REGIONS OF MALTOSE-BINDING PROTEIN THAT INFLUENCE SECB-DEPENDENT AND SECA-DEPENDENT EXPORT IN ESCHERICHIA-COLI

In Escherichia coli, the efficient export of maltose-binding protein ( MBP) is dependent on the chaperone SecB, whereas export of ribose-bind ing protein (RBP) is SecB independent. To localize the regions of MBP involved in interaction with SecB, hybrids between MBP and RBP in secB mutant cells were constructed and analyzed. One hybrid consisted of t he signal peptide and first third of the mature moiety of MBP, followe d by the C-terminal two-thirds of RBP (MBP-RBP112). This hybrid was de pendent upon SecB for its efficient export and exhibited a strong expo rt defect in secA mutant cells. A hybrid between RBP and MBP with the same fusion point was also constructed (RBP-MBP116). The RBP-MBP116 hy brid remained SecB independent and only exhibited a partial export def ect in secA mutant cells. In addition, MBP species with specific alter ations in the early mature region were less dependent on SecB for thei r efficient export. The export of these altered MBP species was also l ess affected in secA mutant cells and in cells treated with sodium azi de. These results present additional evidence for the targeting role o f SecB.