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The immunity protein (Imm) encoded by the Escherichia coli phage T4 effects exclusion of phage superinfecting cells already infected with T4. The 83-residue polypeptide possesses two long lipophilic areas (from residues 3 to 32 and 37 to 65) interrupted by a hydrophilic stretch including two positively charged residues. The charge distribution of the protein very strongly suggested that it is a plasma membrane protein with the C terminus facing the periplasm. While it could be shown that the expected location was correct, fusions of Imm to alkaline phosphatase or beta-galactosidase showed that the C terminus was at the cytosolic side of the membrane. Also, concerning function, 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. Evidence suggesting that Imm is associated with the membrane at specific sites is presented. It is proposed that Imm is localized to the membrane with the help of a receptor and that, therefore, it does not follow the established rules for the topology of other membrane proteins. The results also suggest that Imm acts indirectly, possibly by altering the conformation of a component of a phage DNA injection site.

Location and unusual membrane topology of the immunity protein of the Escherichia coli phage T4