Positively charged residues influence the degree of SecA dependence in protein translocation across the E. coli inner membrane.

The sec machinery catalyzes the translocation of nascent polypeptide chains across the inner membrane of E. coli, yet some inner membrane proteins depend only weakly or not at all on an intact sec function for membrane insertion even though they have stretches of chain protruding into the periplasmic space. Earlier work has demonstrated that the length of a periplasmic loop correlates with its degree of sec-dependence. We now show that the content of positively charged residues in a translocated loop also correlates with the degree of dependence on SecA function, suggesting that arginines and lysines may be inherently difficult to move across the membrane during sec-dependent translocation.