Mass transport of preform of a KDEL-tailed cysteine proteinase (SH-EP) to protein storage vacuoles by endoplasmic reticulum-derived vesicle is involved in protein mobilization in germinating seeds

A vacuolar cysteine proteinase, designated SH-EP, is expressed in the cotyl edon of germinated Vigna mungo seeds and is responsible for the degradation of storage proteins. SH-EP is a characteristic vacuolar proteinase possess ing a COOH-terminal endoplasmic reticulum (ER) retention sequence, KDEL. In this work, immunocytochemical analysis of the cotyledon cells of germinate d V. mungo seeds was performed using seven kinds of antibodies to identify the intracellular transport pathway of SH-EP from ER to protein storage vac uoles.-A preform of SH-EP synthesized in ER accumulated at the edge or midd le region of ER where the transport vesicle was formed. The vesicle contain ing a large amount of proSH-EP, termed KV, budded off from ER, bypassed the Golgi complex, and was sorted to protein storage vacuoles. This massive tr ansport of SH-EP via KV was thought to mediate dynamic protein mobilization in the cotyledon cells of germinated seeds. We discuss the possibilities t hat the KDEL sequence of KDEL-tailed vacuolar cysteine proteinases function as an accumulation signal at ER, and that the mass transport of the protei nases by ER-derived KV-like vesicle is involved in the protein mobilization of plants.