ELECTROSTATIC INTERACTIONS DRIVE SCAFFOLDING COAT PROTEIN-BINDING ANDPROCAPSID MATURATION IN BACTERIOPHAGE-P22/

The first step in assembly of the bacteriophage P22 is the formation o f a T = 7 icosahedral ''procapsid,'' the major components of which are the coat protein and an inner core composed of the scaffolding protei n. Although not present in the mature virion, the scaffolding protein is required for procapsid assembly. Eleven amino-acid residues at the extreme carboxyl terminus of the scaffolding protein are required for binding to the coat protein, and upon deletion of these residues, appr oximately 20. additional residues become disordered. Sequence analysis and NMR data suggest that the 30 residues at the carboxyl terminus fo rm a helix-loop-helix motif which is stabilized by interhelical hydrop hobic interactions. This ''coat protein recognition domain'' presents an unusually high number of positively charged residues on one face, s uggesting that electrostatic interactions between this domain and the coat protein may contribute to recognition and binding. We report here that high ionic strength (1 M NaCl) completely inhibited procapsid as sembly in vitro. When scaffolding protein was added to empty procapsid ''shells'' of coat protein, 1 M NaCl partially inhibited the binding of scaffolding protein to the shells. This suggests that the positivel y charged coal protein recognition domain at the carboxyl terminus of the scaffolding protein binds to a negatively charged region on the co at protein. During DNA packaging, the scaffolding protein exits the pr ocapsid; scaffolding protein exit is followed by the expansion of the procapsid into a mature capsid. Procapsid shells can be induced to und ergo a similar expansion reaction in vitro by heating (45-70 degrees C ); this process was also inhibited by 1 M NaCl. These results are cons istent with a model in which negatively charged scaffold protein-bindi ng domains in the coat proteins move apart during procapsid expansion; this relief of electrostatic repulsion could provide a driving force for expansion and subsequent maturation. High-salt concentrations woul d screen this repulsion, while packaging of DNA (a polyanion) in vivo may increase the instability of the procapsid enough to trigger its ex pansion. (C) 1998 Academic Press