The Cα—H⋅⋅⋅O hydrogen bond: A determinant of stability and specificity in transmembrane helix interactions

Abstract

The C␣OH⅐⅐⅐O hydrogen bond has been given little attention as a determinant of transmembrane helix association. Stimulated by recent calculations suggesting that such bonds can be much stronger than has been supposed, we have analyzed 11 known membrane protein structures and found that apparent carbon ␣ hydrogen bonds cluster frequently at glycine-, serine-, and threonine-rich packing interfaces between transmembrane helices. Parallel righthanded helix-helix interactions appear to favor C␣OH⅐⅐⅐O bond formation. In particular, C␣OH⅐⅐⅐O interactions are frequent between helices having the structural motif of the glycophorin A dimer and the GxxxG pair. We suggest that C␣OH⅐⅐⅐O hydrogen bonds are important determinants of stability and, depending on packing, specificity in membrane protein folding.T he hydrogen bond is a key element in the interplay between stability and specificity in protein folding. The desolvation penalty associated with burial of polar side chains in an aqueous environment is not always fully recovered by hydrogen bond formation, so hydrogen bonds provide a small or even unfavorable net energy contribution to folding. However, the strength and directionality of hydrogen bonds make them an important factor in discriminating between correctly folded and misfolded states. Hence, polar interactions tend to contribute more to specificity than to stability in soluble proteins (1-3). Conversely, in the apolar environment of biological membranes donor and acceptor groups cannot be satisfied by the solvent, and hydrogen bonds strongly stabilize the helical conformation of membrane spanning domains (4) and can stabilize tertiary interactions as well (5-9). We are interested in the role of hydrogen bonds in the association of transmembrane helices, a stage that is pivotal in the folding of membrane proteins (4). Recently, the DeGrado group and our laboratory showed that the substitution of a single polar amino acid residue into model transmembrane helices induces homooligomerization (10 -13); the association driven by hydrogen bonding can be strong and independent of packing details. Thus, in the apolar environment, the strength of hydrogen bonds can stabilize the association of transmembrane helices, although a lack of a need for sequence specificity could create a danger of inducing promiscuous association (10, 13).Weaker hydrogen bonds, such as those between carbon and oxygen atoms (COH⅐⅐⅐O), have received little attention in the membrane protein field, and their occurrence in membrane proteins has never been surveyed. The C␣ is an activated carbon donor because it is bound to the electron-withdrawing amide NOH and CAO groups, and, in soluble proteins, hydrogen bonds between main-chain C␣OH groups and backbone or side-chain oxygen atoms are often observed (14-17). Despite its abundance, the structural contribution of the C␣OH⅐⅐⅐O hydrogen bond has been unclear and its interaction energy has been believed to be small. Recently, by using ab initio calculations, Vargas et al. (18) and Scheiner et al. (19...

The K domain mediates heterodimerization of theArabidopsisfloral organ identity proteins, APETALA3 and PISTILLATA

“…However, the fact that PI G131V and PI G131A interact with AP3 less well than wild-type PI suggests that hydrophobicity is not favored at this putative d position and that a small R group at position 131 may be important (Table 1). Though glycine residues are rare in helical segments of soluble proteins, glycine residues are not uncommon in transmembrane helices (Senes et al, 2001). In transmembrane proteins, such as the glycerol facilitator from E. coli, which contains eight transmembrane a-helices, glycine residues on one a-helix form inter-helical Ca-HÁ Á ÁO hydrogen bonds with the carbonyl or hydroxyl oxygen on other helices.…”

“…In the collagen triple helix, glycine also forms inter-helical Ca-HÁ Á ÁO hydrogen bonds (Bella and Berman, 1996). These unconventional interhelical Ca-HÁ Á ÁO hydrogen bonds are proposed to be a determinant of stabilization and specificity in transmembrane helix interactions (Senes et al, 2001). It is possible that the d-position Gly-131 in PI participates in formation of Ca-HÁ Á ÁO bonds in a similar way to glycine residues in collagen triple helix and transmembrane helices.…”

Transmembrane Domain Interactions and Residue Proline 378 Are Essential for Proper Structure, Especially Disulfide Bond Formation, in the Human Vitamin K-Dependent γ-Glutamyl Carboxylase

“…Recently, three motifs that are important for TMD helix-helix interactions have been identified: the GxxxG motif and its like, polar and hydrogen-bonding motifs, and proline motifs (for review, see ref 23). Glycine motifs have been shown to support a hydrogen bond network that stabilizes domain-domain interactions (41). Among the five TMDs in carboxylase, there is a GxxxG (residues 128 and 132) motif in TMD2.…”

Identification and characterization of the first reptilian CD9, and its expression analysis in response to bacterial infection

“…Some polar residues (such as Asn and Glu) in the TMs were also highly conserved, which were thought to stabilize the tertiary structure by forming strong hydrogen bonds with each other or other residues [27][28][29]. In addition, like most known tetraspanin, turtle CD9 has no typical signaling motif, potentially suggesting that it acts mainly as an adapter protein facilitating the interaction of associated molecules in tetraspanin signaling networks [3].…”