Motifs of two small residues can assist but are not sufficient to mediate transmembrane helix interactions.
Both experimental and statistical searches for specific motifs that mediate transmembrane helix-helix interactions showed that two glycine residues separated by three intervening residues (GxxxG) provide a framework for specific interactions. Further work suggested that other motifs of small residues can mediate the interaction of transmembrane domains, so that the AxxxA-motif could also drive strong interactions of alpha-helices in soluble proteins. Thus, all these data indicate that a motif of two small residues in a distance of four might be enough to provide a framework for transmembrane helix-helix interaction. To test whether GxxxG is equivalent to (small)xxx(small), we investigated the effect of a substitution of either of the two Gly residues in the glycophorin A GxxxG-motif by Ala or Ser using the recently developed GALLEX system. The results of this mutational study demonstrate that, while a replacement of either of the two Gly by Ala strongly disrupts GpA homo-dimerization, the mutation to Ser partly stabilizes a dimeric structure. We suggest that the Ser residue can form a hydrogen bond with a backbone carbonyl group of the adjacent helix stabilizing a preformed homo-dimer. While (small)xxx(small) serves as a useful clue, the context of adjacent side-chains is essential for stable helix interaction, so each case must be tested.
Schneider
Dirk
D
Department of Molecular Biophysics and Biochemistry, Yale University, P.O. Box 208114, New Haven, CT 06520-8114, USA.
Engelman
Donald M
DM
D020816:Amino Acid Motifs; D019281:Dimerization; D006021:Glycophorins / Q000235:genetics / Q000378:metabolism*; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D017510:Protein Folding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary
D006021:Glycophorins; D008565:Membrane Proteins