Identification of a membrane-associated cysteine protease with possible dual roles in the endoplasmic reticulum and protein storage vacuole

SH-EP is a vacuolar cysteine proteinase from germinated seeds of Vigna mung o, The enzyme has a C-terminal propeptide of 1 kDa that contains an endopla smic reticulum (ER) retention signal, KDEL, The KDEL-tail has been suggeste d to function to store SH-EP as a transient zymogen in the lumen of the ER, and the C-terminal propeptide was thought to be removed within the ER or i mmediately after exit from the ER, In the present study, a protease that ma y be involved in the post-translational processing of the C-terminal propep tide of SH-EP was isolated from the microsomes of cotyledons of V. muno see dlings. cDNA sequence for the protease indicated that the enzyme is a membe r of the papain superfamily, Immunocytochemistry and subcellular fractionat ion of cotyledon cells suggested that the protease was localized in both th e ER and protein storage vacuoles as enzymatically active mature form. In a ddition, protein fractionations of the cotyledonary microsome and Sf9 cells expressing the recombinant protease indicated that the enzyme associates w ith the microsomal membrane on the luminal side. The protease was named mem brane-associated cysteine protease, MCP, The possibility that a papain-type enzyme, MCP, exists as mature enzyme in both ER and protein storage vacuol es will be discussed.