Different sorting of Lys-Asp-Glu-Leu proteins in rat liver

Most of the resident soluble proteins of the endoplasmic reticulum (ER) seem to be sorted into this compartment via their COOH-terminal tetrapeptide Lys-Asp-Glu-Leu (KDEL). This sorting is supposed to occur in a post-ER compartment. Three resident soluble ER glycoproteins belonging to the KDEL family are CaBP1, CaBP2, CaBP3 (= calreticulin), and CaBP4 (= grp94) (Nguyen Van, P., Peter, F., and Söling, H.-D. (1989) J. Biol. Chem. 264, 17494-17501). In rat liver, calreticulin possesses a carbohydrate moiety of the complex hybrid type with terminal galactoses (Nguyen Van, P., Peter, F., and Söling, H.-D. (1989) J. Biol. Chem. 264, 17494-17501). We can show now that practically all calreticulin molecules (and not only a fraction) possess terminal galactoses as well as the COOH-terminal KDEL sequence. This as well as pulse-chase experiments performed at 37 and 15 degrees C indicate that calreticulin must have passed through the trans-Golgi. Subcellular fractionations of post-mitochondrial supernatants from isolated rat hepatocytes by sucrose-Nycodenz gradient centrifugation revealed that calreticulin is confined mainly to the rough ER, grp94 mainly to the smooth ER. CaBP1, a member of the thioredoxin family, was recovered in fractions which most likely represent the intermediate compartment. This indicates that KDEL is a sorting signal which leads to the retention of these proteins in the pre-Golgi compartments. However, additional factors, most likely residing within the specific KDEL protein itself, determine the final location of the protein within the pre-Golgi compartments. This is underlined by experiments in which the density dependent distribution of total KDEL proteins was studied using a COOH-terminal KDEL-specific antibody.