Effects of Escherichia coli secB mutations on pre-maltose binding protein conformation and export kinetics.

Mutations affecting the secB gene of Escherichia coli cause a defect in protein export. This report presents the demonstration that the secB mutations caused a defect in co-translational processing of maltose binding protein (MBP). A significant amount of post-translational processing of pre-MBP occurred within 1 min after termination of pulse labeling; at later time points only a small amount of additional processing occurred. The conformation of the intracellular precursor form of MBP was examined in a secB::Tn5 mutant, using protease sensitivity (Randall, L. L., and Hardy, S. J. S. (1986) Cell 46, 921-928) as the assay. In contrast to the isogenic wild type strain, a population of pre-MBP that had folded into a protease resistant conformation was detected in the secB mutant. In addition, sublethal doses of chloramphenicol did not significantly affect protein export in the secB::Tn5 mutant and the secB::Tn5 mutation did not lead to defects in membrane energization.