The Rapid Degradation of Mutant SecA Protein in the Bacillus subtilis secA341 (ts) Mutant Causes a Protein Translocation Defect in the Cell

Abstract

To study the function of SecA protein and the protein translocation system of Bacillus subtilis, wild-type and mutant SecA proteins were characterized in vivo and in vitro. SecA protein was abundant in a wild-type strain (168) and existed in a stable homodimer. In contrast to this, SecA341 (ts) protein having an amino acid replacement from proline to leucine at residue 431 was undetectable by immunoblotting in the cell lysate of a secA341 mutant (TB301) at the nonpermissive temperature, 42°C. Pulse-chase studies using ³⁵S-methionine showed that newly synthesized SecA protein was rapidly degraded in the mutant at 42°C. Purified SecA341 protein was more sensitive to trypsin and subtilisin than purified wild-type SecA protein in the presence of ATP. These results indicate that the secA341 mutation causes the rapid degradation of mutant SecA protein and a concomitant protein translocation defect in the cell.