title: High-affinity binding of Escherichia coli SecB to the signal sequence region of a presecretory protein.
abstract: The Escherichia coli cytosolic homotetrameric protein SecB is known to be involved in protein export across the plasma membrane. A currently prevalent view holds that SecB functions exclusively as a chaperone interacting nonspecifically with unfolded proteins, not necessarily exported proteins, whereas a contrary view holds that SecB functions primarily as a specific signal-recognition factor--i.e., in binding to the signal sequence region of exported proteins. To experimentally resolve these differences we assayed for binding between chemically pure SecB and chemically pure precursor (p) form (containing a signal sequence) and mature (m) form (lacking a signal sequence) of a model secretory protein (maltose binding protein, MBP) that was C-terminally truncated. Because of the C-terminal truncation, neither p nor m was able to fold. We found that SecB bound with 100-fold higher affinity to p (Kd 0.8 nM) than it bound to m (Kd 80 nM). As the presence of the signal sequence in p is the only feature that distinguished p from m, these data strongly suggest that the high-affinity binding of SecB is to the signal sequence region and not the mature region of p. Consistent with this conclusion, we found that a wild-type signal peptide, but not an export-incompetent mutant signal peptide of another exported protein (LamB), competed for binding to p. Moreover, the high-affinity binding of SecB to p was resistant to 1 M salt, whereas the low-affinity binding of SecB to m was not. These qualitative differences suggested that SecB binding to m was primarily by electrostatic interactions, whereas SecB binding to p was primarily via hydrophobic interactions, presumably with the hydrophobic core of the signal sequence. Taken together our data strongly support the notion that SecB is primarily a specific signal-recognition factor.
mesh_terms: D018528:ATP-Binding Cassette Transporters; D001426:Bacterial Proteins / Q000302:isolation & purification / Q000378:metabolism*; D002352:Carrier Proteins / Q000096:biosynthesis / Q000302:isolation & purification / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000378:metabolism*; D029968:Escherichia coli Proteins; D007700:Kinetics; D008320:Maltose / Q000378:metabolism; D058619:Maltose-Binding Proteins; D018832:Molecular Chaperones / Q000378:metabolism; D009004:Monosaccharide Transport Proteins; D021382:Protein Sorting Signals / Q000302:isolation & purification / Q000378:metabolism*; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000302:isolation & purification / Q000378:metabolism
chemical_list: D018528:ATP-Binding Cassette Transporters; D001426:Bacterial Proteins; D002352:Carrier Proteins; D029968:Escherichia coli Proteins; D058619:Maltose-Binding Proteins; D018832:Molecular Chaperones; D009004:Monosaccharide Transport Proteins; D021382:Protein Sorting Signals; D011994:Recombinant Proteins; C057842:SecB protein, Bacteria; C461599:maltose transport system, E coli; D008320:Maltose