A Cysteine Endopeptidase (SH-EP) in Germinated Vigna mungo Seeds: Post-translational Processing and Intracellular Transport

Abstract

A plant cysteine endopeptidase, designated SH-EP, is a major protease occurring in cotyledons of Vigna mungo seedlings, and acts to degrade seed globulin stored in protein bodies (protein storage vacuoles). SH-EP is synthesized on membrane-bound ribosomes as a 43-kDa intermediate through the cotranslational cleavage of a signal sequence, and the intermediate is processed further to the 33-kDa mature enzyme via 39- and 36-kDa intermediates, (a) N-terminalprocessing— Experiments of in vitro processing of the SH-EP intermediates revealed that two processing enzymes, VmPE-1 and VmPE-2 (V. mungo processing enzymes 1 and 2), are involved in the processing. VmPE-1 was purified from the day-3 cotyledons. The enzyme is the same type of protease as asparaginyl endopeptidases in terms of the primary structure and substrate specificity, (b) C-terminal processing — The amino acid sequence of SH-EP deduced from the cDNA contains C-terminus with Lys-Asp-Glu-Leu (KDEL) tail, which is known as a retention signal for endoplasmic reticulum (ER) while mature SH-EP is localized in protein bodies. The analysis for C-terminal amino acid residues of SH-EP indicated that the C-terminal propeptide of 10 amino acid residues containing the KDEL tail is processed to form mature SH-EP.