Characterization of subunit structural changes accompanying assembly of the bacteriophage P22 procapsid

P22 serves as a model for the assembly and maturation of icosahedral double -stranded DNA viruses. The viral capsid precursor, or procapsid, is assembl ed from 420 copies of a 47 kDa coat protein subunit (gp5) that is rich in b eta -strand secondary structure. Maturation to the capsid, which occurs in vivo upon DNA packaging, is accompanied by shell expansion and a large incr ease in the level of protection against deuterium exchange of amide NH grou ps. Accordingly, shell maturation resembles the final step in protein foldi ng, wherein domain packing and an exchange-protected core become more fully developed [Tuma, R., Prevelige, P. E., Jr., and Thomas, G. J., Jr. (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 9885-9890]. Here, we exploit recent advan ces in Raman spectroscopy to investigate the P22 coat protein subunit under conditions which stabilize the monomeric state, viz., in solution at very low concentrations. Under these conditions, the monomer exhibits an elongat ed shape, as demonstrated by small-angle X-ray scattering. Raman spectra al low the identification of conformation-sensitive marker bands of the monome r, as well as the characterization of NH exchange dynamics for comparison w ith procapsid and capsid shell assemblies. We show that procapsid assembly involves significant ordering of the predominantly beta -strand backbone. W e propose that such ordering may mediate formation of the distinct subunit conformations required for assembly of a T = 7 icosahedral lattice. However , the monomer, like the subunit within the procapsid lattice, exhibits a mo derate level of protection against low-temperature NH exchange, indicative of a nascent folding core. The environments and exchange characteristics of key side chains are also similar for the monomeric and procapsid subunits, and distinct from corresponding characteristics of the capsid subunit. The monomer thus represents a compact but metastable folding intermediate alon g the pathway to assembly of the procapsid and capsid.