Recognition of ligands by SecB, a molecular chaperone involved in bacterial protein export

SecB is a molecular chaperone involved in protein export from Escherichia coli. It is a highly negatively charged, soluble, tetrameric protein with a monomer molecular mass of 16,400 kDa. It has two functions: it maintains precursors of some exported proteins in a conformation compatible with export, by preventing them from aggregating or from folding into their thermodynamically stable state in the cytoplasm, and it delivers both nascent and completed precursors to SecA, one of the components of the export apparatus that are on and in the plasma membrane. SecB recognizes completed precursors of soluble proteins, not by direct interaction with leader sequences but by virtue of the property, imposed by their leader sequences, that they fold slowly: i.e. there is a kinetic partitioning between folding and interaction with SecB. Only those polypeptides that fold slowly interact significantly with this molecular chaperone even though it is able to bind a wide variety of non-native proteins. Binding studies with purified peptides indicate that each SecB monomer has a binding site that can interact with flexible peptides having a net positive charge and a length of about ten residues, which may depend on the charge density. Binding of the hydrophobic fluorescent probe 1-anilino-naphthalene-8-sulphonate (ANS) indicates that simultaneous interaction of multiple peptides causes a conformational change that exposes a hydrophobic site on SecB. This hydrophobic region is thought to contribute an extra binding site for physiological ligands of SecB. A model of SecB binding to nonnative precursors is presented.