Abstract
The distribution of mannose 6-phosphate (Man6P) receptors for lysosomal enzymes was investigated in human normal and mucolipidosis II (I-cell) disease fibroblasts by light and electron microscopic immunocytochemistry. In normal fibroblasts whose lysosomal enzymes contain Man6P recognition markers required for binding to the receptor, intracellular immunoreactive receptors were concentrated in one or two cisternae located on one side of the Golgi stack, in coated vesicles, and in lysosomes and endosomes. Presumably, this distribution delineates the Man6P-dependent transport pathway for newly synthesized lysosomal enzymes. In I-cell fibroblasts whose lysosomal enzymes lack the Man6P recognition marker, intracellular receptors were found almost exclusively in one or two Golgi cisternae and in numerous coated vesicles located near the immunoreactive cisternae but were rarely detected in lysosomes and endosomes. The results obtained on I-cell fibroblasts suggest that in the absence of endogenous ligands (lysosomal enzymes bearing Man6P residues), Man6P receptors do not constitutively shuttle-via coated vesicles-between the Golgi complex and lysosomes or endosomes. Rather, they accumulate in coated vesicles concentrated at the presumptive sorting site in the Golgi complex. The findings suggest that a specific subpopulation of coated vesicles (primary lysosomes) is involved in Man6P receptor transport of lysosomal enzymes from the Golgi complex to lysosomes and that their relocation is triggered by ligand binding.