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 la ttice with a triangulation number of 7. Prohead I, the first shell str ucture in the assembly pathway, is composed of 42 kDa coat protein sub units that have not yet undergone the proteolytic cleavage, conformati onal changes, and covalent cross-linking steps that occur later in the assembly of mature heads. Prohead I can be efficiently dissociated in to 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 ro om temperature or in the course of self folding and assembly in in vit ro 0 degrees C. Pentamer and hexamer capsomeres can be interconverted in vitro by manipulating solvent conditions, and this makes it possibl e 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 hex amer ratio. Assembly is most efficient when the pentamer to hexamer ra tio is 1:5 (a mass ratio of 1:6), or the same as the capsomere ratio i n a T = 7 shell. (C) 1995 Academic Press Limited