Molecular cloning and characterization of Vigna mungo processing enzyme 1 (VmPE-1), an asparaginyl endopeptidase possibly involved in post-translational processing of a vacuolar cysteine endopeptidase (SH-EP).

Asparaginyl endopeptidase is a cysteine endopeptidase that has strict substrate specificity toward the carboxy side of asparagine residues. Vigna mungo processing enzyme 1, termed VmPE-1, occurs in the cotyledons of germinated seeds of V. mungo, and is possibly involved in the post-translational processing of a vacuolar cysteine endopeptidase, designated SH-EP, which degrades seed storage protein. VmPE-1 also showed a substrate specificity to asparagine residues, and its enzymatic activity was inhibited by NEM but not E-64. In addition, purified VmPE-1 had a potential to process the recombinant SH-EP precursor to its intermediate in vitro. cDNA clones for VmPE-1 and its homologue, named VmPE-1A, were identified and sequenced, and their expressions in the cotyledons of V. mungo seedlings and other organs were investigated. VmPE-1 mRNA and SH-EP mRNA were expressed in germinated seeds at the same stage of germination although the enzymatic activity of VmPE-1 rose prior to that of SH-EP. The level of VmPE-1A mRNA continued increasing as germination proceeded. In roots, stems and leaves of fully grown plants, and in hypocotyls, VmPE-1 and VmPE-1A were little expressed. We discuss possible functions of VmPE-1 and VmPE-1A in the cotyledons of germinated seeds.