Genetic and biochemical analysis of serine carboxypeptidase-like proteins in Arabidopsis thaliana

The Arabidopsis genome encodes a family of 51 proteins that are homologous to known serine carboxypeptidases. Phylogenetic analysis shows that these serine carboxypeptidase-like (SCPL) proteins can be divided into five major clades, with three clades having representatives in plants, animals, and fungi, and two clades having representatives only in plants. RT-PCR analysis demonstrates that the Arabidopsis SCPL genes are diverse in their expression patterns. A subgroup of 21 SCPL proteins includes two that have been shown to function as acyltransferases in plant secondary metabolism: sinapoylglucose: malate sinapoyltransferase (SMT) and sinapoylglucose:choline sinapoyltransferase (SCT). SMT is encoded by one of five SCPL genes arranged in tandem to form a cluster on chromosome 2. Analysis of deletion mutant lines lacking one or more genes in this SCPL gene cluster reveals that three of the genes also encode sinapoylglucose acyltransferases: At2g23000, At2g23010, and At2g22980. At2g23000 encodes sinapoylglucose:anthocyanin sinapoyltransferase (SAT), an enzyme that is required for the synthesis of the sinapoylated anthocyanins in Arabidopsis. At2g23010 encodes an enzyme with sinapoylglucose:sinapoylglucose sinapoyltransferase (SST) activity. SST activity is also exhibited by SMT and the protein encoded by At2g22980. The study of the Arabidopsis SCPL proteins thus provides an opportunity to examine structure-function relationships in enzyme evolution and plant secondary metabolism