VACUOLAR PROCESSING ENZYME RESPONSIBLE FOR MATURATION OF SEED PROTEINS

A vacuolar processing enzyme responsible for maturation of seed protei ns was isolated from the castor bean and soybean. The processing enzym e belongs to a novel cysteine proteinase with a molecular mass of 37 k Da for castor bean and 39 kDa for soybean. The enzyme splits a peptide bond on the C-terminal side of an exposed asparagine residue of the p roprotein precursors to produce their mature seed proteins such as 11S globulin and 25 albumin. Immunocytochemical localization bf the enzym e in the vacuolar matrix of maturing castor bean endosperm indicates t hat the maturation of the seed proteins occurs in the vacuoles. Molecu lar characterization revealed that the enzyme is synthesized as an ina ctive precursor with a larger molecular mass. The results of immunoele ctron microscopic analysis suggested that the precursor is transported to vacuoles via dense vesicles together with proproteins of seed prot eins. After arriving at the vacuoles, the inactive precursor is conver ted into an active enzyme. This suggests that a cascade for proprotein processing is involved in the maturation of seed proteins. Vacuolar p rocessing enzyme activity was found in various plant tissues and sever al cDNA homologues of the enzyme were isolated from different plants. Thus a similar processing enzyme is widely distributed in plant tissue s and plays a crucial role in the maturation of a variety of proteins in plant vacuoles.