HK97 is a tailed temperate bacteriophage of E. coli that builds an icosahedral capsid using steps that include regulated assembly, proteolysis, radical conformational changes and the formation of novel covalent bonds (Fig. 1). This pathway is being exploited as a model system to explore how the formation of multiprotein complexes can be regulated by each of these mechanisms. We have identified and purified at least four intermediates (Prohead I, Prohead II, Head I and Head II) and examined them by cryo-electron microscopy and three dimensional reconstruction procedures (Fig. 2). Comparison of particle reconstructions at resolution of about 25 - 30 A have lead to major insights into the causes and purposes of the regulated changes that we have also characterized biochemically and genetically.

Prohead I consists of 420 copies of the 42 kDa gp5 capsid protein arranged as 72 blister-shaped morphological capsomers in a thick walled hollow T=7 icosahedral particle with a diameter of -470 Å.