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Topological analysis of the transhydrogenase in Escherichia coli membranes using proteolytic probes Tong, Raymond Cheuk Wa


Using proteolytic probes, the pyridine nucleotide transhydrogenase (EC from Escherichia coli was analyzed for its native topography in the cytoplasmic membrane. Before analyses could be performed, the isolation of transhydrogenase-enriched ISO (inside-out) cytoplasmic membrane vesicles was accomplished by modification of the procedure followed by Clarke (Clarke, D. M. and Bragg, P. D. (1985) Eur. J. Biochem. 149, 517-523) in purifying the enzyme from overexpressing E.coli JM83pDC21 cells. Two major changes were made. One was that the solubilization of the bacterial membrane and subsequent purification steps were omitted. The other was the separation of outer membranes from the cytoplasmic membrane preparation by sucrose gradient density centrifugation. This was essential owing to the contaminating presence of a 30 kD protein in the outer membrane of the original preparation. Transhydrogenase-enriched RSO (right-side-out) membrane vesicles were isolated by a different procedure using lysozyme-mediated breakage of E.coli spheroplasts and subsequent vesicular reformation. To identify possible transhydrogenase fragments arising from proteolytic cleavage, anti-E.coli transhydrogenase polyclonal antibodies were generated in rabbits. Two sets of polyclonal antibodies were produced. One set cross-reacted with both the α (52 kD) and β (48 kD) subunits of the transhydrogenase. The other reacted with the α subunit only. Trypsin and proteinase K were the main proteolytic probes used against both ISO and RSO cytoplasmic membrane vesicles, although chymotrypsin was also used in preliminary experiments with ISO membrane vesicles. Identification of fragments resulting from proteolytic cleavage of the enzyme was obtained using anti-transhydrogenase antibodies and by N-terminal sequencing and/or C-terminal sequencing. In some of these experiments, isolation of the proteolytic fragments was necessary prior to analysis. This was done using a number of different methods. The particular methods applied, which included column chromatography strategies and elution procedures from SDS-Polyacrylamide gels, depended on the type of analysis carried out. The analyses indicated that the α subunit has at least a 41 kD sequence extending from its N-terminus which is exposed to the cytoplasmic side of the membrane. This sequence may contain an active site of the enzyme. This is suggested by the binding of this fragment to a NAD-affinity column. The membrane-imbedded region of the α subunit anchoring the 41 kD region predicted by hydropathy plotting (Clarke, D. M., Loo, Tip W., Gilliam, S. and Bragg, P. D. (1986), Eur. J. Biochem. 158, 647-653) could not be detected by our methods. Susceptible tryptic cleavage sites along the 41 kD region were identified by partial proteolysis and may reflect areas in the subunit's tertiary or quaternary structure that are exposed to the surrounding medium. Major cleavage sites were at arg₁₅, Iys₂₂₇, Iys₂₆₄, arg₂₆₈, Iys₂₇₅, arg₃₅₅, and arg₃₆₁. There do not appear to be significant portions of the subunit protruding into the periplasm as neither trypsin nor proteinase K had any effect on the subunit in RSO-oriented membrane vesicles. Proteinase K experiments with ISO and RSO membrane vesicles suggest that a 20 kD portion of the β subunit is protected from cleavage and is imbedded in the membrane. The identity of this fragment could not be confirmed. Hydropathy analysis of the transhydrogenase gene-derived amino acid sequence (Clarke, D. M., Loo, Tip W., Gilliam, S. and Bragg, P. D. (1986), Eur. J. Biochem. 158, 647-653) suggests that this could be a sequence extending from the N-terminus of the β subunit. This is a hydrophobic sequence containing 7 possible transmembranous helices and having a theoretical molecular weight in the range of 20 kD. The proteinase K results also indicate that the rest of the β subunit is exposed to the cytoplasmic side of themembrane rather than the periplasmic side. The results obtained here are consistent with hydropathy predictions made with regard to this subunit. In addition, two different experiments indicate that an α-α subunit interaction may be present in the oligomeric structure of the membrane-bound enzyme (Hou, C, Potier, M. and Bragg, P. D. (1990), Biochim. Biophys. Acta 1018, 61-66). Substrates of the enzyme did not appear to affect the transhydrogenase's general conformation upon binding as detected by experiments using partial tryptic proteolysis. Partial trypsinolysis also revealed that selective detergent extraction of transhydrogenase-enriched ISO vesicles with Triton X-100 and sodium cholate did not affect the overall conformation of the membrane-bound enzyme despite greatly reducing the enzymatic activity.

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