results highlight the importance of the GxxxG+beta-branched motif in transmembrane helix-helix interactions. In addition, the special role for the beta-branched residues Ile and Val suggested here is consistent with the hypothesis that residues with constrained rotameric freedom in helical conformation might reduce the entropic cost of folding in transmembrane proteins. Additional material is available at http://engelman.csb.yale. edu/tmstat and http://bioinfo.mbb.yale. edu/tmstat.     

Membrane topology of gp41 and amyloid precursor protein: Interfering transmembrane interactions as potential targets for HIV and Alzheimer treatment

The amyloid precursor protein (APP), that plays a critical role in the development of senile plaques in Alzheimer disease (AD), and the gp41 envelope protein of the human immunodeficiency virus (HIV), the causative agent of the acquired immunodeficiency syndrome (AIDS), are single-spanning type-1 transmembrane (TM) glycoproteins with the ability to form homo-oligomers. In this review we describe similarities, both in structural terms and sequence determinants of their TM and juxtamembrane regions. The TM domains are essential not only for anchoring the proteins in membranes but also have functional roles. Both TM segments contain GxxxG motifs that drive TM associations within the lipid bilayer. They also each possess similar sequence motifs, positioned at the membrane interface preceding their TM domains. These domains are known as cholesterol recognition/interaction amino acid consensus (CRAC) motif in gp41 and CRAC-like motif in APP. Moreover, in the cytoplasmic domain of both proteins other α-helical membranotropic regions with functional implications have been identified. Recent drug developments targeting both diseases are reviewed and the potential use of TM interaction modulators as therapeutic targets is discussed.     

Self-association of models of transmembrane domains of ErbB receptors in a lipid bilayer

Association of transmembrane (TM) helices is facilitated by the close packing of small residues present along the amino-acid sequence. Extensive studies have established the role of such small residue motifs (GxxxG) in the dimerization of Glycophorin A (GpA) and helped to elucidate the association of TM domains in the epidermal growth factor family of receptors (ErbBs). Although membrane-mediated interactions are known to contribute under certain conditions to the dimerization of proteins, their effect is often considered nonspecific, and any potential dependence on protein sequence has not been thoroughly investigated. We recently reported that the association of GpA is significantly assisted by membrane-induced contributions as quantified in different lipid bilayers. Herein we extend our studies to explore the origin of these effects and quantify their magnitude using different amino-acid sequences in the same lipid environment. Using a coarse-grained model that accounts for amino-acid specificity, we perform extensive parallel Monte Carlo simulations of ErbB homodimerization in dipalmitoyl-phosphatidylcholine lipid bilayers. A detailed characterization of dimer formation and estimates of the free energy of association reveal that the TM domains show a significant affinity to self-associate in lipid bilayers, in qualitative agreement with experimental findings. The presence of GxxxG motifs enhances favorable protein-protein interactions at short separations. However, the lipid-induced attraction presents a more complex character than anticipated. Depending on the interfacial residues, lipid-entropic contributions support a decrease of separation or a parallel orientation to the membrane normal, with important implications for protein function.     

Properties of the intracellular transient receptor potential (TRP) channel in yeast, Yvc1

Transient receptor potential (TRP) channels are found among mammals, flies, worms, ciliates, Chlamydomonas, and yeast but are absent in plants. These channels are believed to be tetramers of proteins containing six transmembrane domains (TMs). Their primary structures are diverse with sequence similarities only in some short amino acid sequence motifs mainly within sequences covering TM5, TM6, and adjacent domains. In the yeast genome, there is one gene encoding a TRP-like sequence. This protein forms an ion channel in the vacuolar membrane and is therefore called Yvc1 for yeast vacuolar conductance 1. In the following we summarize its prominent features.     

Transmembrane Recognition of the Semaphorin Co-Receptors Neuropilin 1 and Plexin A1: Coarse-Grained Simulations

The cancer associated class 3 semaphorins require direct binding to neuropilins and association to plexins to trigger cell signaling. Here, we address the role of the transmembrane domains of neuropilin 1 and plexin A1 for the dimerization of the two receptors by characterizing the assembly in lipid bilayers using coarse-grained molecular dynamics simulations. From experimental evidence using a two-hybrid system showing the biochemical association of the two receptors transmembrane domains, we performed molecular simulations in DOPC and POPC demonstrating spontaneously assembly to form homodimers and heterodimers with a very high propensity for right-handed packing of the helices. Inversely, left-handed packing was observed with a very low propensity. This mode of packing was observed uniquely when the plexin A1 transmembrane domain was involved in association. Potential of mean force calculations were used to predict a hierarchy of self-association for the monomers: the two neuropilin 1 transmembrane domains strongly associated, neuropilin 1 and plexin A1 transmembrane domains associated less and the two plexin A1 transmembrane domains weakly but significantly associated. We demonstrated that homodimerization and heterodimerization are driven by GxxxG motifs, and that the sequence context modulates the packing mode of the plexin A1 transmembrane domains. This work presents major advances towards our understanding of membrane signaling platforms assembly through membrane domains and provides exquisite information for the design of antagonist drugs defining a novel class of therapeutic agents.     

Motifs of serine and threonine can drive association of transmembrane helices

Known sequence motifs containing key glycine residues can drive the homo-oligomerization of transmembrane helices. To find other motifs, a randomized library of transmembrane interfaces was generated in which glycine was omitted. The TOXCAT system, which measures transmembrane helix association in the Escherichia coli inner membrane, was used to select high-affinity homo-oligomerizing sequences in this library. The two most frequently occurring motifs were SxxSSxxT and SxxxSSxxT. Isosteric mutations of any one of the serine and threonine residues to non-polar residues abolished oligomerization, indicating that the interaction between these positions is specific and requires an extended motif of serine and threonine hydroxyl groups. Computational modeling of these sequences produced several chemically plausible structures that contain multiple hydrogen bonds between the serine and threonine residues. While single serine or threonine side-chains do not appear to promote helix association, motifs can drive strong and specific association through a cooperative network of interhelical hydrogen bonds.     

A conserved membrane-spanning amino acid motif drives homomeric and supports heteromeric assembly of presynaptic SNARE proteins

Assembly of the SNARE proteins synaptobrevin/VAMP, syntaxin, and SNAP-25 to binary and ternary complexes is important for docking and/or fusion of presynaptic vesicles to the neuronal plasma membrane prior to regulated neurotransmitter release. Despite the well characterized structure of their cytoplasmic assembly domains, little is known about the role of the transmembrane segments in SNARE protein assembly and function. Here, we identified conserved amino acid motifs within the transmembrane segments that are required for homodimerization of synaptobrevin II and syntaxin 1A. Minimal motifs of 6-8 residues grafted onto an otherwise monomeric oligoalanine host sequence were sufficient for self-interaction of both transmembrane segments in detergent solution or membranes. These motifs constitute contiguous areas of interfacial residues assuming alpha-helical secondary structures. Since the motifs are conserved, they also contributed to heterodimerization of synaptobrevin II and syntaxin 1A and therefore appear to constitute interaction domains independent of the cytoplasmic coiled coil regions. Interactions between the transmembrane segments may stabilize the SNARE complex, cause its multimerization to previously observed multimeric superstructures, and/or be required for the fusogenic activity of SNARE proteins.     

GxxxG motifs, phenylalanine, and cholesterol guide the self-association of transmembrane domains of ErbB2 receptors

GxxxG motifs are common in transmembrane domains of membrane proteins and are often introduced to artificial peptides to inhibit or promote association to stable structures. The transmembrane domain of ErbB2 presents two separate such motifs that are proposed to be connected to stability and activity of the dimer. Using molecular simulations, we show that these sequences play a critical role during the recognition stage, forming transient complexes that lead to stable dimers. In pure phospholipid bilayers association occurs by contacts formed at the C-terminus promoted by the presence of phenylalanine residues. Helices subsequently rotate to eventually pack at short separations favored by lipid entropic contributions. In contrast, at intermediate cholesterol concentrations, a different pathway is followed that involves dimers with a weaker interface toward the N-terminus. However, at high cholesterol content, a switch toward the C-terminus is observed with an overall nonmonotonic change of the dimerization affinity. This conformational switch modulated by cholesterol has important implications on the thermodynamic, structural, and kinetic characteristics of helix-helix association in lipid membranes.     

A Transmembrane Domain and GxxxG Motifs within L2 Are Essential for Papillomavirus Infection

During cellular invasion, human papillomavirus type 16 (HPV16) must transfer its viral genome (vDNA) across the endosomal membrane prior to its accumulation at nuclear PML bodies for the establishment of infection. After cellular uptake, the capsid likely undergoes pH-dependent disassembly within the endo-/lysosomal compartment, thereby exposing hidden domains in L2 that facilitate membrane penetration of L2/vDNA complexes. In an effort to identify regions of L2 that might physically interact with membranes, we have subjected the L2 sequence to multiple transmembrane (TM) domain prediction algorithms. Here, we describe a conserved TM domain within L2 (residues 45 to 67) and investigate its role in HPV16 infection. In vitro, the predicted TM domain adopts an alpha-helical structure in lipid environments and can function as a real TM domain, although not as efficiently as the bona fide TM domain of PDGFR. An L2 double point mutant renders the TM domain nonfunctional and blocks HPV16 infection by preventing endosomal translocation of vDNA. The TM domain contains three highly conserved GxxxG motifs. These motifs can facilitate homotypic and heterotypic interactions between TM helices, activities that may be important for vDNA translocation. Disruption of some of these GxxxG motifs resulted in noninfectious viruses, indicating a critical role in infection. Using a ToxR-based homo-oligomerization assay, we show a propensity for this TM domain to self-associate in a GxxxG-dependent manner. These data suggest an important role for the self-associating L2 TM domain and the conserved GxxxG motifs in the transfer of vDNA across the endo-/lysosomal membrane.     

Sequence-dependent backbone dynamics of a viral fusogen transmembrane helix

The transmembrane domains of membrane fusogenic proteins are known to contribute to lipid bilayer mixing as indicated by mutational studies and functional reconstitution of peptide mimics. Here, we demonstrate that mutations of a GxxxG motif or of Ile residues, that were previously shown to compromise the fusogenicity of the Vesicular Stomatitis virus G-protein transmembrane helix, reduce its backbone dynamics as determined by deuterium/hydrogen-exchange kinetics. Thus, the backbone dynamics of these helices may be linked to their fusogenicity which is consistent with the known over-representation of Gly and Ile in viral fusogen transmembrane helices. The transmembrane domains of membrane fusogenic proteins are known to contribute to lipid bilayer mixing. Our present results demonstrate that mutations of certain residues, that were previously shown to compromise the fusogenicity of the Vesicular Stomatitis virus G-protein transmembrane helix, reduce its backbone dynamics. Thus, the data suggest a relationship between sequence, backbone dynamics, and fusogenicity of transmembrane segments of viral fusogenic proteins.     

Motifs of two small residues can assist but are not sufficient to mediate transmembrane helix interactions

Both experimental and statistical searches for specific motifs that mediate transmembrane helix-helix interactions showed that two glycine residues separated by three intervening residues (GxxxG) provide a framework for specific interactions. Further work suggested that other motifs of small residues can mediate the interaction of transmembrane domains, so that the AxxxA-motif could also drive strong interactions of alpha-helices in soluble proteins. Thus, all these data indicate that a motif of two small residues in a distance of four might be enough to provide a framework for transmembrane helix-helix interaction. To test whether GxxxG is equivalent to (small)xxx(small), we investigated the effect of a substitution of either of the two Gly residues in the glycophorin A GxxxG-motif by Ala or Ser using the recently developed GALLEX system. The results of this mutational study demonstrate that, while a replacement of either of the two Gly by Ala strongly disrupts GpA homo-dimerization, the mutation to Ser partly stabilizes a dimeric structure. We suggest that the Ser residue can form a hydrogen bond with a backbone carbonyl group of the adjacent helix stabilizing a preformed homo-dimer. While (small)xxx(small) serves as a useful clue, the context of adjacent side-chains is essential for stable helix interaction, so each case must be tested.     

Sequence context modulates the stability of a GxxxG-mediated transmembrane helix-helix dimer

To quantify the relationship between sequence and transmembrane dimer stability, a systematic mutagenesis and thermodynamic study of the protein-protein interaction residues in the glycophorin A transmembrane helix-helix dimer was carried out. The results demonstrate that the glycophorin A transmembrane sequence dimerizes when its GxxxG motif is abolished by mutation to large aliphatic residues, suggesting that the sequence encodes an intrinsic propensity to self-associate independent of a GxxxG motif. In the presence of an intact GxxxG motif, the glycophorin A dimer stability can be modulated over a span of -0.5 kcal mol(-1) to +3.2 kcal mol(-1) by mutating the surrounding sequence context. Thus, these flanking residues play an active role in determining the transmembrane dimer stability. To assess the structural consequences of the thermodynamic effects of mutations, molecular models of mutant transmembrane domains were constructed, and a structure-based parameterization of the free energy change due to mutation was carried out. The changes in association free energy for glycophorin A mutants can be explained primarily by changes in packing interactions at the protein-protein interface. The energy cost of removing favorable van der Waals interactions was found to be 0.039 kcal mol(-1) per A2 of favorable occluded surface area. The value corresponds well with estimates for mutations in bacteriorhodopsin as well as for those mutations in the interiors of soluble proteins that create packing defects.     

Folding of helical membrane proteins: the role of polar, GxxxG-like and proline motifs

Helical integral membrane proteins share several structural determinants that are widely conserved across their universe. The discovery of common motifs has furthered our understanding of the features that are important to stability in the membrane environment, while simultaneously providing clues about proteins that lack high-resolution structures. Motif analysis also helps to target mutagenesis studies, and other experimental and computational work. Three types of transmembrane motifs have recently seen interesting developments: the GxxxG motif and its like; polar and hydrogen bonding motifs; and proline motifs.     

TOPDOM: database of domains and motifs with conservative location in transmembrane proteins

The TOPDOM database is a collection of domains and sequence motifs located consistently on the same side of the membrane in alpha-helical transmembrane proteins. The database was created by scanning well-annotated transmembrane protein sequences in the UniProt database by specific domain or motif detecting algorithms. The identified domains or motifs were added to the database if they were uniformly annotated on the same side of the membrane of the various proteins in the UniProt database. The information about the location of the collected domains and motifs can be incorporated into constrained topology prediction algorithms, like HMMTOP, increasing the prediction accuracy. AVAILABILITY: The TOPDOM database and the constrained HMMTOP prediction server are available on the page http://topdom.enzim.hu CONTACT: tusi@enzim.hu; lkalmar@enzim.hu.