Three-Dimensional Structure of Isometric Capsids of Bacteriophage T4

Abstract Coliphage T4 is a large, double-stranded DNA virus (family Myoviridae) which infects Escherichia coli. It is the classic example of a contractile-tailed phage and a wealth of information on viral morphogenesis processes have been obtained on it. Extensive studies of T4 by negative-stain and metal-shadow electron microscopy have shown that mature virions, which contain over 40 different polypeptides, have a prolate capsid that encapsidates the genome (∼175 × 106Da), and several other structural components, including the collar with whiskers, neck, a contractile sheath, and a baseplate with fibers. A complete understanding of processes such as assembly of the viral capsids will require atomic-scale knowledge of the various protein interactions. Details of the process can be extrapolated from knowledge of the initial, transitional, and final states of a structure. We have initiated an examination of T4 head morphogenesis by studying mutant, isometric capsids ('isometrics’) using cryo-electron microscopy and three-dimensional (3D) image reconstruction methods as applied to particles with icosahedral symmetry.