Structure:

There are two main parts building bacteriophage T4, namely: head and tail with the fibres.

Head: "The head of bacteriophage T4 has a weight of 194 MDa and is 1150 Ǻ long and 850 Ǻ wide. The head is an icosahedron consisting of 160 hexamers of gp23 (major capsid protein, 48.4 kDa), 11 pentamers of gp24 (pentamic corner protein, 46 kDa), and 1 dodecamer of gp20 (Table 1). During the formation of the prohead, scaffold and shell proteins undergo proteolytic cleavage by the active form of gp21 (prohead protease, T4PPase). The amino termini of proteins gp23, gp24, IPI, IPII, IPIII, and gpalt are cleaved, while proteins gp22, gp21, gp67, and gp68 are extensively digested (Leiman et al. 2003). The distance between the gp23 hexamer centres is ~140Ǻ (Fokine et al. 2004). These two proteins form an approximately 30-Ǻ-thick shell protecting the nucleic acid. Head corners are occupied by gp24, which forms pentamers and interacts with the borders of the gp23 hexamers (Fig. 1). Another structural element of the T4capsid is a small protein, Soc (small outercapsid protein, 9 kDa). It forms an almost continuous mesh on the surface of the gp23 hexamers. It binds two gp23 subunits, but it cannotbind around the gp24 pentamers or between gp23 and gp24. Some studies also revealed that Soc-gp23 interactions are favoured over Soc-Soc interactions (Olson et al. 2001). The lattice created by gp Soc encircles each gp23 hexamer which does not border on a gp24 pentamer. When gp23 is neighbouring gp24, Soc molecules occupy only five of the six sides of the hexagon. It is not incorporated into the side which is attached to the gp24 pentamer. Most of the gp23 molecules are in contact with two Soc proteins, except those which are closest to a gp24, which interact with only one Soc particle. The location of the Soc molecules seems to confirm one of their proposed functions, which is reinforcing the vaults between gp23 subunits. When located between gp23 subunits, gp Soc forms trimers, but when purified from the phage or expressed in vitro it has the form of a monomer. Although the function of Soc protein has not yet been fully characterised, it is considered to stabilise the phage's capsid against thermal denaturation and exposure to detergent or alkaline pH, so it is responsible for preserving phage viability in unfavourable conditions (Olson et al. 2001).