DETECTION AND CHARACTERIZATION OF A VACUOLAR PROTEIN (VP24) IN ANTHOCYANIN-PRODUCING CELLS OF SWEET-POTATO IN SUSPENSION-CULTURE

Vacuoles containing large amounts of protein and anthocyanin were isol ated and purified from anthocyanin-producing cultured cells of sweet p otato (Ipomoea batatas). A 24 kDa protein (VP24), identified as a majo r protein in the isolated vacuoles, was detected by SDS-polyacrylamide gel electrophoresis. Neither VP24 nor anthocyanin was detectable in d ark-cultured cells, but VP24 appeared three days after the start of ir radiation with light and the level of VP24 increased for up to a week. High concentrations of 2,4-D markedly inhibited the accumulation of b oth VP24 and anthocyanin. These results indicated that the expression of VP24, which was induced by exposure to light, was closely accompani ed by the accumulation of anthocyanin in the vacuoles. VP24 was recove red as an insoluble reddish precipitate after ultracentrifugation of a lysate of anthocyanin-containing vacuoles. Immunoblot analysis indica ted that this vacuolar protein was distinct from sporamin, a major sto rage protein of sweet potato tuberous root. These results suggest that VP24 is not an integral protein of the vacuolar membrane but tends to form aggregates via interactions with anthocyanin during extraction. VP24 may be involved in the formation of intravacuolar pigmented struc tures that develop in vivo in the anthocyanin-containing vacuoles of s weet potato cells in culture.