A signal peptide with a proline next to the cleavage site inhibits leader peptidase when present in a <i>sec</i>‐independent protein

Proline residues are rarely found in the three most C‐terminal positions of bacterial signal peptides, and have never been found in position +1 immediately following the cleavage site. It was recently shown that a Pro +1 mutation in the E. coli maltose binding protein precursor not only prevents cleavage of the signal peptide but also inhibits the leader peptidase enzyme, resulting in cessation of cell growth (Barkocy‐Gallagher, G.A. and Bassford, P.J. (1992) J. Biol. Chem. (in press)). Since maltose binding protein is dependent on the sec machinery for translocation across the inner membrane, it was not clear if this ‘Pro +1 ’ effect was restricted to sec ‐dependent proteins, or whether it applies also to proteins that do not require the sec functions for translocation. We now present data suggesting that the striking phenotypic effects of Pro +1 mutations can be elicited also by sec ‐independent proteins.