Biochemical characterization of sinapoylglucose:choline sinapoyltransferase,a serine carboxypeptidase-like protein that functions as an acyltransferase in plant secondary metabolism

Recently, serine carboxypeptidase-like (SCPL) proteins that catalyze transacylation reactions in plant secondary metabolism have been identified from wild tomato and Arabidopsis. These include sinapoylglucose: choline sinapoyltransferase (SCT), an enzyme that functions in Arabidopsis sinapate ester synthesis. SCT and the other known SCPL acyltransferases all share the conserved serine, aspartic acid, and histidine residues employed for catalysis by classical serine carboxypeptidases, although the importance of these residues and the mechanism by which this class of SCPL proteins catalyze acyltransferase reactions is unknown. To characterize further SCT and its catalytic mechanism, we have employed the Saccharomyces cerevisiae vacuolar protein localization 1 mutant, which secretes the serine carboxypeptidase, carboxypeptidase Y, and other proteins normally targeted to the vacuole. When expressed in this strain, SCT is similarly secreted. SCT has been purified from the yeast medium and used for kinetic characterization of the protein. Immunological analysis of SCT has revealed that the expected 50-kDa mature protein is proteolytically processed in yeast and in planta, most likely resulting in the production of a heterodimer derived from a 30- and 17-kDa polypeptide.