AMINO-TERMINAL REGION OF SECA IS INVOLVED IN THE FUNCTION OF SECG FORPROTEIN TRANSLOCATION INTO ESCHERICHIA-COLI MEMBRANE-VESICLES

Protein translocation across the cytoplasmic membrane of Escherichia c oli is accomplished by concerted actions of the translocation ATPase S ecA and the membrane-embedded SecE/Y/G; complex. SecA interacts with p reproteins and undergoes ATP-driven cycles of membrane insertion-deins ertion. To address how SecA interacts functionally with other componen ts in the translocation machinery, we characterized a SecA mutant lack ing amino-terminal 8 amino acid residues (SecA N-8). Although the abse nce of the 8 residues did not grossly affect the interaction of SecA w ith 8 preprotein, ATP, or phospholipids, nor did it affect the intrins ic ATPase activity, it gave differential effects on the translocation of different preproteins. It also affected the translocation ATPase ac tivity, the ability of membrane insertion, and the topology inversion of SecG coupled with the membrane insertion-deinsertion of SecA, Most noteworthy, SecA N-8 was pronouncedly defective in the translocation o f proton motive force-dependent preproteins, in which SecG might have a role. We propose that the amino-terminal region of SecA is important for the functional interaction with SecG.