1.E.9 The T4 Immunity (T4 Imm) Family
The Imm protein of the Escherichia coli phage T4 effects exclusion of phage superinfecting cells already infected with T4. The 83 residue protein possesses two putative TMSs (residues 3-32 and 37-65). The C-terminus is probably in the cytoplasm (Lu et al., 1993). It has been reported to resemble holins (Young and Bläsi, 1995). This inner membrane protein functions to block DNA entry into the bacterial cytoplasm (Labrie et al., 2010). Homologues are encoded in many phage and bacterial genomes.
The immunity (imm) gene was placed under the control of the lac regulatory elements in a pUC plasmid. Expression of the imm gene caused exclusion of T4. The fraction of phage which was excluded upon infection of cells possessing the plasmid-encoded Imm protein ejected only about one-half of their DNA. Therefore, the Imm protein inhibited, directly or indirectly, DNA ejection (Lu and Henning 1989).
The two long lipophilic TMSs are interrupted by a hydrophilic stretch including two positively charged residues. Fusions of Imm to alkaline phosphatase or beta-galactosidase showed that the C terminus is at the cytosolic side of the membrane. There was almost no structural specificity to this part of the protein; even removal of the two positively charged residues did not completely abolish function. It was proposed that Imm is localized to the membrane with the help of a receptor, and that, therefore, it does not follow the positive inside rule. Imm may act indirectly, possibly by altering the conformation of a component of a phage DNA injection site (Lu et al. 1993).