Two distinct regions of the LamB signal sequence function in different steps in export.

The hydrophobic core of the Escherichia coli LamB signal sequence contains two structurally distinct regions. One region forms a helix in nonpolar environments, and the other is less structured. These regions seem to be of special importance for export, as judged by the magnitude of the defect caused by their mutational inactivation. To gain insight into the mechanistic importance of these two regions, we examined the ability of precursors to pass partially through the export pathway when each region is mutated. The results demonstrate that mutations in the helical and unstructured regions of the signal sequence block different steps in the export pathway.