A truncated Bacillus subtilis SecA protein consisting of the N-terminal 234 amino acid residues forms a complex with Escherichia coli SecA51(ts) protein and complements the protein translocation defect of the secA51 mutant.

Although wild-type Bacillus subtilis SecA barely complements the growth and protein translocation defect of Escherichia coli secA51(ts) at the non-permissive temperature, an N-terminal peptide of B. subtilis SecA complements the defects. To elucidate the mechanism of this complementation, a series of plasmids encoding truncated SecA proteins was constructed and their products were analyzed in E. coli cells. The truncated B. subtilis SecA protein consisting of the N-terminal 234 amino acid residues (BN234) complemented the growth and protein translocation defects of E. coli secA51 but not those of another secA amber mutant, E. coli secA13(ts). BN234 existed in both a soluble form, possibly as a homodimer, and a higher-molecular-weight complex in E. coli strain MM52 (secA51). The purified complex, consisting of at least BN234, SecA51, and ATP-dependent protease La, was held together by a cross-linking reagent, EDAC. The other truncated proteins consisting of the N-terminal 584 or 396 amino acid residues and the C-terminal 607 residues of B. subtilis SecA did not complement the two E. coli mutants or form a complex with SecA51. These results suggest that BN234 and SecA51 proteins form a functional complex in vivo and complement the defects of E. coli MM52.