Neutron diffraction studies of the structure of filamentous bacteriophage Pf1. Demonstration that the coat protein consists of a pair of alpha-helices with an intervening, non-helical surface loop.

The structure of filamentous bacteriophage Pf1 has been studied using neutron diffraction from magnetically oriented gels of native and specifically deuterated phage. These methods have been used to determine the positions of the two methionine, two tyrosine and six isoleucine residues of the coat protein. Combined with the positions of the five valine residues previously determined, they represent one third (15 of 46) of the residues of the coat protein. These 15 amino acid residue positions have been used as the basis for constructing a model for the protein consisting of two alpha-helices with an intervening surface loop. The first helix extends from near the amino terminus to Ile12. The second helix extends from Lys20 to at least Met42, and may contain a bend between Ile32 and Val35. The two helices are tilted by about 15 degrees relative to one another, and are positioned in such a way that they appear to be bound end-to-end by main-chain hydrogen bonds. The intervening, non-helical loop, made up of Thr13 to Met19, connects the two helices without disrupting the pattern of main-chain hydrogen bonding, but does not result in a bend in the otherwise continuous helical structure. This model is used to predict the approximate positions of all amino acid residues in the Pf1 protein coat, providing a basis for further understanding of a number of viral properties including the symmetry transitions, the non-isomorphism of heavy-atom derivatives, and the protein-protein and protein-DNA interactions in the virion.