Coiled coils 9-to-5: Rational de novo design of α-helical barrels with tunable oligomeric states

The rational design of linear peptides that assemble controllably and predictably in water is challenging. Sequences must encode unique target structures and avoid alternative states. However, the stabilizing and discriminating non-covalent forces available are weak in water. Nonetheless, for -helical coiled-coil assemblies considerable progress has been made in rational de novo design. In these, sequence repeats of nominally hydrophobic (h) and polar (p) residues, hpphppp, direct the assembly of amphipathic helices into dimeric to tetrameric bundles. Expanding this pattern to hpphhph can produce larger -helical barrels. Here, we show that pentamers to nonamers are achieved simply by varying the residue at one of these h sites. In L/I-K-E-I-A-x-Z repeats, decreasing the size of Z from threonine to serine to alanine to glycine gives progressively larger oligomers. X-ray crystal structures of the resulting -helical barrels rationalize this: side chains at Z point directly into the helical interfaces, and smaller residues allow closer helix contacts and larger assemblies.