Characterization of a region in mature LamB protein that interacts with a component of the export machinery of Escherichia coli

It has been shown that the synthesis of an export-defective protein can interfere with the normal export process in Escherichia coli by limiting the availability of SecB protein, a component of the export apparatus (Collier, D.N., Bankaitis, V.A., Weiss, J.B., and Bassford, P.J. (1988) Cell 53, 273-283). Consistent with this observation, we find that the interference elicited by an export-defective LamB protein is a titratable response resulting from the limitation of a single ligand. We have mapped the interfering region in LamB to between amino acids 320 and 380 of the mature protein. Expression of this sequence in the form of a LacZ-LamB-LacZ fusion protein elicits the export interference phenotype. Deletion of the sequence from an export-defective LamB protein eliminates the ability of this protein to interfere with the export of other secreted proteins. Together, these findings show that this sequence is both necessary and sufficient to cause export interference. Surprisingly, deletion of this sequence from an otherwise wild-type LamB protein does not cause the mutant LamB product to exhibit any obvious export defect. Based on our results, we propose that SecB interacts with both amino acids 320-380 of mature LamB and the LamB signal sequence during initiation of the export process.