The coat and scaffolding proteins of bacteriophage
P22 procapsids have been purified in soluble form. By incubating both purified
proteins with a mutant-infected cell extract lacking procapsids, but competent
for DNA packaging in vitro (Poteet et al., 1979), we were able to
obtain assembl of biologically active procapsids in vitro. The active
species for complementation in vitro in both protein preparations
copurified with the soluble subunits, indicating that these subunits represent
precursors in procapsid polymerization.
When the purified coat and scaffolding subunits were mixed directly, they
polymerized into double-shelled procapsid-like structures during dialysis
from 1.5 m-guanidine hydrochloride to buffer. When dailyzed separately under
the same conditions, the scaffolding subunits did not polymerize but remained
as soluble subunits, as did most of the coat subunits. No evidence was found
for self-assembly of the scaffolding protein in the absence of the coat
protein.
The unassembled coat subunits sedimented at 3.9 S and the unassembled scaffolding
subunits sedimented at 2.4 S in sucrose gradients. The Stokes' radius, determined
by gel filtration, was 24 Å for
the coat subunits and 34 Å for the scaffolding subunits. These results
indicate taht the scafolding subunits are relatively slender elongated molecules,
whereas the coat subunits are more globular.
The experiments suggest that the procapsid is built by copolymerization
of the two protein species. Their interaction on the growing surface of
the shell structure, and not in solution, appears to regulate successive
binding interactions.
