The first step in assembly of the bacteriophage P22 is the formation of a T=7 icosahedral 'procapsid,' the major components of which are the coat protein and an inner core composed of the scaffolding protein. 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, approximately 20 additional residues become disordered. Sequence analysis and NMR data suggest that the 30 residues at the carboxyl terminus form a helix-loop-helix motif which is stabilized by interhelical hydrophobic interactions. This 'coat protein recognition domain' presents an unusually high number of positively charged residues on one face, suggesting 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 assembly 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 positively charged coat protein recognition domain at the carboxyl terminus of the scaffolding protein binds to a negatively charged region on the coat protein. During DNA packaging, the scaffolding protein exits the procapsid; scaffolding protein exit is followed by the expansion of the procapsid into a mature capsid. Procapsid shells can be induced to undergo a similar expansion reaction in vitro by heating (45-70°C); this process was also inhibited by 1 M NaCl. These results are consistent with a model in which negatively charged scaffold protein-binding 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 would screen this repulsion, while packaging of DNA (a polyanion) in vivo may increase the instability of the procapsid enough to trigger its expansion.