THE C-TERMINAL HDEL SEQUENCE IS SUFFICIENT FOR RETENTION OF SECRETORYPROTEINS IN THE ENDOPLASMIC-RETICULUM (ER) BUT PROMOTES VACUOLAR TARGETING OF PROTEINS THAT ESCAPE THE ER

Proteins are co-translationally transferred into the endoplasmic retic ulum (ER) and then either retained or transported to different intrace llular compartments or to the extracellular space. Various molecular s ignals necessary for retention in the ER or targeting to different com partments have been identified. In particular, the HDEL and KDEL signa ls used for retention of proteins in yeast and animal ER have also bee n described at the C-terminal end of soluble ER processing enzymes in plants. The fusion of a KDEL extension to vacuolar proteins is suffici ent for their retention in the ER of transgenic plant cells. However, recent results obtained using the same strategy indicate that HDEL doe s not contain sufficient information for full retention of phaseolin e xpressed in tobacco. In the present study, an HDEL C-terminal extensio n was fused to the vacuolar or extracellular (Delta pro) forms of spor amin. The resulting SpoHDEL or BproHDEL, as well as Spo and Delta pro, were expressed at high levels in transgenic tobacco cells (Nicotiana tabacum cv BY2). The intracellular location of these different forms o f recombinant sporamin was studied by subcellular fractionation. The r esults clearly indicate that addition of an HDEL extension to either S po or Delta pro induces accumulation of these sporamin forms in a comp artment that co-purifies with the ER markers NADH cytochrome C reducta se, binding protein (BiP) and calnexin. In addition, a significant Spo HDEL or Delta proHDEL fraction that escapes the ER retention machinery is transported to the vacuole. From these results, it may be proposed that, in addition to its function as an ER retention signal, HDEL cou ld also act in quality control by targeting chaperones or chaperone-bo und proteins that escape the ER to the plant lysosomal compartment for degradation.