Characterization of a membrane-bound C-glucosyltransferase responsible for carminic acid biosynthesis in Dactylopius coccus Costa.

Carminic acid， a glucosylated anthraquinone found in scale insects like Dactylopius coccus， has since ancient times been used as a red colorant in various applications. Here we show that a membrane-bound C-glucosyltransferase， isolated from D. coccus and designated DcUGT2， catalyzes the glucosylation of flavokermesic acid and kermesic acid into their respective C-glucosides dcII and carminic acid. DcUGT2 is predicted to be a type I integral endoplasmic reticulum (ER) membrane protein， containing a cleavable N-terminal signal peptide and a C-terminal transmembrane helix that anchors the protein to the ER， followed by a short cytoplasmic tail. DcUGT2 is found to be heavily glycosylated. Truncated DcUGT2 proteins synthesized in yeast indicate the presence of an internal ER-targeting signal. The cleavable N-terminal signal peptide is shown to be essential for the activity of DcUGT2， whereas the transmembrane helix/cytoplasmic domains， although important， are not crucial for its catalytic function.