Asparagine-mediated self-association of a model transmembrane helix

In membrane proteins, the extent to which polarity, hydrogen bonding, and van der Waals packing interactions of the buried, internal residues direct protein folding and association of transmembrane segments is poorly understood. The energetics associated with these various interactions should differ substantially between membrane versus water-soluble proteins. To help evaluate these energetics, we have altered a water-soluble, two-stranded coiled-coil peptide to render its sequence soluble in membranes. The membrane-soluble peptide associates in a monomer-dimer-trimer equilibrium, in which the trimer predominates at the highest peptide/detergent ratios. The oligomers are stabilized by a buried Asn side chain. Mutation of this Asn to Val essentially eliminates oligomerization of the membrane-soluble peptide. Thus, within a membrane-like environment, interactions involving a polar Asn side chain provide a strong thermodynamic driving force for membrane helix association.