A mutation downstream from the signal peptidase cleavage site affects cleavage but not membrane insertion of phage coat protein.

Morphogenesis of filamentous phage includes synthesis of the phage major coat protein in precursor form, its insertion into the host cell plasma membrane, its cleavage to the mature form of the protein, and its assembly there into virions. The M13 mutant am8H1R6 encodes a coat protein in which leucine replaces glutamic acid as residue 2 of the mature protein [Boeke, J. D., Russel, M. & Model, P. (1980) J. Mol. Biol. 144, 103-116]. The coat protein precursor produced by this variant is a poor substrate for the Escherichia coli signal peptidase both in vivo and in vitro. This pre-coat protein, which is eventually processed and assembled into viable phage particles, is associated with the membrane fraction of the infected cell. We conclude that the domain recognized by the signal peptidase extends beyond the signal peptide itself. Furthermore, membrane association and signal peptide cleavage can be separated temporally under conditions that permit membrane insertion, cleavage, and phage assembly.