Cold-sensitive mutations in phage P22 coat protein cause the accumulation of precursor capsids in cells growing at the nonpermissive temperature (16 degrees C). The assembly of coat proteins which carry the substitutions threonine at position 10 to isoluecine (T10I), arginine at position 101 to cysteine (R101C), or asparagine at position 414 to serine (N414S) which cause cold-sensitivity has been investigated. All three proteins were found to fold into a monomeric species. Coat proteins carrying the amino acid substitutions T10I and R101C were not able to interact with scaffolding protein appropriately to initiate assembly in vitro while coat protein carrying the substitution N414S was able to assemble; however, capsids formed of this protein had an increased affinity for scaffolding protein. These amino acid substitutions define two regions in coat protein that are essential for the interaction of coat protein with scaffolding protein at different stages in capsid maturation.