MATURATION OF THE HEAD OF BACTERIOPHAGE T4 V. A Possible DNA Packaging Mechanism: In Vitro Cleavage of the Head Proteins and the Structure of the Core of the Polyhead

The most recent developments in studies on the maturation of the head of bacteriophage T4 are described and discussed. The major features of the maturation steps of the head are the following: (a) The viral DNA is pulled into an empty head in a series of events. (b) Cleavage of two core proteins, P22 (MW = 31,000), to small fragments and the internal protein IPIII (MW = 23,000) to IPIII* (MW = 21,000) appears to be intimately linked to the DNA packaging event, whereas the cleavage of the major head protein of the viral coat, P23 (MW = 55,000), to P23* (MW = 45,000) precedes the DNA packaging event. The P22 core proteins appear to be the precursors of the well-known, highly acidic internal peptides. We have tested the idea that these internal peptides collapse DNA by a repulsive interaction as various polymers like polyethylene oxide (PeO) and polyacrylate(PAA) do. We found that high concentrations of the internal peptides, polyaspartic acid, and polyglutamic acid, collapse DNA. This supports the idea that repulsive interactions with the internal peptides may collapse the DNA inside the head, and thus pull the DNA in. The structure of the DNA collapsed by PeO was studied with the electron microscope and contrasted with the structure of DNA collapsed by polylysine. We find PeO collapses T4 DNA into compact particles best described as a ball of string, of about the size of the T4 head. Two structures are seen in preparations of polylysine-collapsed DNA. One has the shape of a donut and the DNA strand appears to be radially distributed as a spiral; the other is a stemlike structure in which the DNA is folded back and forth in a pleated structure. The aberrant tubular polyhead contains the precursor protein P23, P22, and the internal proteins IPIII and IPII. Addition of chloroform to a polyhead preparation extracts the proteins P22, IPIII, and IPII. This 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. Structural studies reveal that the core of the polyhead is highly organized into a helical structure consisting of 4–6 helical chains wound about a hollow center of approximately 150 a diameter. Cleavage of the various head proteins occurs when partially purified polyheads are incubated at 37°C. In a 100 minute incubation, about 60–70% of P23 (MW = 55,000) is converted to P23* (MW = 45,000) and a significant conversion of IPIII (MW = 23,000) 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 the in vivo products, suggesting that the protease cleavage is specific. However, several other protein fragments are generated during the in vitro cleavage reaction which have not been observed in vivo. Appropriate mutant studies reveal that the products of genes 21 and 22 are required for these in vitro cleavage reactions.