Intracellular transport and processing of plant proteases

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SH-EP is a cysteine protease from germinating mung bean (Vigna mungo) cotyledons that functions to degrade vacuolar storage proteins to supply nitrogen and carbon for the early stages of the new plant's growth. Although SH-EP appears to be a typical member of the papain superfamily cysteine proteases the cDNA of proSH-EP shows it also possesses a carboxyterminal ER retention sequence KDEL found on ER-lumen resident proteins. In order to examine the function of the ER retention sequence we expressed a full-length cDNA of SH-EP and a minus-KDEL control in Sf-9 cells using the baculovirus sytem. Our observations on the synthesis, processing and trafficking of SH-EP in Sf-9 cells support a model where the KDEL ER-retention sequence is posttranslationally removed either within the ER or immediately after its exit from the ER resulting in the accumulation of proSH-EP minus its KDEL signal. It is this intermediate form that appears to progress through the endomembrane system and is subsequent … More ly processed to form mature active SH-EP.The function of the KDEL ER-rentention sequence may be to retard the exit of proSH-EP from the ER leading to its temporary accumulation within the ER making proSH-EP with its KDEL retention sequence a transient zymogen. This pattern of posttranslational processing in Sf-9 cells provides an explaination for previous results on the carboxyterminal sequenching of mature SH-EP purified from mung bean seeds which was shown to lack the KDEL.SH-EP with its KDEL in the seed cotyledon cells may serve to provide a posttranslational control of vacuole protein degradation by allowing sufficient proSH-EP to be synthesized and stored to rapidly degrade storage proteins by allowing the cell to regulate the delivery of the stored inactive form of proSH-EP to its activation site and substrate. The removal of an ER retention sequence with its potential to regulate protein delivery to a functional site presents an alternative role for ER retention sequences in addition to its well established role in maintaining the protein composition of the ER lumen. Less