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The determinants on the cytoplasmic tail of the cation-dependent mannose 6-phosphate receptor (CD-MPR) required for lysosomal enzyme sorting have been analyzed. Mouse L cells deficient in the mannose 6-phosphate/insulin-like growth factor-II receptor were transfected with normal bovine CD-MPR cDNA or cDNAs containing mutations in the 67-amino acid cytoplasmic tail and assayed for their ability to target the lysosomal enzyme cathepsin D to lysosomes. Cells expressing the wild-type bovine CD-MPR sorted 67 +/- 2% of newly synthesized cathepsin D compared with the base-line value of 47 +/- 1%. The presence of mannose 6-phosphate in the medium did not affect the efficiency of cathepsin D sorting, indicating that the routing of the ligand-receptor complex is completely intracellular. Mutant receptors with the carboxyl-terminal His-Leu-Leu-Pro-Met67 residues deleted or replaced with alanines sorted cathepsin D below the base-line value. A mutant receptor with the outermost Pro-Met residues replaced with alanines sorted cathepsin D better than the wild-type receptor, indicating that the essential residues for sorting are the His-Leu-Leu sequence. Disruption of a putative casein kinase II phosphorylation site at Ser57 had no detectable effect on sorting. The mutant receptor with the five-amino acid deletion was able to bind to a phosphopentamannose affinity column, proving that its ligand binding site was grossly intact. Resialylation experiments showed that this mutant receptor recycled from the cell surface to the Golgi at a rate similar to the normal CD-MPR, indicating that the defect in sorting is at the level of the Golgi.
Johnson et al. (Thu,) studied this question.