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Index >> Bacteriophages - Part One >> Head Assembly of Phage T4

Head Assembly of Phage T4

	Head Assembly of Phage T4 - The principal protein of phage T4 heads is gp23 (MW 56.000), a primary product of gene 23 About 10,000 daltons of this protein are cleaved from the A,-terminal to form gp23, Other components of the head are gp18 (MW 69,000) gp20 (MW 63,000) gp19 (MW 18,000) and gp24 (MW 43,500). The last protein is cleaved from gp24 (MW 45,000).
The Y genes (20, 21, 22, 23, 24, 31, 40) control the cleavage of head proteins to their mature forms. Mutants of anyone of these proteins result in aberrant structures (lumps, polyheads, tau particles), which are formed of the uncleaved gp23 rather than gp23*. The T4 head also contains internal components like the polyamines putrescine and spermidine, acid soluble peptides and' acid insoluble proteins.
Acid soluble components are the three oligopeptides VI,. VII, and II, the last two of which are found in mature particles The oligopeptides may be the products of gene 22 cleavage. Mutations in anyone of the Y genes block the appearance of these polypeptides. The acid insoluble proteins are IPI, IPII and IPIII. They account for S% of the phage particle proteins. These proteins are present in the mature head in a cleaved form designated as IPI, IPII, and IPIII., respectively.
Cleavage of 15 amino acid from the N-terminal of IPI results in the formation or IPII, and a reduction in molecular weight from 11,750, to 10,000. Cleavage of IPUI to IPIII reduces the molecular weight from 21,000 to 18,000. Cleavage of IPI to IPI* from the N-terminal end also occurs.
	Cleavage of IPIII, and also possibly of IPH and IPI, is prevented by mutation in any one of, the Y genes. In all three proteins the cleavage probably takes place by the rupture of the same peptide linkage between glutamic acid and alanine. Another internal protein found in the phage head is BI. (MW 61,000). This protein is formed by the cleavage of its precursor BI (MW 79,000). BI appears to be injected into the host cell along with the DNA, and may be responsible for altering the host RNA polymerase. The assembly of phage T4 head takes place in four stages, which have been called prohead I. prohead II, prohead III and the mature head.

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