PROTEOLYTIC AND CONFORMATIONAL CONTROL OF VIRUS CAPSID MATURATION - THE BACTERIOPHAGE-HK97 SYSTEM

Bacteriophage capsid assembly pathways provide excellent model systems to study large-scale conformational changes and other mechanisms that regulate the formation of macromolecular complexes. These capsids are formed from proheads: relatively fragile precursor particles which ma ture by undergoing extensive remodeling. Phage HK97 employs novel feat ures in its strategy for building capsids, including assembly without a scaffolding protein, and the formation of a network of covalent cros s-links between neighboring subunits in the mature virion. In addition , proteolytic cleavage of the capsid protein from 42 kDa to 31 kDa is essential for maturation. To investigate the structural bases for prot eolysis and cross-linking, we have used cryo-electron micrographs to r econstruct the three-dimensional structures of purified particles from four discrete stages in the assembly pathway: Prohead I, Prohead II, Head I and Head II. Prohead I has icosahedral T = 7 packing of blister -shaped pentamers and hexamers. The pentamers are 5-fold symmetric, bu t the hexamers exhibit an unusual departure from 6-fold symmetry, as i f two trimers had undergone a shear dislocation of about 25 Angstrom. Proteolytic conversion to Prohead II leaves the outer surface largely unchanged, but a major loss of density from the inner surface is obser ved, which we infer to represent the excision of the amino-terminal do mains of the capsid protein. Upon expansion to the Head I state, the c apsid becomes markedly larger, thinner walled, and more polyhedral: mo reover, the capsomer shapes change radically; especially notable is th e disappearance of the large hexon dislocation. No differences between Head I and the covalently cross-linked Head II could be observed at t he current resolution of about 25 Angstrom, from which we infer that i t is the conformational rearrangements effected by expansion that crea te the micro-environments needed for the autocatalytic formation of th e isodipeptide bonds found in the mature virions (''pseudo-active site s''). (C) 1995 Academic Press Limited