Preprotein Translocation by a Hybrid Translocase Composed of Escherichia coli and Bacillus subtilis Subunits

Bacterial protein translocation is mediated by translocase, a multisubunit membrane protein complex that consists of a peripheral ATPase SecA and a preprotein-conducting channel with SecY, SecE, and SecG as subunits. Like Escherichia coli SecG, the Bacillus subtilis homologue, YvaL, dramatically stimulated the ATP-dependent translocation of precursor PhoB (prePhoB) by the B. subtilis SecA-SecYE complex. To systematically determine the functional exchangeability of translocase subunits, all of the relevant combinations of the E. coli and B. subtilis secY, secE, and secG genes were expressed in E. coli. Hybrid SecYEG complexes were overexpressed at high levels. Since SecY could not be overproduced without SecE, these data indicate a stable interaction between the heterologous SecY and SecE subunits. E. coli SecA, but not B. subtilis SecA, supported efficient ATP-dependent translocation of the E. coli precursor OmpA (proOmpA) into inner membrane vesicles containing the hybrid SecYEG complexes, if E. coli SecY and either E. coli SecE or E. coli SecG were present. Translocation of B. subtilis prePhoB, on the other hand, showed a strict dependence on the translocase subunit composition and occurred efficiently only with the homologous translocase. In contrast to E. coli SecA, B. subtilis SecA binds the SecYEG complexes only with low affinity. These results suggest that each translocase subunit contributes in an exclusive manner to the specificity and functionality of the complex