Dephosphorylation of the mannose-6-phosphate recognition marker is localized in later compartments of the endocytic route. Identification of purple acid phosphatase (uteroferrin) as the candidate phosphatase.

The mannose 6-phosphate (Man6P) recognition marker in lysosomal proteins is known to be dephos- phorylated after the delivery of lysosomal proteins to the endosomeflysosome compartment. The rate of Man6P recognition marker inactivation depends on the cell type and lysosomal protein. In the present study we show that in BHK 21 cells, which rapidly dephosphorylate lysosomal proteins, the recognition marker is stable in the endosomal compartment, to which lysosomal enzymes such as arylsulfatase A are delivered during endocytosis at 20°C. Dephosphorylation depends on the transfer of internalized lyso- somal enzymes from the 20°C compartment to later compartments, most likely lysosomes. This transfer is sensitive to NH,CI and nocodazole. In vitro experiments identified purple acid phosphatase (uteroferrin) as a candidate for the lysosomal phosphatase catalyzing in vivo the dephosphorylation of Man6P recogni- tion marker. Keywords: mannose 6-phosphate ; mannose 6-phosphatase ; uteroferrin.