Folding and stability of mutant scaffolding proteins defective in P22 capsid assembly.

Bacteriophage P22 scaffolding subunits are elongated molecules that interact through their C termini with coat subunits to direct icosahedral capsid assembly. The soluble state of the subunit exhibits a partially folded intermediate during equilibrium unfolding experiments, whose C-terminal domain is unfolded (Greene, B., and King, J. (1999) J. Biol. Chem. 274, 16135-16140). Four mutant scaffolding proteins exhibiting temperature-sensitive defects in different stages of particle assembly were purified. The purified mutant proteins adopted a similar conformation to wild type, but all were destabilized with respect to wild type. Analysis of the thermal melting transitions showed that the mutants S242F and Y214W further destabilized the C-terminal domain, whereas substitutions near the N terminus either destabilized a different domain or affected interactions between domains. Two mutant proteins carried an additional cysteine residue, which formed disulfide cross-links but did not affect the denaturation transition. These mutants differed both from temperature-sensitive folding mutants found in other P22 structural proteins and from the thermolabile temperature-sensitive mutants described for T4 lysozyme. The results suggest that the defects in these mutants are due to destabilization of domains affecting the weak subunit-subunit interactions important in the assembly and function of the virus precursor shell.