Molecular organization of the bacteriophage P22 coat protein shell

Abstract

P22 is a moderately large double-stranded DNA-containing bacteriophage. The mature virion contains several minor protein species and a single major coat protein species. Freeze-etch electron microscopic evidence is presented here to show that the coat protein is present on the exterior of the virion in a T = 7 levo type lattice with icosahedral symmetry. Each of the coat protein subunits participates in the formation of two morphological units visible in the electron microscope, one at a local 3-fold axis of symmetry, and one at a 5-fold or local 6-fold axis of symmetry, thus giving the appearance of a T = 21 dextro surface lattice. The fact that the precursor particles (proheads) have previously been shown to have a surface structure similar to phage (Earnshaw et al., 1976) suggests a model for the coat protein shell expansion during DNA encapsidation.