ΔμH+ dependency of in vitro protein translocation into Escherichia coli inner-membrane vesicles varies with the signal-sequence core-region composition

Signal sequences frequently contain α-helix-destabilizing amino acids in the hydrophobic core. Nuclear magnetic resonance studies on the conformation of signal sequences in membrane mimetic environments revealed that these residues cause a break in the α-helix. In the precursor of the Escherichia coli outer membrane protein PhoE (pre-PhoE), a glycine residue at position -10 (Gly^?10) is thought to be responsible for the break in the α-helix. We investigated the role of this glycine residue in the translocation process by employing site-directed mutagenesis. SDS-PAGE analysis showed drastic variations in the electrophoretic mobilities of the mutant precursor proteins, suggesting an important role of the glycine residue in determining the conformation of the signal sequence. In vivo, no drastic differences in the translocation kinetics were observed as compared with wild-type PhoE, except when a charged residue (Arg) was substituted for Gly^?10. However, the in vitro translocation of all mutant proteins into inverted inner-membrane vesicles was affected. Two classes of precursors could be distinguished. Translocation of one class of mutant proteins (Ala, Cys and Leu for Gly^?10) was almost independent of the presence of a ΔμH⁺, whereas translocation of the other class of precursors (wild type or Ser) was strongly decreased in the absence of the ΔμH⁺. Apparently, the ΔμH⁺ dependency of in vitro protein translocation varies with the signal-sequence core-region composition. Furthermore, a proline residue at position -10 resulted in a signal sequence that did not prevent the folding of the precursor in an in vitro trimerization assay.