Polar residues drive association of polyleucine transmembrane helices

Abstract

Although many polar residues are directly involved in transmembrane protein functions, the extent to which they contribute to more general structural features is still unclear. Previous studies have demonstrated that asparagine residues can drive transmembrane helix association through interhelical hydrogen bonding We have studied the ability of other polar residues to promote helix association in detergent micelles and in biological membranes. Our results show that polyleucine sequences with Asn, Asp, Gln, Glu, and His, residues capable of being simultaneously hydrogen bond donors and acceptors, form homo-or heterooligomers. In contrast, polyleucine sequences with Ser, Thr, and Tyr do not associate more than the polyleucine sequence alone. The results therefore provide experimental evidence that interactions between polar residues in the helices of transmembrane proteins may serve to provide structural stability and oligomerization specificity. Furthermore, such interactions can allow structural flexibility required for the function of some membrane proteins.

Sequence-specific Dimerization of the Transmembrane Domain of the “BH3-only” Protein BNIP3 in Membranes and Detergent

“…However, the histidine is not sufficient to drive strong association of BNIP3 TM domains because several mutations at positions 176 or 180 almost completely abolish dimerization in SDS-PAGE and in E. coli membranes. Thus, the interactions supported by His 173 within the BNIP3 TM domain depend strongly upon the flanking sequences, unlike the interactions observed between polar residues in model TM domains (27)(28)(29)(30).…”

“…It has been shown previously that the GXXXG motif can occur in different contexts, with multiple copies of the motif aligned in tandem (34,35) or displayed on different faces of the TM helix, providing alternate interaction sites (36). Our identification of His 173 , Ala 176 , and Gly 180 as interfacial residues demonstrates that the previously described (27)(28)(29)(30) ability of polar residues to drive interactions between TM domains can be exploited by biology in the context of an AXXXG motif to give additional sequence specificity and stability to helix-helix interactions in membranes.…”

“…We targeted position His 173 within the TM domain because of previous reports (27)(28)(29)(30) that polar side chains can drive helix-helix association. Because mutations at His 173 have graded effects on dimerization in both TOXCAT and SDS-PAGE assays, our results demonstrate that this polar residue does help to stabilize TM domain interactions.…”

“…Strongly polar residues within hydrophobic TM domains have been shown to drive oligomerization in membranes and detergents (27)(28)(29)(30), and the GXXXG motif has been shown to mediate interactions between TMs in several biological systems (31-37). Mutational analysis shows that the TM domain interaction is mediated by both polar interactions and the motif AXXXG, and that the detergentsolubilized dimer is disrupted by the same mutations that disrupt the membrane-embedded complex.…”

Membrane protein folding: beyond the two stage model

“…Part of the stability may come from K-carbon hydrogen bonds [12]. Hydrogen bonding generally contributes to helix association, and can be strong [13,14]. The free energy of glycophorin helix dimerization in a detergent environment, which should be weaker than in a bilayer, is about 9 kcal/mol [15].…”