Residues essential for the function of SecE, a membrane component of the Escherichia coli secretion apparatus, are located in a conserved cytoplasmic region.

Protein export in Escherichia coli is absolutely dependent on two integral membrane proteins, SecY and SecE. Previous deletion mutagenesis of the secE gene showed that only the third of three membrane-spanning segments and a portion of the second cytoplasmic region are necessary for its function in protein export. Here we further define the residues important for SecE function. Alignment of the SecE homologues of various eubacteria reveals that they all contain one membrane-spanning segment, compared with three in E. coli SecE, and that the most conserved region among them lies in their putative cytoplasmic amino termini; little homology exists in their membrane-spanning segments. The SecE homologue of the extreme thermophilic bacterium Thermotoga maritima was cloned and found to complement a deletion of secE in E. coli. Deletion or replacement of the cytoplasmic region of E. coli SecE eliminated SecE function, indicating that this sequence is essential for a functional secretion machinery. Mutant analysis suggests that the most important function of the third membrane-spanning segment is to maintain the proper topological arrangement of the conserved cytoplasmic domain.