VP24 found in anthocyanic vacuolar inclusions (AVIs) of sweet potato cells is a member of a metalloprotease family

Abstract A 24 kDa vacuolar protein (VP24) was found to accumulate abundantly in anthocyanic vacuolar inclusions (AVIs) in anthocyanin-producing sweet potato cells grown in suspension culture. To clarify the function of VP24, we have isolated and characterized a cDNA clone encoding VP24 (Accession No. AB025531 ). Sequence analysis revealed that the 2.9 kbp VP24 cDNA encodes a protein of 893 amino acid residues with a predicted molecular mass of 96.3 kDa. VP24 is synthesized as a large precursor protein which possesses an N-terminal extension composed of a signal peptide and a propeptide, plus the polypeptide of the mature VP24 and its C-terminal propeptide, that contains the multiple transmembrane domains [Plant Physiol. 125 (2001) 447]. Based on sequence identities, the mature VP24 domain most likely belongs to the zinc metalloprotease family. In order to confirm whether VP24 actually has peptidase activity we overexpressed the mature VP24 domain in Escherichia coli . The expressed fusion protein clearly showed aminopeptidase activity in vitro. Native VP24 purified from the anthocyanin-accumulating cells also showed aminopeptidase activity. These results suggest that VP24 expressed in the anthrocyanin-producing sweet potato cells could be a novel vacuolar localized aminopeptidase.