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.