Lysosomal acid phosphatase is not involved in the dephosphorylation of mannose 6-phosphate containing lysosomal proteins.

The mannose 6-phosphate (Man6P) residues that are necessary for the targeting of newly synthesized lysosomal proteins are dephosphorylated after delivery of lysosomal proteins to lysosomes. To examine the role of lysosomal acid phosphatase (LAP) for the dephosphorylation of Man6P residues in lysosomal proteins, the phosphorylation of endogenous lysosomal proteins and of internalized arylsulfatase A was analyzed in mouse L-cells that overexpress human LAP. Non-transfected L-cells dephosphorylate endogenous lysosomal proteins slowly (half time approximately 13 h) as well as internalized arylsulfatase A. A more than 100-fold overexpression of LAP in these cells did not affect the dephosphorylation rate. Control experiments showed that the internalized arylsulfatase A and overexpressed LAP partially colocalize and that under in vitro conditions purified LAP does not dephosphorylate arylsulfatase A. Taken together, these results indicate that LAP is not the mannose 6-phosphatase that dephosphorylates lysosomal proteins after their delivery to lysosomes.