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.