Assembly in vitro of bacteriophage HK97 proheads.

Bacteriophage HK97 is a lambdoid phage with a head assembled from 415 copies of a 42 kDa subunit arranged in an icosahedrally symmetrical lattice with a triangulation number of 7. Prohead I, the first shell structure in the assembly pathway, is composed of 42 kDa coat protein subunits that have not yet undergone the proteolytic cleavage, conformational changes, and covalent cross-linking steps that occur later in the assembly of mature heads. Prohead I can be efficiently dissociated into capsomeres by treatment with 2 M KCl. The resulting capsomeres are a mixture of two species, identified as pentamers and hexamers of the 42 kDa subunit. These capsomeres were also detected as the products of chaperonin-assisted renaturation of 42 kDa polypeptide in vitro at room temperature or in the course of self folding and assembly in vitro at 0 degrees C. Pentamer and hexamer capsomeres can be interconverted in vitro by manipulating solvent conditions, and this makes it possible to carry out the in vitro shell assembly reaction at different input ratios of hexamer to pentamer. The Prohead I structures produced are always the normal (T = 7) size regardless of the input pentamer to hexamer ratio. Assembly is most efficient when the pentamer to hexamer ratio is 1:5 (a mass ratio of 1:6), or the same as the capsomere ratio in a T = 7 shell.