The forces stabilizing the three-dimensional structures of mem- brane proteins are currently not well understood. Previously. it was shown that a single Asn side chain in a transmembrane segment can mediate the dimerization and trimerization of a variety of hydrophobic helices. Here, we examine the tendencies of a representative set of amino acids (Asn. Gln, Asp, Glu, Lys, Ala, Val, Leu, Ser. Thr) to direct the oligomerization of a model transmem- brane helix. The model peptide is entirely hydrophobic throughout a 20-residue segment and contains a single central site for the introduction of various amino acid "guests." Analytical ultracen- trifugation and gel electrophoresis were used to determine the stoichiometry and free energy of association of the entire set of peptides within micelles. Variants with two polar atoms at the guest site--Asn, Gln, Asp, and Glu-formed stable trimers, whereas residues with one or fewer polar atoms showed a much weaker tendency to associate. The data are examined in light of the frequencies of occurrence of various amino acid side chains in membrane proteins and provide insight into the role of polar interactions in directing transmembrane helix association. These data also suggest an approach to the design of variants of natural single-span transmembrane proteins with various potentials to associate in the bilayer.
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