Plant aspartic proteinases: enzymes on the way to a function

Plant aspartic proteinases have been characterized from seeds, flowers and leaves of a number of different species, The enzymes are generally either m onomeric or heterodimeric, containing two peptides processed from the same precursor protein. The plant enzymes, like their mammalian and microbial co unterparts, are active at acidic pH and inhibited by a class specific inhib itor pepstatin A, Plant aspartic proteinases are generally either secreted or targeted to the vacuolar/protein storage body compartment, The primary s equences of many of these enzymes have been determined and are very homolog ous with each other as well as with enzymes from mammalian and microbial or igins. Plant aspartic proteinases, however, have a very unique plant specif ic region, which is not found in mammalian, microbial, or viral aspartic pr oteinases, The function of this region has not been elucidated, A role for these plant enzymes in protein processing or degradation has been proposed, however, more studies are required to confirm their in vivo functions. Rec ent intriguing results suggest possible roles for these enzymes in programm ed cell-death of tissues and in pathogen resistance.