Purification of the Escherichia coli secB gene product and demonstration of its activity in an in vitro protein translocation system

Isolation of a secY homologue from Bacillus subtilis: evidence for a common protein export pathway in eubacteria

Genetic and biochemical studies have shown that the product of the Escherichia coli secY gene is an integral membrane protein with a central role in protein secretion. We found the Bacillus subtilis secY homologue within the spc-alpha ribosomal protein operon at the same position occupied by E. coli secY. B. subtilis secY coded for a hypothetical product 41% identical to E. coli SecY, a protein thought to contain 10 membrane-spanning segments and 11 hydrophilic regions, six of which are exposed to the cytoplasm and five to the periplasm. We predicted similar segments in B. subtilis SecY, and the primary sequences of the second and third cytoplasmic regions and the first, second, fourth, fifth, seventh, and tenth membrane segments were particularly conserved, sharing greater than 50% identity with E. coli SecY. We propose that the conserved cytoplasmic regions interact with similar cytoplasmic secretion factors in both organisms and that the conserved membrane-spanning segments actively participate in protein export. Our results suggest that despite the evolutionary differences reflected in cell wall architecture, Gram-negative and Gram-positive bacteria possess a similar protein export apparatus.

The ATPase activity of SecA is regulated by acidic phospholipids, SecY, and the leader and mature domains of precursor proteins

The ATPase activity of SecA is stimulated by E. coli plasma membrane vesicles bearing SecY protein and a precursor protein such as proOmpA. This activity is termed "translocation ATPase". Liposomes alone can also stimulate SecA ATPase, but membrane proteins block this stimulation in native inner membranes. We define the stimulation of SecA ATPase by lipid as "SecA/lipid ATPase". SecA/lipid ATPase, translocation ATPase, and translocation into inner membrane vesicles require acidic phospholipids, suggesting an underlying unity of mechanism. ProOmpA and ATP stabilize liposome-bound SecA. Full SecA/lipid ATPase activity and stability are also seen when a mixture of a leader peptide and either OmpA or maltose binding protein (MBP) are added instead of proOmpA, while neither the leader peptide alone nor OmpA or MBP suffice. Cytosolic proteins in conjuction with a leader peptide are less active in this reaction, indicating that liposome-bound SecA protein recognizes both leader and mature domains.

Effects of inhibitors of membrane signal peptide peptidase on protein translocation into membrane vesicles

The effect of the removal of signal peptides after cleavage of precursor molecules by the signal peptidase I was examined in an in vitro translocation system with Escherichia coli membrane vesicles. The translocation of periplasmic alkaline phosphatase precursors was significantly inhibited by the protease inhibitors antipain, elastatinal and leupeptin. Antipain and leupeptin enhanced the translocation of precursors of outer membrane protein OmpA, but inhibited the processing. However, antipain did not inhibit the processing of precursors mediated by signal peptidase I in the soluble form. Moreover, the inhibition by antipain was not due to the disruption of membrane integrity, but occurred during the process of protein translocation. Since these small peptide inhibitors are known to inhibit membrane protease IV, a signal peptide peptidase, these results suggest that the hydrolysis of signal peptides is an important step in the recycles of the overall translocation process, and that the prevention of degradation of signal peptides feedback inhibits the preceding steps in the translocation pathway.

Mutations that improve export of maltose-binding protein in SecB- cells of Escherichia coli

It previously has been proposed that the Escherichia coli SecB protein promotes the export of the maltose-binding protein (MBP) from the cytoplasm by preventing the folding of the precursor MBP (preMBP) into a translocation-incompetent conformation. The export of wild-type MBP is only partially blocked in SecB- cells. In contrast, the export of MBP16-1, an MBP species with a defective signal peptide, is totally dependent on SecB; hence, SecB- cells that synthesize MBP16-1 are unable to utilize maltose as a sole carbon source. The selection of Mal+ revertants primarily yielded mutants with alterations in the MBP16-1 signal peptide that permitted SecB-independent MBP export to the periplasm to various extents. Although each of these alterations increased the overall hydrophobicity of the signal peptide, it was not possible to strictly equate changes in hydrophobicity with the degree of SecB-independent export. Somewhat unexpectedly, two mutants were obtained in which MBP export in SecB- cells was markedly superior to that of the wild-type MBP. Although wild-type MBP is not cotranslationally translocated in SecB- cells, the two mutant proteins designated MBP172 and MBP173 exhibited significant cotranslational export in the absence of SecB. Thus, the role of SecB was partially supplanted by a signal peptide that promoted more rapid movement of MBP through the export pathway. When preMBP included the MBP172 signal peptide as well as an alteration in the mature moiety that slows folding, the SecB requirement for maximal MBP export efficiency was almost totally eliminated. These results provide additional strong support for the proposed antifolding role of SecB in MBP export.

SecB functions as a cytosolic signal recognition factor for protein export in E. coli

A purified 64 kd protein, consisting of four identical subunits of the 16 kd SecB, binds to the signal sequence of preproteins prior to their translocation across inverted vesicles (INV) derived from the E. coli plasma membrane. The purified SecB tetramer competes with canine signal recognition particle (SRP) in signal sequence binding and thus behaves as a prokaryotic equivalent of SRP. As shown by cell fractionation and immunoblot analysis with anti-SecB antibodies, SecB is a cytosolic protein. An E. coli supernatant depleted of SecB after passage through an anti-SecB Sepharose column retains full translation activity but is unable to support translocation into added INV. Translocation into INV is fully restored by readdition of purified SecB.

The secE gene encodes an integral membrane protein required for protein export in Escherichia coli

Genetic screening and selection procedures employing a secA-lacZ fusion strain repeatedly have yielded mutations in four genes affecting the protein export pathway of Escherichia coli. These genes are secA, secD, prlA/secY, and secE. We discuss the significance of the failure to find new sec genes after extensive use of this approach. One of the genes, secE, has been characterized in some detail. From the DNA sequence of the gene and analysis of alkaline phosphatase fusions to the SecE protein, we propose that it is a 13,600-dalton integral cytoplasmic membrane protein. The data presented here and in the accompanying paper strongly suggest that secE has an important role in E. coli protein export.