The structural proteins of the tubular, aberrant polyheads of bacteriophage T4 have been analyzed. The major structural constituents of polyheads are the precursor proteins P23, P22, and the internal proteins IPIII and IPII. Addition of chloroform to a polyhead preparation extracts three of these proteins (P22, IPIII, and IPII). Such treatment removes the inside material (core) seen in polyheads prior to the chloroform extraction as judged by electron microscopy. We conclude that P22. IPIII, and IPII (and supposedly IPI) are the major structural constituents of the core of polyheads, while P23 is the major constituent of the outer coat.

Cleavage of the various head proteins occurs when partially purified polyheads are incubated at 37°. In a 100-min incubation, about 60%–70% of P23 (MW = 56,000) is converted to P23∗ (MW = 46,000) and a significant conversion of IPIII (MW = 23,500) to IPIII∗ (MW = 21,000) is seen. The protein P22 (MW = 31,000) disappears during this incubation and is supposedly cleaved to small fragments.

The in vitro products, P23∗ and IPIII∗, have the same molecular weight as their in vivo product, suggesting that the cleavage is specific. However, several other protein fragments are generated during the in vitro cleavage reaction, which have not been observed in vivo. Appropriate studies with mutants reveal that the products of genes 21 and 22 are required for these in vitro cleavage reactions. The cleavage reaction is strongly inhibited by the addition of chloroform to the reaction mixture and is partially inhibited by the addition of TPCK (tosyl-phenylamine-chloromethylketone).