Escherichia coli SecY and SecE proteins appear insufficient to constitute the SecA receptor

Chaperone-like activities of the CsaA protein of Bacillus subtilis

The growth and protein export defects of Escherichia coli secA51(Ts) strains can be suppressed by the CsaA protein of Bacillus subtilis. The present studies indicate that this effect can be attributed to chaperone-like activities of CsaA. First, CsaA stimulated protein export in secB, groES and dnaJ mutant strains of E. coli. Second, CsaA suppressed the growth defects of dnaK, dnaJ and grpE mutants of E. coli. Third, and most importantly, CsaA exhibited chaperone-like properties by stimulating the reactivation of heat-denatured firefly luciferase in groEL, groES, dnaK and grpE mutant strains of E. coli, and by preventing the aggregation of heat-denatured luciferase in vitro. Thus, it seems that CsaA suppresses the growth and secretion defects of E. coli secA(Ts) strains either by improving the translocation competence of exported pre-proteins, thereby making them better substrates for mutant SecA proteins, or by stimulating the translocation activity of mutant SecA proteins.

A component of the chloroplast protein import apparatus functions in bacteria

Toc36 is a family of 44-kDa envelope polypeptides previously identified as components of the chloroplast protein import apparatus by virtue of their close physical proximity to translocating proteins. An indication of their function thus remains at large. A heterologous in vivo approach for studying the function of Toc36 was developed in this study by introducing a member of Toc36 into E. coli to assess its effect on bacterial protein translocation. The presence of Toc36 enhances the translocation of two bacterial periplasmic proteins in a manner resembling the chloroplast system. Translocation of the two bacterial periplasmic proteins was less sensitive to sodium azide, resembling more the azide-insensitive nature of the chloroplast protein import process. Mutated Toc36 proteins were not capable of causing the same effect as that observed for unaltered Toc36. Toc36 was also capable of complementing bacterial strains with temperature-sensitive secA mutations that affected protein translocation. The combined results provide evidence that Toc36 plays a central role in the chloroplast protein translocation process.

Topology of the integral membrane form of Escherichia coli SecA protein reveals multiple periplasmically exposed regions and modulation by ATP binding

SecA insertion and integration into the Escherichia coli inner membrane is a critical step for the catalysis of protein translocation across this layer. To understand this step further, SecA topology was investigated. To determine which regions of SecA are periplasmically exposed, right-side out membrane vesicles were prepared from strains synthesizing monocysteine SecA variants produced by mutagenesis and probed with a membrane-impermeant sulfhydryl-labeling reagent. To determine which regions of SecA contain membrane-integration determinants, inverted inner membrane vesicles were subjected to proteolysis, and integral-membrane fragments of SecA were identified with region-specific antibodies. The membrane association properties of various truncated SecA species produced in vivo were also determined. Our analysis indicates that the membrane topology of SecA is complex with amino-terminal, central, and carboxyl-terminal regions of SecA integrated into the membrane where portions are periplasmically accessible. Furthermore, the insertion and penetration of the amino-terminal third of SecA, which includes the proposed preprotein-binding domain, is subject to modulation by ATP binding. The importance of these studies to the cycle of membrane insertion and de-insertion of SecA that promotes protein translocation and SecA's proximity to the preprotein channel are discussed.

In vivo cross-linking of the SecA and SecY subunits of the Escherichia coli preprotein translocase

Precursor protein translocation across the Escherichia coli inner membrane is mediated by the translocase, which is composed of a heterotrimeric integral membrane protein complex with SecY, SecE, and SecG as subunits and peripherally bound SecA. Cross-linking experiments were conducted to study which proteins are associated with SecA in vivo. Formaldehyde treatment of intact cells results in the specific cross-linking of SecA to SecY. Concurrently with the increased membrane association of SecA, an elevated amount of cross-linked product was obtained in cells harboring overproduced SecYEG complex. Cross-linked SecA copurified with hexahistidine-tagged SecY and not with SecE. The data indicate that SecA and SecY coexist as a stable complex in the cytoplasmic membrane in vivo.

SecYEG and SecA are the stoichiometric components of preprotein translocase

The transport of large preproteins across the Escherichia coli plasma membrane is catalyzed by preprotein translocase, comprised of the peripherally bound SecA subunit and an integrally bound heterotrimeric domain consisting of the SecY, SecE, and SecG subunits. We have now placed the secY, secE, and secG genes under the control of an arabinose-inducible promoter on a multicopy plasmid. Upon induction, all three of the proteins are strongly overexpressed and recovered in the plasma membrane fraction. These membranes show a strong enhancement of 1) translocation ATPase activity, 2) preprotein translocation, 3) capacity for SecA binding, and 4) formation of the membrane-inserted form of SecA. These data establish that SecY, SecE, and SecG constitute the integral membrane domain of preprotein translocase.

SecA protein is exposed to the periplasmic surface of the E. coli inner membrane in its active state

E. coli cells harboring pCG169 containing the secD secF locus possessed SecA protein almost entirely in an integral membrane form in which it displayed normal protein translocation activity. These results imply that integral membrane SecA is the catalytically active form of this enzyme and that products of the secD secF locus regulate SecA association with the inner membrane. Protease and biotinylation accessibility studies of right side-out and inside-out membrane vesicles derived from this strain revealed that SecA was exposed to the periplasmic surface of the inner membrane. These studies suggest a model of bacterial protein secretion, whereby insertion of SecA into the inner membrane and its association with SecY/E/G promotes assembly of active protein-conducting channels comprised in part of integral membrane SecA protein, and products of the secD secF locus regulate the channel assembly-disassembly reaction by modulating the SecA insertion-deinsertion step.