Visualization of the maturation transition in bacteriophage P22 by electron cryomicroscopy

Large-scale conformational transitions are involved in the life-cycle of ma ny types of virus. The dsDNA phages, herpesviruses, and adenoviruses must u ndergo a maturation transition in the course of DNA packaging to convert a scaffolding-containing precursor capsid to the DNA-containing mature virion . This conformational transition converts the procapsid, which is smaller, rounder, and displays a distinctive skewing of the hexameric capsomeres, to the mature virion, which is larger and more angular, with regular herons. We have used electron cryomicroscopy and image reconstruction to obtain 15 Angstrom structures of both bacteriophage P22 procapsids and mature phage. The maturation transition from the procapsid to the phage results in severa l changes in both the conformations of the individual coat protein subunits and the interactions between neighboring subunits. The most extensive conf ormational transformation among these is the outward movement of the trimer clusters present at all strict and local 3-fold axes on the procapsid inne r surface. As the trimer tips are the sites of scaffolding binding, this he lps to explain the role of scaffolding protein in regulating assembly and m aturation. We also observe DNA within the capsid packed in a manner consist ent with the spool model. These structures allow us to suggest how the bind ing interactions of scaffolding and DNA with the coat shell may act to cont rol the packaging of the DNA into the expanding procapsids. (C) 2000 Academ ic Press.