The proton motive force, acting on acidic residues, promotes translocation of amino-terminal domains of membrane proteins when the hydrophobicity of the translocation signal is low.

We have shown previously that the first transmembrane segment of leader peptidase can function to translocate the polar amino-terminal Pf3 domain across the membrane into the periplasm independently of the proton motive force (pmf) (Lee, J. I., Kuhn, A., and Dalbey, R. E. (1992) J. Biol. Chem. 267, 938-943). We now show that when the first transmembrane segment lacks a strong hydrophobic character, the pmf is required for translocation. In addition, we find that the amino-terminal acidic residue proximal to the transmembrane domain plays a critical role in pmf-dependent amino-terminal translocation. Moreover, the pmf is required to hold the amino-terminal domain in the periplasm to prevent it from slipping such that the amino terminus is no longer exposed to the periplasm. In all cases, translocation occurs under conditions in which the function of the Sec machinery is impaired. These studies show that the low hydrophobicity of the first apolar domain (the translocation signal) can be compensated for by a negative charge in the amino-terminal region, upon which the pmf acts.