Structure of bacteriophage φ29 head fibers has a supercoiled triple repeating helix-turn-helix motif

The tailed bacteriophage phi 29 capsid is decorated with 55 fibers attached to quasi-3-fold symmetry positions. Each fiber is a homo-trimer of gene product 8.5 (gp8.5) and consists of two major structural parts, a pseudohexagonal base and a protruding fibrous portion that is about 110 angstrom in length. The crystal structure of the C-terminal fibrous portion (residues 112-280) has been determined to a resolution of 1.6 angstrom. The structure is about 150 angstrom long and shows three distinct structural domains designated as head, neck, and stem. The stem region is a unique three-stranded helix-turn-helix supercoil that has not previously been described. When fitted into a cryoelectron microscope reconstruction of the virus, the head structure corresponded to a disconnected density at the distal end of the fiber and the neck structure was located in weak density connecting it to the fiber. Thin section studies of Bacillus subtilis cells infected with fibered or fiberless phi 29 suggest that the fibers might enhance the attachment of the virions onto the host cell wall.