High-affinity binding of Escherichia coli SecB to the signal sequence region of a presecretory protein - PubMed (original) (raw)

Comparative Study

High-affinity binding of Escherichia coli SecB to the signal sequence region of a presecretory protein

M Watanabe et al. Proc Natl Acad Sci U S A. 1995.

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.

PubMed Disclaimer

Similar articles

Cited by

References

    1. Gene. 1989 Jan 30;75(1):167-75 - PubMed
    1. Proc Natl Acad Sci U S A. 1989 Apr;86(8):2728-32 - PubMed
    1. Nature. 1993 Nov 25;366(6453):351-4 - PubMed
    1. Cell. 1989 Aug 25;58(4):695-705 - PubMed
    1. J Biol Chem. 1989 Oct 15;264(29):17293-7 - PubMed

Publication types

MeSH terms

Substances

LinkOut - more resources