Posttranslational removal of the carboxyterrninal kdel of the cysteine protease sh-ep occurs prior to maturation of the enzyme

SH-EP is ft cysteine protease from germinating mung bean (Vigna inungo) (otyledons that functions to degrade vacuolar storage proteins to supply nitrogen and carbon for the early stages of the new plant's growth. SH-EP possesses a carboxyterrninal ER retention sequence KDEL. In order to examine the function of the ER retention sequence, we expressed a cDNA of SH-EP and a minus-KDEL control in Sf-9 cells using the bacuiovirus system. The synthesis, processing and trafficking of SH-EP in Sf-9 relis support a model where the KDEL EK retention sequence is posltranslationally removed either within the ER oi immediately after its exit from the ER resulting in the accumulation of pioSH-EP minus its KDEL signal. U is this intermediate form that appears to progress through the endoiiiembrane system and is subsequently processed to form mature active SH-EP. Lhe function of the KDEL sequence may be to relard the exit of proSH-EP from the ER leading to its temporary accumulation within the ER making SH-EP with its KDEI retention sequence a transient /ymögen. SH-EP with ils KDEL may provide a posttranslational control of vacuole protein degradation by allowing sufficient proSH EP to be synthesized and stored to Uter degrade storage proteins, ['he removal of an ER retention sequence To regulate protein delivery to a functional site presents an alternative role for EH retention sequences in addition u> its well established role in maintaining the protein composition of the ER lumon.