Secretion and membrane integration of a filamentous phage-encoded morphogenetic protein.

The filamentous phage-encoded gene IV protein is required at high levels for virus assembly, although it is not a constituent of the virion. It is an integral membrane protein that does not contain an extended hydrophobic region of the kind often required for stable integration in the inner membrane. Rather, like a number of Escherichia coli outer membrane proteins, pIV is rich in charged amino acid residues and is predicted to consist of extensive beta-sheet structures. In phage-producing cells, pIV is primarily detected in the outer membrane, while in cells that produce it from the cloned gene, pIV is found in both the inner and outer membranes. The protein is synthesized as a precursor. Following cleavage of the signal sequence and translocation into the periplasm, the mature form is initially found as a soluble species. Soluble pIV then integrates into the membrane with a half-time of one to two minutes. Neither phage assembly nor other phage proteins are needed for this membrane integration, and phage assembly does not require the presence of the soluble form. The gene IV protein may be part of the structure through which the assembling phage is extruded.