Binding of scaffolding subunits within the P22 procapsid lattice.

The capsid assembly pathways of the dsDNA bacteriophages, herpesviruses, and adenoviruses all proceed through a precursor shell lacking DNA. These procapsids contain scaffolding proteins required for assembly but absent from mature virions. The bacteriophage P22 procapsid contains approximately 300 molecules of the 33-kDa gene 8 scaffolding protein, in addition to the 420 molecules of gene 5 coat protein. During the process of DNA packaging and phage maturation, all 300 scaffolding molecules are released intact to participate in subsequent rounds of procapsid assembly. Low concentrations of guanidine hydrochloride (GuHCl) reproduce the release of scaffolding from procapsids in vitro, in the absence of DNA. The release was reversible; when the GuHCl was removed by dialysis, the scaffolding subunits reentered the extracted capsids to regenerate morphologically normal procapsids. The subunits presumably exited and reentered through the channels recently observed at the centers of the pentamers and hexamers (Prasad, B. V. V., Prevelige, P. E., Marietta, E., Chen, R. O., Thomas, D., King, J., and Chiu, W. (1993). J. Mol. Biol. 231 65-74). We have utilized this reaction to investigate the binding of scaffolding within normal procapsids and to other large structures of coat protein. Procapsids contained two classes of scaffolding subunits, which may represent binding of scaffolding to different specific positions within the T = 7 procapsid lattice. These sites became lost or inaccessible upon phage maturation.