Functional Identification of Arabidopsis ATSIP2 (At3g57520) as an Alkaline alpha-Galactosidase with a Substrate Specificity for Raffinose and an Apparent Sink-Specific Expression Pattern

Arabidopsis ATSIP2 has recently been suggested to be a raffinose synthase gene. However, it has high amino acid identity to functionally characterized alkaline alpha-galactosidases from Cucumis melo and Zea mays. Using the Sf9 insect cell expression system, we demonstrate that recombinant ATSIP2 is a genuine alkaline alpha-galactosidase with a distinct substrate specificity for raffinose, and not a raffinose synthase. A beta-glucuronidase reporter construct using the ATSIP2 promoter shows that ATSIP2 is strongly expressed in sink tissues of Arabidopsis, i.e. sink leaves and non-xylem parts of the root stele, suggesting a physiological function in raffinose phloem unloading.