Determination of the sites required for the allosteric inhibition of serine acetyltransferase by L-cysteine in plants.

Serine acetyltransferase (SATase; EC 2.3.1.30) catalyzes the formation of O-acetylserine from L-Ser and acetyl-CoA in plants and bacteria. In plants, two types of SATase have been described. One is allosterically inhibited by L-Cys, and the second is not sensitive to L-Cys inhibition. However, the allosteric site in SATase has not been identified. To understand better the mechanism of L-Cys inhibition of plant SATases, we constructed several chimeric SATase enzymes from watermelon SATase (WaSATase) (sensitive type) and Arabidopsis SAT-p (insensitive type). These enzymes were expressed in Escherichia coli, and inhibition of the mutated SATase activity by L-Cys was analyzed. Mutated WaSATase, in which Met280 was changed to Ile, was no longer inhibited by L-Cys. Analysis of the inhibition the chimeric enzymes indicated that the C-terminal region of WaSATase from Pro276 to Phe285, in which five amino acids are different from those of SAT-p, was responsible for the determination of the sensitivity to L-Cys. In particular, Gly277 in the C-terminal region of WaSATase was primarily responsible for the L-Cys inhibition. The N-terminal half of the protein, which does not contain the catalytic domain, was also important for the sensitivity to L-Cys. These results indicate that the sensitivity of SATase to L-Cys is due to the N-terminal and C-terminal regions rather than to the catalytic domain.