Bacteriophage P22 scaffolding protein forms oligomers in solution.

The scaffolding protein of Salmonella typhimurium bacteriophage P22 is a 33.6 kDa protein required both in vivo and in vitro for the polymerization of the viral coat protein into closed T = 7 icosahedral procapsids. In vitro assembly reaction kinetics have previously been found to vary between second and third order with respect to scaffolding protein concentration, suggesting that dimers and/or higher-order oligomers may be the active species in assembly. Analytical ultracentrifugation experiments suggest that scaffolding protein undergoes a rapidly-reversible monomer/dimer/tetramer equilibrium, with higher association constants at 4 degrees C than at 20 degrees C. Under conditions in which in vitro assembly reactions are carried out (30 to 1000 microg/ml scaffolding protein, 20 degrees C), monomers are the predominant species, but the concentration of dimers is significant. A mutant scaffolding protein, R74C/L177I, which forms disulfide-linked dimers, catalyzed procapsid assembly at a higher rate than did the wild-type scaffolding protein; preincubation in dithiothreitol had little effect on the wild-type protein, but greatly reduced the activity of the mutant. These findings suggest that dimers and/or higher-order oligomers of scaffolding protein are active species in the assembly of P22.