Scaffolding mutants identifying domains required for P22 procapsid assembly and maturation

Assembly of the icosahedral shells of the dsDNA bacteriophages, herpesviruses, and adenoviruses requires proteins not found in the mature virion, termed scaffolding proteins. The bacteriophage P22 precursor procapsid contains approximately 300 scaffolding molecules within a shell composed of 420 coat protein subunits. Though nonsense mutants are common, few mutants affecting the functions of the scaffolding protein have been recovered. We report here the isolation and characterization of new missense mutants unable to form infectious virions under restrictive conditions. These mutant scaffolding subunits were competent for protein folding and capsid assembly under restrictive conditions. Two mutants were defective in assembly into the procapsid of the portal complex, which serves as the channel through which DNA is packaged. These mutations may identify a region of the scaffolding protein required for interaction with the portal subunits. Two mutants in a different region of the sequence were impaired in scaffolding release from the procapsid both in vivo and in vitro. These mutations may identify a new domain required for scaffolding release. Scaffolding release appeared to be required for capsid expansion; in turn, scaffolding release seemed to depend upon the presence of a portal. This may help to order the pathway of events in phage maturation.