Expression of rat liver S-adenosylhomocysteinase cDNA in Escherichia coli and mutagenesis at the putative NAD binding site.

The cDNA for rat liver S-adenosylhomocysteinase has been cloned, and the nucleic acid sequence has been determined. By comparison of the deduced amino acid sequence for S-adenosylhomocysteinase with that of the dinucleotide binding region for other proteins, the sequence from amino acids 213 to 244 in rat liver S-adenosylhomocysteinase was proposed to be part of the NAD binding site (Ogawa, H., Gomi, T., Mueckler, M. M., Fujioka, M., Backlund, P. S., Jr., Aksamit, R. R., Unson, C. G., and Cantoni, G. L. (1987) Proc. Natl. Acad. Sci. U. S. A. 84, 719-723). A vector has been constructed that expresses S-adenosylhomocysteinase in Escherichia coli in the presence of isopropyl beta-D-thiogalactopyranoside by inserting the coding sequence of rat liver S-adenosylhomocysteinase cDNA downstream from the lac promoter of plasmid pUC118. The enzyme that is produced comprises as much as 10% of the soluble cellular proteins. The purified enzyme is a tetramer, contains 4 mol of tightly bound NAD, and has kinetic properties indistinguishable from those of the liver enzyme. Tryptic peptide mapping and NH2-terminal sequence analysis indicate that the recombinant enzyme is structurally identical to the liver enzyme except for the absence of the NH2-terminal blocking group. The rat liver enzyme has a blocked NH2-terminal alanine residue (Ogawa, H., Gomi, T., Mueckler, M. M., Fujioka, M., Backlund, P. S., Jr., Aksamit, R. R., Unson, C. G., and Cantoni, G. L. (1987) Proc. Natl. Acad. Sci. U. S. A. 84, 719-723). By oligonucleotide-directed mutagenesis mutant vectors have been generated that express proteins in which each of the glycines in the Gly-Xaa-Gly-Xaa-Xaa-Gly sequence of the putative NAD binding site (residues 219-224) was changed to valine. Immunoblot analysis of extracts of the cells transformed with these vectors reveals the presence of immunoreactive proteins with the subunit molecular weight of S-adenosylhomocysteinase. The mutant proteins have no catalytic activity, contain no bound NAD, and do not form the same quaternary structure as the recombinant S-adenosylhomocysteinase.