Structure of filamentous bacteriophage PF1

Abstract

The structure of filamentous bacteriophage Pfl has been determined to 7 A resolution by analysis of x-ray diffraction data from partially oriented fibers of virus particles. Continuous intensity distributions along layer lines were measured by numerically separating contributions from layer lines overlapping due to disorientation in the specimen. The data were phased by an iterative technique that utilized the known spatial extent and high ..cap alpha..-helical content of the virus coat protein to refine structural models. Refinement of these models converges to a unique structural solution that is both consistent with the x-ray data and with information derived from physical and chemical studies. Examination of the resulting electron density map shows that the coat protein is made up to two ..cap alpha..-helical segments. One helical segment, almost parallel to the particle axis, is centered at a radius of approx. 15 A. The other, at 25 A radius, is tilted by 25/sup 0/ to the particle axis. This arrangement s consistent with every generalization about ..cap alpha..-helical packing. The inner and outer segments interlock along most of their length with a crossing angle of 20.5/sup 0/. The inner ..cap alpha..-helical segments also interact with symmetry related copies of themselves as domore » the outer segments. The double layer of tightly packed, intricately interlocked ..cap alpha..-helices forms a stable, 20-A thick protein coat about the viral DNA.« less