VACUOLAR PROCESSING ENZYMES IN PROTEIN-STORAGE VACUOLES AND LYTIC VACUOLES

Vacuolar processing enzymes (VPEs), which are responsible for maturati on of various vacuolar proteins belong to a novel family of cysteine p roteinases. Molecular characterization of castor bean VPE revealed tha t the latent precursor of VPE (proVPE) is converted into an active VPE by self-catalytic proteolysis. Thus, no other factor is necessary to produce active VPE and VPE itself is a key enzyme in determining the f inal conformation of the vacuolar proteins. The VPE-mediated system is widely distributed in various plant organs. The temporal and spatial expression of the VPE system has been examined with three Ambidopsis V PEs, alpha VPE, beta VPE and gamma VPE. The beta VPE gene is expressed in seeds, suggesting that beta VPE plays a role in maturation of seed proteins in protein storage vacuoles. On the other hand, both the alp ha VPE and gamma VPE genes are expressed in senescent tissues and thei r expression patterns are correlated with programmed cell death. The v egetative VPEs might regulate the activation of senescence-associated hydrolytic enzymes in the lytic vacuoles of cells preparing for death.