The molecular chaperone SecB is released from the carboxy-terminus of SecA during initiation of precursor protein translocation

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Abstract
The chaperone SecB keeps precursor proteins in a translocation‐competent state and targets them to SecA at the translocation sites in the cytoplasmic membrane of Escherichia coli. SecA is thought to recognize SecB via its carboxy‐terminus. To determine the minimal requirement for a SecB‐binding site, fusion proteins were created between glutathione‐S‐transferase and different parts of the carboxy‐terminus of SecA and analysed for SecB binding. A strikingly short amino acid sequence corresponding to only the most distal 22 aminoacyl residues of SecA suffices for the authentic binding of SecB or the SecB–precursor protein complex. SecAN880, a deletion mutant that lacks this highly conserved domain, still supports precursor protein translocation but is unable to bind SecB. Heterodimers of wild‐type SecA and SecAN880 are defective in SecB binding, demonstrating that both carboxy‐termini of the SecA dimer are needed to form a genuine SecB‐binding site. SecB is released from the translocase at a very early stage in protein translocation when the membrane‐bound SecA binds ATP to initiate translocation. It is concluded that the SecB‐binding site on SecA is confined to the extreme carboxy‐terminus of the SecA dimer, and that SecB is released from this site at the onset of translocation.