title: The position of the Gly-xxx-Gly motif in transmembrane segments modulates dimer affinity.
abstract: 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 alpha-helical TM segments.
authors.affiliation: Division of Molecular Structure and Function, Research Institute, Hospital for Sick Children, Toronto, ON M5G 1X8, Canada.
authors.forename: Rachel M
authors.initials: RM
authors.lastname: Johnson
authors.forename: Arianna
authors.initials: A
authors.lastname: Rath
authors.forename: Charles M
authors.initials: CM
authors.lastname: Deber
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D036022:Capsid Proteins / Q000737:chemistry; D002462:Cell Membrane / Q000737:chemistry*; D019281:Dimerization; D005998:Glycine / Q000737:chemistry*; D006021:Glycophorins / Q000737:chemistry; D007930:Leucine / Q000737:chemistry; D008565:Membrane Proteins / Q000737:chemistry*; D008969:Molecular Sequence Data; D017510:Protein Folding; D017386:Sequence Homology, Amino Acid
chemical_list: D036022:Capsid Proteins; D006021:Glycophorins; D008565:Membrane Proteins; C025668:coat protein, Bacteriophage M13; D007930:Leucine; D005998:Glycine