Evidence that luminal ER proteins are sorted from secreted proteins in a post-ER compartment.
Several soluble proteins that reside in the lumen of the ER contain a specific C-terminal sequence (KDEL) which prevents their secretion. This sequence may be recognized by a receptor that either immobilizes the proteins in the ER, or sorts them from other proteins at a later point in the secretory pathway and returns them to their normal location. To distinguish these possibilities, I have attached an ER retention signal to the lysosomal protein cathepsin D. The oligosaccharide side chains of this protein are normally modified sequentially by two enzymes to form mannose-6-phosphate residues; these enzymes do not act in the ER, but are thought to be located in separate compartments within (or near) the Golgi apparatus. Cathepsin D bearing the ER signal accumulates within the ER, but continues to be modified by the first of the mannose-6-phosphate forming enzymes. Modification is strongly temperature-dependent, which is also a feature of ER-to-Golgi transport. These results support the idea that luminal ER proteins are continuously retrieved from a post-ER compartment, and that this compartment contains N-acetylglucosaminyl-1-phosphotransferase activity.
Pelham
H R
HR
MRC Laboratory of Molecular Biology, Cambridge, UK.
D000818:Animals; D002402:Cathepsin D / Q000032:analysis / Q000235:genetics*; D002460:Cell Line; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D006801:Humans; D008247:Lysosomes / Q000378:metabolism; D010761:Phosphorus Radioisotopes; D010766:Phosphorylation; D010957:Plasmids; D011499:Protein Processing, Post-Translational; D011993:Recombinant Fusion Proteins / Q000032:analysis; D014162:Transfection
D010761:Phosphorus Radioisotopes; D011993:Recombinant Fusion Proteins; D002402:Cathepsin D