Vacuolar Processing Enzyme Responsible for Maturation of Seed Proteins

Abstract

Summary A vacuolar processing enzyme responsible for maturation of seed proteins was isolated from the castor bean and soybean. The processing enzyme belongs to a novel cysteine proteinase with a molecular mass of 37 kDa 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 proprotein precursors to produce their mature seed proteins such as 11S globulin and 2S albumin. Immunocytochemical localization of the enzyme in the vacuolar matrix of maturing castor bean endosperm indicates that the maturation of the seed proteins occurs in the vacuoles. Molecular characterization revealed that the enzyme is synthesized as an inactive precursor with a larger molecular mass. The results of immunoelectron microscopic analysis suggested that the precursor is transported to vacuoles via dense vesicles together with proproteins of seed proteins. After arriving at the vacuoles, the inactive precursor is converted into an active enzyme. This suggests that a cascade for proprotein processing is involved in the maturation of seed proteins. Vacuolar processing enzyme activity was found in various plant tissues and several cDNA homologues of the enzyme were isolated from different plants. Thus a similar processing enzyme is widely distributed in plant tissues and plays a crucial role in the maturation of a variety of proteins in plant vacuoles.