The molecular chaperone SecB is released from the carboxy-terminus of SecA during initiation of precursor protein translocation.
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.
Fekkes
P
P
Department of Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Kerklaan 30, 9751 NN Haren, The Netherlands.
van der Does
C
C
Driessen
A J
AJ
D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000378:metabolism*; D001665:Binding Sites; D001692:Biological Transport; D019281:Dimerization; D004926:Escherichia coli; D029968:Escherichia coli Proteins; D026901:Membrane Transport Proteins; D008958:Models, Molecular; D018832:Molecular Chaperones / Q000378:metabolism*; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000378:metabolism; D011485:Protein Binding; D011498:Protein Precursors / Q000378:metabolism*; D000069816:SEC Translocation Channels; D000081416:SecA Proteins; D017386:Sequence Homology, Amino Acid
D001426:Bacterial Proteins; D029968:Escherichia coli Proteins; D026901:Membrane Transport Proteins; D018832:Molecular Chaperones; D010446:Peptide Fragments; D011498:Protein Precursors; D000069816:SEC Translocation Channels; C057842:SecB protein, Bacteria; D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases; D000081416:SecA Proteins