Lytic activity localized to membrane-spanning region of phi X174 E protein

Lytic activity of the phi X174 E (lysis) protein had previously been localized to the amino terminal 51 amino acids (a.a.) of the molecule (Blasi and Lubitz 1985). This E gene lytic activity has here been further localized to the amino terminal 29 a.a., a region of the protein which is thought to just span the cell membrane (Young and Young 1982). phi X174 E gene fusions to both the lacZ gene and the chloramphenicol acetyl transferase (CAT) gene resulted in fusion proteins with lytic activity. Fusion to a third protein, trpE, did not result in lytic activity. These results support a model of oligomerization of the phi X174 E protein for lytic activity since both beta-galactosidase and CAT exist as tetramers in their native state. A difference in the composition of the charged amino acids at the cytoplasmic boundary between the various fusion proteins could also account for these results, since these amino acids may play a role in proper anchoring of the E protein in the cell membrane. In a spontaneous E gene mutant, which introduces a proline residue at position 9 of the E protein, lytic activity of the E protein was decreased, but not abolished. The presence of the helix-breaking proline at this position may interfere with insertion of the lysis protein into the cell membrane, leading to the decreased functional activity of the protein.