The biosynthesis of membrane-bound M13 coat protein. Energetics and assembly intermediates.

The major coat protein of bacteriophage M13 spans the plasma membrane of infected cells prior to its assembly into extruding virus. It is initially made as a precursor, termed procoat, with a 23-residue leader sequence at its NH2 terminus. Procoat is found bound to the inner surface of the plasma membrane. The electrical potential of the cell membrane is required for procoat insertion and conversion to coat protein, although the order of these events has been unknown. We now report studies of the conversion of a mutant procoat (procoat-R6) from the virus M13am8H1R6 to mutant coat (coat R6). The behavior of procoat-R6 differs from that of the wild type procoat in three respects. (i) Pulse-labeled procoat-R6 is largely found inserted across the cell membrane. This suggests that the active site of leader peptidase is on the periplasmic membrane face and that insertion normally precedes processing for wild type procoat as well. (ii) Despite the greater abundance of inserted procoat-R6 in M13am8H1R6-infected cells than inserted procoat in wild type infections, procoat-R6 is processed to coat-R6 more slowly than procoat is converted to coat. (iii) The membrane insertion and proteolytic processing of procoat-R6 are almost completely insensitive to uncouplers. We present a working model for the energetics and assembly intermediates of coat protein biosynthesis.