MULTIPLE CONFORMATIONAL STATES OF THE BACTERIOPHAGE-T4 CAPSID SURFACELATTICE INDUCED WHEN EXPANSION OCCURS WITHOUT PRIOR CLEAVAGE

The maturation pathway of bacteriophage T4 capsid provides a model sys tem for the study of large-scale conformational changes, in that the p recursor capsid progresses through four long-lived and widely differin g states. The surface lattice first assembled (uncleaved/unexpanded st ate: hexagonal lattice constant, a = 11.8 nm) undergoes proteolytic cl eavage (cleaved/unexpanded state), then expands (cleaved/expanded stat e: a = 14.0 nm), and then binds accessory proteins. The most profound change, expansion, normally follows cleavage of the major capsid prote in gp23 to gp23 (the 65-residue N-terminal ''Delta-domain'' is remove d), but can be induced in vitro in the absence of cleavage by treatmen t with 0.25 M guanidine-HCl (uncleaved/expanded state). We have studie d this alternative pathway by negative staining electron microscopy of polyheads (tubular capsid variants). We find that uncleaved/expanded polyheads encompass four discrete states, called G1-G4, distinguished by their lattice constants of 12.6 nm (G1), 13.4 nm (G2), and 14.0 nm (G3, G4) and by the structures of their hexameric capsomers. Viewed in projection, the G4 capsomer differs from the cleaved/expanded capsome r only in the presence of additional mass at one site per protomer. Th is mass correlates with the presence of the Delta-domain, which transl ocates from the inner to the outer surface when the uncleaved lattice expands. Based on proximity of resemblance among these capsomers, we s uggest that G1 to G4 represent a sequence of transitional states whose endpoint is G4. G1, G2, and G3 may correspond to intermediates that a re too short-lived to be observed when the cleaved lattice expands, bu t are trapped by the retention of Delta-domains at the interfaces betw een subunits in the uncleaved lattice. (C) 1997 Academic Press.