Mutations of the molecular chaperone protein SecB which alter the interaction between SecB and maltose-binding protein

SecB is a 16-kDa cytosolic chaperone protein that is required for efficient export of particular proteins in Escherichia coli. To identify regions of SecB that contribute to efficient protein export, we isolated secB point mutants that are defective for protein export in vivo. We obtained missense mutations at residues Leu75 (SecBL75Q), Cys76 (SecBC76Y), and Glu77 (SecBE77K) in the center of the secB gene. In vivo, mutant SecBL75Q and SecBE77K proteins are capable of binding to precursor maltose-binding protein (MBP) and preventing the formation of export-incompetent precursor MBP; however, export of MBP is still defective. In vitro, purified SecBL75Q and SecBE77K proteins bound to unfolded MBP and blocked its refolding. SecBL75Q and SecBE77K were more effective than wild-type SecB at blocking the refolding of unfolded MBP, suggesting that SecBL75Q and SecBE77K have a higher affinity for unfolded MBP.