SecA-independent Translocation of the Periplasmic N-terminal Tail of an Escherichia coli Inner Membrane Protein

We have shown previously that the 100-residue-long periplasmic N-terminal tail of the <i>Escherichia coli</i> inner membrane protein ProW can be translocated across the inner membrane in a <i>sec</i>-independent manner and that its translocation is blocked by the introduction of three positively charged residues near its C-terminal end (Whitley, P., Zander, T., Ehrmann, M., Haardt, M., Bremer, E., and von Heijne, G.(1994) <i>EMBO J.</i> 13, 4653-4661). We have now further analyzed the requirements for translocation of the N-terminal tail and found that the introduction of even a single arginine can block translocation. Position-specific differences in the effects on translocation of arginine insertions suggest that the C-terminal end of the N-terminal tail is more critical for translocation than the central and N-terminal regions. We also show that the N-terminal tail is translocated in a truncation mutant where a stop codon is placed immediately after the first transmembrane segment, provided that the transmembrane segment is flanked on its C-terminal end by positively charged residues. Thus, <i>sec</i>-independent translocation of a relatively large domain can be induced by a translocation signal located at the extreme C terminus of a protein.