Identification and characterization of the domain structure of bacteriophage P22 coat protein

The bacteriophage P22 serves as a model for assembly of icosahedral dsDNA v iruses. The P22 procapsid, which constitutes the precursor for DNA packagin g, is built from 420 copies of a single coat protein with the aid of stoich iometric amounts of scaffolding protein. Upon DNA entry, the procapsid shel l expands and matures into a stable virion. It was proposed that expansion is mediated by hinge bending and domain movement. We have used limited prot eolysis to map the dynamic stability of the coat protein domain structures. The coat protein monomer is susceptible to proteolytic digestion, but limi ted proteolysis by small quantities of elastase or chymotrypsin yielded two metastable fragments (domains). The N-terminal domain (residues 1-180) is linked to the C-terminal domain (residues 205-429) by a protease-susceptibl e loop (residues 180-205). The two domains remain associated after the loop cleavage. Although only a small change of secondary structure results from the loop cleavage, both tertiary interdomain contacts and subunit thermost ability are diminished. The intact loop is also required for assembly of th e monomeric coat protein into procapsids. Upon assembly, coat protein becom es largely protease-resistant, baring cleavage within the loop region of ab out half of the subunits. Loop cleavage decreases the stability of the proc apsids and facilitates heat-induced shell expansion. Upon expansion, the lo op becomes protease-resistant. Our data suggest the loop region becomes mor e ordered during assembly and maturation and thereby prays an important rol e in both of these stages.