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).

Gp Hoc (Highly Immunogenic Outer Capsid Protein, 39.1 kDa) is probably the most characteristic protein of the T4 capsid because of its regular location in the middle of each gp23 hexagon and because it protrudes considerably from the surface of the phage's head. This protrusion extends about 50 Ǻ over the capsid shell and it has a molecular weight of about 12 kDa. Hoc protein has a multidomain structure. It consists of a ~1.9-Ǻ-high round base, a thin neck region, and a ~20-Ǻ-wide and 24-Ǻ-high globular head. The function of gp Hoc is unknown (Leiman et al. 2003).

Fig 1. Protein structure of bacteriophage T4 head. A. General structure of bacteriophage T4 virion. B. Bacteriophage T4 head. Arrangement of proteins on head surface. Pentamers of gp24 are placed on head's corners while the rest of the surface is occupied by units made of  gp23, gpsoc and gphoc. C. Reciprocal arrangement of head proteins. Hexamers of gp23 are surrounded by gpSoc except borders with gp24 pentamers. The middle of each hexamer occupies gpHoc.

Table 1. The protein structure of the head of bacteriophage T4 (modified from Fokine et al. 2004, and Leimann et al. 2003)

italics - name of the gene

♦ - proteins forming a scaffold, coated with gp23 and gp24 during the maturation process. After prohead assembly, the scaffolding core structure is proteolytically removed and the core proteins are either totally or partially degraded. The degradation products of some core proteins remain inside the mature head. Only the internal proteins (IPI, IPII, and IPIII) are found in comparable amounts in the prohead and in their processed form (Kuhn et al. 1986).

Tail with fibres: "The T4 tail and fibres are important tools for bacteriophage interactions with its host. This part of the capsid determines phage specificity and enables the infecting of bacteria. The tail is composed of two concentric protein cylinders. The outer cylinder is contractile and the inner one builds the channel for the nucleic acid that is stored in the head. Such a syringe-like construction of the tail enables the injection of DNA into the bacterial cell. The inner tube has a 90-Ǻ outer and 40-Ǻ inner diameter and is constructed of 144 copies of gp19 (Table 2). The outer part of the tail is called the tail sheath. It is built by 144 copies of gp18 (in accordance with the number of gp19). The length of the tail is probably determined by the "ruler protein" (or template) gp29 (Leiman et al. 2003). The uncontracted tail is 1000 Ǻ long and 210 Ǻ in diameter (Mesyanshinov et al. 2004), which corresponds to the dimensions of the tail sheath. When contracted, the tail sheath is only 360 Ǻ long and 270 Ǻ wide. The length of the tail tube does not change during contraction (Leiman et al. 2003). At either end of the cylinders are the baseplate and fibres. The baseplate is a multiprotein structure 270Ĺ high and 520 Ĺ in diameter at its widest part. The proteins form six wedges surrounding the central hub with the help of two trimeric proteins, (gp9)₃ and (gp12)₃. Gp11, gp10, gp7, gp8, gp6, gp53, and gp25 combine sequentially to build up the wedges. Gp5, gp27, gp29, and probably gp26 and gp28 form the baseplate's hub (Leiman et al. 2003). Gp5 has a lysozyme domain which is necessary for the digestion of the bacterial peptidoglycan layer during the infection process.

One of bacteriophage T4's most useful device are the fibres: long tail fibres (LTFs) and short tail fibres (STFs) located on the distal part of the tail and whiskers extending outwardly from the collar region of the virion (Conley and Wood 1975). The long tail fibres, which are responsible for the recognition of specific receptors on the bacteria's surface, are ~1450 Ĺ long and ~40 Ĺ in diameter (Leiman et al. 2003). Each fibre consists of two halves: the proximal half encoded by gene 34 and the distal encoded by genes 36 and 37. These halves are connected by gp35, which interacts with gp34 and gp36. The protein connecting the LTFs with the baseplate is gp9. Association of the proximal part of the fibre with gp9 is assisted by gp63. gp9 has an extremely important role during infection. After the binding of an LTF to the LPS (lipopolysaccharide) on the bacteria's wall, it initiates the transition of the baseplate structure into a star-like conformation and the tail sheath contraction that enables injection of the phage's DNA into the cell. Besides that, gp9 is also responsible for the collective movements of the fibres and for preventing the baseplate from abortive triggering. The short tail fibres are gp12 trimers attached to the baseplate by gp11. They are 340-Ĺ-long club-like structures with a narrowing in the middle where the fibre can bend up to 90ş. STFs are responsible for binding the phage particle to the bacteria's surface. During the infection process the C-termini of gp12 molecule bind to the core region of the LPS cell-surface receptor (Mesyanzhinov et al. 2004).

Table 2. The protein structure of the T4 phage tail and fibres (modified from Mesyanzhinov et al. 2004 and Leimann et al. 2003)

italics - name of the gene

n.d. - not determined

* the number of copies of fibre proteins is given per fibre. There are six fibres of each type per T4 virion"

Molecular modification of T4 bacteriophage proteins and its potential application: Kurzepa A., Dabrowska K., Switala-Jelen K., Gorski A.