Several of the structural proteins of phage P2 and its satellite P4 undergo proteolytic processing during development of mature phage particles. Here, we report that uncleaved shell protein, gpN, is present in immature capsids of both P2 and P4, showing that assembly precedes processing. This excludes the possibility that processing of gpN is involved in capsid size determination. We also find that N*, the fully processed version of gpN, produced from a plasmid, can assemble into both P2- and P4-sized particles, implying that the amino-terminal end of gpN is not required for assembly initiation nor for the formation of a T = 4 shell. As may be expected for a scaffolding protein, we find that gpO coexists with gpN in immature P2, as well as P4, capsids. This result supports the conclusion that gpO is required for both phages and strongly suggests that the O derivative, h7 (found in mature capsids), results from proteolytic cleavage after gpN/gpO coassembly. © 1994 Academic Press, Inc.