Procoat, the precursor of M13 coat protein, requires an electrochemical potential for membrane insertion.

The coat protein of coliphage M13 spans the host cell cytoplasmic membrane prior to its assembly into extruding virus. It is made as a soluble cytoplasmic precursor, termed "procoat," with 23 extra amino acid residues at the NH2 terminus. Procoat binds to the cell membrane and is converted proteolytically to coat protein. When the electrochemical gradient of an infected cell is rapidly dissipated by uncouplers, procoat still binds to the plasma membrane but is not converted to coat. We report here that membrane-bound procoat is only detected at the inner face of the cytoplasmic membrane and that uncouplers prevent it from integrating into a transmembrane conformation.