title: Conformational transition between four and five-stranded phenylalanine zippers determined by a local packing interaction.
abstract: Alpha-helical coiled coils play a crucial role in mediating specific protein-protein interactions. However, the rules and mechanisms that govern helix-helix association in coiled coils remain incompletely understood. Here we have engineered a seven heptad "Phe-zipper" protein (Phe-14) with phenylalanine residues at all 14 hydrophobic a and d positions, and generated a further variant (Phe-14(M)) in which a single core Phe residue is substituted with Met. Phe-14 forms a discrete alpha-helical pentamer in aqueous solution, while Phe-14(M) folds into a tetrameric helical structure. X-ray crystal structures reveal that in both the tetramer and the pentamer the a and d side-chains interlock in a classical knobs-into-holes packing to produce parallel coiled-coil structures enclosing large tubular cavities. However, the presence of the Met residue in the apolar interface of the tetramer markedly alters its local coiled-coil conformation and superhelical geometry. Thus, short-range interactions involving the Met side-chain serve to preferentially select for tetramer formation, either by inhibiting a nucleation step essential for pentamer folding or by abrogating an intermediate required to form the pentamer. Although specific trigger sequences have not been clearly identified in dimeric coiled coils, higher-order coiled coils, as well as other oligomeric multi-protein complexes, may require such sequences to nucleate and direct their assembly.
authors.affiliation: Department of Biochemistry, Weill Medical College of Cornell University, New York, NY 10021, USA.
authors.forename: Jie
authors.initials: J
authors.lastname: Liu
authors.forename: Qi
authors.initials: Q
authors.lastname: Zheng
authors.forename: Yiqun
authors.initials: Y
authors.lastname: Deng
authors.forename: Neville R
authors.initials: NR
authors.lastname: Kallenbach
authors.forename: Min
authors.initials: M
authors.lastname: Lu
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D018360:Crystallography, X-Ray; D008969:Molecular Sequence Data; D010649:Phenylalanine / Q000737:chemistry*; D011487:Protein Conformation; D015202:Protein Engineering; D017433:Protein Structure, Secondary
chemical_list: D010649:Phenylalanine