Biochemical studies of bacterial protein export

Overproduction, purification and characterization of SecD and SecF, integral membrane components of the protein translocation machinery of Escherichia coli

SecD and SecF proteins were overproduced by means of recombinant DNA technology. Immunoblot and amino-acid sequencing analysis revealed that the overproduced proteins are SecD and SecF. The SecD- or SecF-overproduced membrane fraction was subjected to differential solubilization. The SecD protein was then purified through ion-exchange and size-exclusion chromatographies. The SecF protein was purified through size exclusion chromatography. Proteoliposomes reconstituted from the purified SecD and SecF together with SecE and SecY were used to analyze the translocation activity. SecD and SecF did not exhibit significant effects on the translocation activity of proteoliposomes. The amounts of SecD and SecF in overproducers were determined densitometrically on a stained SDS gel and their overproduction (fold) was determined by means of immunoblot analysis. Then the number of these molecules in one normal cell were estimated. From these numbers, together with those of other Sec proteins, the number of the translocation machinery existing in one Escherichia coli cell was inferred to be around 500.

Temperature-dependent insertion of prolipoprotein into Escherichia coli membrane vesicles and requirements for ATP, soluble factors, and functional SecY protein for the overall translocation process

The requirements for the translocation of prolipoprotein into membrane vesicles were examined in an in vitro system. As measured by the eventual modification and processing of the prolipoprotein to form mature lipoprotein, the overall translocation process was found to require ATP hydrolysis, the presence of some heat-labile soluble cytoplasmic translocation factors, and the function of a cytoplasmic membrane protein, SecY/PrlA. However, the initial step of complete insertion of prolipoprotein into the membrane vesicles occurred without apparent requirements of a nucleotide, cytoplasmic translocation factors, or a functional SecY/PrlA membrane protein. Immunopurified prolipoprotein spontaneously inserted into membrane vesicles at elevated temperatures and required ATP and cytoplasmic translocation factors to form mature lipoprotein. The prolipoprotein inserted most efficiently into liposomes made of negatively charged phospholipids, indicating the importance of phospholipids in protein translocation. These results suggest that ATP hydrolysis and the actions of both cytoplasmic translocation factors and a functional SecY/PrlA membrane protein occur at a step(s) after the insertion of the precursors into membrane vesicles. The initial step of spontaneous insertion of prolipoprotein into membranes is in good agreement with membrane trigger hypothesis proposed by W. Wickner (Annu. Rev. Biochem. 48:23-45, 1979) and the helical hairpin hypothesis proposed by D. M. Engleman and T. A. Steitz (Cell 23:411-422, 1981).

SecA suppresses the temperature-sensitive SecY24 defect in protein translocation in Escherichia coli membrane vesicles

Genetic analysis of protein secretion in Escherichia coli has identified secY/prlA and secA as components of the secretory apparatus. We have examined the roles of the secY(prlA) gene product (an integral membrane protein) and the soluble secA gene product in translocation of OmpA and alkaline phosphatase precursors in an in vitro system. The protein translocation defect of the secY24 mutation was recently demonstrated in vitro as was its suppression by an S300 extract. We show here that the extract was essentially inactive in SecY24 suppression when SecA protein was removed from it by immunoaffinity chromatography. Furthermore, purified SecA protein suppressed the SecY24 defect. Preincubation of the inactivated SecY24 membrane vesicles either with S300 containing SecA or with purified SecA protein reconstituted the membranes and restored the translocation activity when assayed in the absence of additional soluble proteins. These results suggest that the SecY24 translocation defect is suppressed by SecA interacting, directly or indirectly, with SecY24 on the cytoplasmic membrane.

Secretion of proteins by Bacillus subtilis 168 grown in the presence of membrane active agents (alcohols)

Sodium dodecyl sulphate-polyacrylamide gel electrophoresis was used to study the composition of proteins secreted by the Gram-positive microorganism Bacillus subtilis strain 168 when the latter was grown in the presence of primary alcohols (methanol, ethanol, 1-propanol and 1-butanol). These membrane-active agents had different effects on the pattern of proteins exported by B. subtilis. The secretion of some proteins was inhibited by the alkanols while that of others was stimulated, depending on the type of drug used. All alcohols were found to decrease the activity of extracellular enzymes such as alpha-amylase and serine protease without affecting significantly the activity of these enzymes when tested in vitro. The observed effects might be due to the ability of these agents to perturb the structure of biological membranes, thus interfering with the process of protein translocation through the lipid bilayers.

Instability of protease production in a rel+/rel- -pair of Bacillus licheniformis and associated morphological and physiological characteristics

A naturally occurring relaxed/protease-producing (A-type) versus stringent/not protease-producing (B-type) pair of an industrial Bacillus licheniformis has been characterized; either of the two types can convert into the other. Both types can sporulate, grow anaerobically, grow at 56 degrees C and reduce nitrate; morphologically, they can easily be distinguished by cell- and colony-shape. They differ in the ability to use 12 substrates, as determined in API-tests. The two types are remarkably different in their content of extrachromosomal elements (A-type: 2; B-type: 4); furthermore, they differ in their rel-status (A-type: relaxed; B-type: stringent). We propose that the differences in the ability of the two types to use different substrates probably are due to integration/extrusion of the extrachromosomal elements in and out of the chromosome, distorting or restoring a number of genes, together with induction of certain catabolic genes that are under control of the rel-system.

Requirement of heat-labile cytoplasmic protein factors for posttranslational translocation of OmpA protein precursors into Escherichia coli membrane vesicles

The involvement of possible cytoplasmic factors in ATP-dependent postttranslational translocation of proteins into Escherichia coli membrane vesicles was examined. The precursor of OmpA protein was partially purified by DEAE-cellulose chromatography, and its translocation was found to require material from the soluble cytoplasmic fraction. The fractionated active cytoplasmic translocation factor (CTF) was protease sensitive, micrococcal nuclease insensitive, N-ethylmaleimide resistant, and heat labile. The heat sensitivity of the CTF allowed its specific and preferential inactivation in the crude-precursor synthesis mixture, which provided a simple and rapid assay procedure for the factor during purification. Two active fractions were detected upon further fractionation: the major one was about 8S in sucrose gradient centrifugation and 120 kilodaltons by Sephadex filtration, whereas the other was about 4S and 60 kilodaltons in sucrose gradient centrifugation and by Sephadex filtration, respectively. The active fractions could also be fractionated by DEAE-Sepharose chromatography. These CTFs are apparently different from the previously reported 12S export factor (M. Muller and G. Blobel, Proc. Natl. Acad. Sci. USA 81:7737-7741, 1984).

Biochemical evidence for the secY24 defect in Escherichia coli protein translocation and its suppression by soluble cytoplasmic factors

The secY (prlA) gene product is an integral membrane protein that has been identified genetically as one of the central components of the Escherichia coli protein translocation machinery. We have examined the effect of the secY24 (temperature-sensitive) mutation on the protein translocation activity of E. coli inverted membrane vesicles. Vesicles isolated from cells carrying this allele and grown at the nonpermissive temperature (42 degrees C) were less than 1% as active in translocation as vesicles isolated from an isogenic secY+ strain under the same conditions. Vesicles from the mutant strain grown at the permissive temperature (32 degrees C) were partially active, but those vesicles preincubated at 40 degrees C lost 90% of their activity. Moreover, the secY24 translocation defect on in vivo- or in vitro-inactivated vesicles was suppressed, or compensated, by an S300 soluble fraction from wild-type cells or from secY24 cells grown at nonpermissive temperature. The suppressing factor(s) was heat-labile and sensitive to proteinase K. These results provide biochemical evidence for the essential role of SecY in the translocation process and indicate that the translocation defect of SecY24 membranes can be compensated for by supplementing with additional soluble cytoplasmic proteins.