Filamentous bacteriophage Pf1 structure determined at 7 Å resolution by refinement of models for the α-helical subunit

Abstract

The molecular structure of filamentous bacteriophage Pf1 has been determined to 7 Å resolution by analysis of X-ray diffraction data from partially oriented fibers of virus particles. The continuous intensity distribution along layer-lines was measured by numerically separating contributions from overlapping layer-lines. The data were phased by an iterative refinement technique that used the known spatial extent and high α-helical content of the virus particle to refine a structural model. This refinement converges to a unique structural solution that is consistent with the X-ray data and with information derived from physical and chemical studies. The coat protein consists of two α-helical segments: one, almost parallel to the particle axis, is centered at a radius of about 15 Å; the other, at about 25 Å radius, is tilted by about 25 ° to the particle axis. This structure is consistent with every generalization about α-helical packing. The inner and outer segments interlock along most of their length with a crossing angle of 20.5 °. The inner α-helical segments also interact with symmetry-related copies of themselves, as do the outer segments. The double layer of tightly packed, intricately interlocked α-helices forms a stable, 20 Å thick protein coat around the viral DNA.