Posttranslational removal of the carboxyl-terminal KDEL of the cysteine protease SH-EP occurs prior to maturation of the enzyme

SH-EP is a cysteine protease from germinating mung bean (Vigna mungo) that possesses a carboxyl-terminal endoplasmic reticulum (ER) retention sequence , KDEL. In order to examine the function of the ER retention sequence, we e xpressed a full-length cDNA of SH-EP and a minus-KDEL control in insect Sf- 9 cells using the baculovirus system. Our observations on the synthesis, pr ocessing, and trafficking of SH-EP in Sf-9 cells suggest that the KDEL ER-r etention sequence is posttranslationally removed either while the protein i s still in the ER or immediately after its exit from the ER, resulting in t he accumulation of proSH-EP minus its KDEL signal. It is this intermediate form that appears to progress through the endomembrane system and is subseq uently processed to form mature active SH-EP, The removal of an ER retentio n may regulate protein delivery to a functional site and present an alterna tive role for ER retention sequences in addition to their well established role in maintaining the protein composition of the ER lumen.