The relation between the soluble factor associated with H(+)-transhydrogenase of Rhodospirillum rubrum and the enzyme from mitochondria and Escherichia coli.

Although in mitochondria, Escherichia coli and Rhodobacter capsulatus the H(+)-transhydrogenases are intrinsic membrane proteins, in Rhodospirillum rubrum a water-soluble component (Ths) and a membrane-bound component are together required for activity. Ths was selectively removed from chromatophore membranes of Rhs. rubrum and was purified to homogeneity by precipitation with (NH4)2SO4 and ion-exchange, affinity dye and gel exclusion chromatography. The latter indicated an Mr of approx. 74,000 under non-denaturing conditions but analysis of the pure protein by SDS-PAGE revealed a single polypeptide, Mr 43,000. Antibodies against this polypeptide inhibited transhydrogenase activity of chromatophores and decreased the capacity of Ths to restore activity to depleted membranes. They reacted with a polypeptide of Mr 43,000 in crude cell extract, chromatophore membranes and chromatophore washings but not with transhydrogenase polypeptides from the membranes of E. coli, Rb. capsulatus or animal mitochondria. The N-terminal amino acid sequence of the 43,000 polypeptide was strongly homologous with the reported N-terminal regions of mitochondrial transhydrogenase and the alpha subunit of the E. coli protein. The break between the alpha and beta polypeptides of E. coli transhydrogenase is such that both components are membrane-associated. In contrast, these results suggest that in the Rhs. rubrum enzyme Ths has been formed by a break closer to the N-terminus, thus avoiding the putative trans-membrane helical segments and yielding a relatively hydrophilic subunit, which is water-soluble. There is a predicted similarity between Ths and the reported sequence of alanine dehydrogenase from Bacillus but Ths did not have any alanine dehydrogenase activity.