The position of the Gly-xxx-Gly motif in transmembrane segments modulates dimer affinityThis paper is one of a selection of papers published in this Special Issue, entitled CSBMCB — Membrane Proteins in Health and Disease

Although the intrinsic low solubility of membrane proteins presents challenges to their high-resolution structure determination, insight into the amino acid sequence features and forces that stabilize their folds has been provided through study of sequence-dependent helix–helix interactions between single transmembrane (TM) helices. While the stability of helix–helix partnerships mediated by the Gly-xxx-Gly (GG4) motif is known to be generally modulated by distal interfacial residues, it has not been established whether the position of this motif, with respect to the ends of a given TM segment, affects dimer affinity. Here we examine the relationship between motif position and affinity in the homodimers of 2 single-spanning membrane protein TM sequences: glycophorin A (GpA) and bacteriophage M13 coat protein (MCP). Using the TOXCAT assay for dimer affinity on a series of GpA and MCP TM segments that have been modified with either 4 Leu residues at each end or with 8 Leu residues at the N-terminal end, we show that in each protein, centrally located GG4 motifs are capable of stronger helix–helix interactions than those proximal to TM helix ends, even when surrounding interfacial residues are maintained. The relative importance of GG4 motifs in stabilizing helix–helix interactions therefore must be considered not only in its specific residue context but also in terms of the location of the interactive surface relative to the N and C termini of α-helical TM segments.