Attachment of tail fibers in bacteriophage T4 assembly. Purification, properties, and site of action of the accessory protein coded by gene 63.

The attachment of tail fibers to tail-fiberless T4 particles in extracts of mutant-infected cells proceeds slowly in the absence of gene product Cgp) 63, but can be stimulated up to 50-fold by its presence, supporting the view that gp63 acts catalytically to increase the rate of tail fiber attachment. The active protein has been purified about loo-fold to yield a nearly homogeneous preparation. Polyacrylamide gel electrophoresis of the purified material in the presence of sodium dodecyl sulfate gives a single major component of molecular weight about 42,000. Production of infectious phage proceeds efficiently in the presence of purified gp63, tail fibers, and tail-fiberless particles, suggesting that no additional cofactors or unidentified gene products are required for the attachment reaction. Tail fiber attachment involves transient interaction of the distal half of the tail fiber with the collar region of the particle as well as the joining of the proximal end of the fiber with the phage baseplate. To determine which of these interactions involves gp63, the slow and probably nonphysiological attachment of free proximal half-fibers to the baseplate was investigated and shown to be stimulated about 5-fold by gp63. Subsequent conversion of particles with attached proximal half-fibers to infectious phage by treatment with free distal half-fibers was stimulated less than I.&fold bygp63. We conclude thatgp63 acts to promote the noncovalent association of the proximal end of the tail fiber with the phage baseplate. We cannot yet conclude whether or not its action represents a true catalysis.