The role of the adsorption complex in the termination of filamentous phage assembly.

The adsorption complex of filamentous phage fd consists of two minor coat proteins, g3p and g6p, and is considered to be not only a structural entity, but also a functional unit to terminate phage assembly. Cells were infected with phage M13am8H1, which cannot assemble because it lacks the major coat protein g8p, although producing all of the other minor coat proteins. The membranes of infected cells were solubilized and analysed by non-denaturing PAGE and gel filtration. The data suggest the presence of the adsorption complex in these membranes. Furthermore, the non-polar gene 3 amber-mutant phage R171 was shown to lack g6p in the phage coat as well. The termination of assembly of this phage is disturbed, resulting in synthesis of polyphages. Electron micrographs and transient electrical birefringence show that these polyphages are eight times longer as compared to unit length phage. From these results, we conclude that the formation of the g3p-g6p complex is essential for correct termination of filamentous phage assembly.