Structure Formation of the Neck which Links the Head and the Tail of Bacteriophage

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During the assembly of phage T4, the neck is formed to block the DNA leakage and to prepare for the tail association after DNA packaging into the head is completed. The neck is composed of gp13 (gp: gene product) and gp14 each consists of 309 and 256 amino acid residues with the molecular weight of 41,226 and 32,868, respectively. In order to elucidate the structure and the subunit arrangement of the neck, genes coding for the two proteins were cloned by PCR method, an overexpression system was constructed and the genes were induced by IPTG. The two proteins were purified by ammonium sulfate fractionation, ion exchange chromatography, gen filtration as a single band in SDS-PAGE. Far-ultraviolet circular dichroism spectra of the two proteins indicated that gp13 is rich in α-helix, whereas gp13 is rich in β-sheet. On the other hand, sedimentation velocity indicated that both proteins are monomers with the sedimentation coefficient of 2.80S and 2.00S, respectively and that the frictional ratios were 1.22 and 1.73, respectively, which indicated that gp14 is more elongated than gp13. The two proteins did not interact each other under normal physiological conditions, but it was found that the two proteins formed a specific complex with the sedimentation coefficient of 16.6S. The complex had the molecular weight of 497,000 which together with the molar ratio of 2:1 based on SDS-PAGE indicated that the complex is a hetero 15-mer, (gp 13)10(gp 14)5. This complex is considered to be identical with the complex which was observed when the two proteins were co-expressed. Electron microscopic observations revealed that the complex formed a ring with the diameter of 156 Å which coincided with the electron density of the three-dimensional image of the neck reconstructed from electron micrograph.