Regulation of the phage phi 29 prohead shape and size by the portal vertex

Bacteriophage phi 29 of Bacillus subtilis packages its double-stranded DNA into a preformed prohead during morphogenesis. The prohead is composed of the scaffold protein gp7, the capsid protein pg8, the portal protein gp10, and the dispensable head fiber protein gp8.5. Our objective was to elucidate the phi 29 prohead assembly pathway and to define the factors that determine prohead shape and size. The structural genes of the phi 29 prohead were cloned and expressed in Escherichia coli individually or in combination to study form determination. The scaffold protein was purified from E. coli as a soluble monomer. In vivo and in vitro studies showed that the scaffolding protein interacted with both the portal vertex and capsid proteins. When the scaffold protein interacted only with the capsid protein in vivo, particles were formed with variable size and shape. However, in the presence of the portal vertex protein, particles with uniform size and shape were produced in vivo. SDS-PAGE analysis showed that the latter particles contained the proteins of the scaffold, capsid, head fiber, and portal vertex. These results suggest that the scaffolding protein serves as the linkage between the portal vertex and the capsid proteins, and that the portal vertex plays a crucial role in regulating the size and shape of the prohead.