Structure of T4 polyheads: II. A pathway of polyhead transformations as a model for T4 capsid maturation

We have studied the aberrant tubular polyheads of bacteriophages T4D and T2L as a model system for capsid maturation. Six different types of polyhead surface lattice morphology, and the corresponding protein compositions are reported and discussed. Using in vitro systems to induce transformations between particular polyhead types, we have deduced that the structural classes represent successive points in a transitional pathway. In the first step, coarse polyheads (analogous to the prohead τ-particle) are proteolytically cleaved by a phagecoded protease, a fragment of the gene 21 product. This cleavage of P23 to P23¹ induces a co-operative lattice transformation in the protein of the surface shell, to a conformation equivalent to that of T2L giant phage capsids. These polyheads (derived either from T4 or T2L lysates) can accept further T4-coded proteins. In doing so, they pass through intermediate structural states, eventually reaching an end point whose unit cell morphology is indistinguishable from that of the giant T4 capsids. At least one protein (called soc (Ishii & Yanagida, 1975)) is bound stoichiometrically to P23¹ in the end-state conformation. The simulation of several aspects of capsid maturation (cleavage of P23 to P23¹, stabilization, and lattice expansion) in the polyhead pathway suggest that it parallels the major events of phage T-even capsid maturation, decoupled from any involvement of DNA packaging.