Compensatory expression of human N-Acetylglucosaminyl-1-phosphotransferase subunits in mucolipidosis type III gamma

The N-Acetylglucosaminyl-1-phosphotransferase plays a key role in the generation of mannose 6-phosphate (M6P) recognition markers essential for efficient transport of lysosomal hydrolases to lysosomes. The phosphotransferase is composed of six subunits (α2, β2, γ2). The α- and β-subunits are catalytically active and encoded by a single gene, GNPTAB, whereas the γ-subunit encoded by GNPTG is proposed to recognize conformational structures common to lysosomal enzymes. Defects in GNPTG cause mucolipidosis type III gamma, which is characterized by missorting and cellular loss of lysosomal enzymes leading to lysosomal accumulation of storage material. Using plasmon resonance spectrometry, we showed that recombinant γ-subunit failed to bind the lysosomal enzyme arylsulfatase A. Additionally, the overexpression of the γ-subunit in COS7 cells did not result in hypersecretion of newly synthesized lysosomal enzymes expected for competition for binding sites of the endogenous phosphotransferase complex. Analysis of fibroblasts exhibiting a novel mutation in GNPTG (c.619insT, p.K207IfsX7) revealed that the expression of GNPTAB was increased whereas in γ-subunit overexpressing cells the GNPTAB mRNA was reduced. The data suggest that the γ-subunit is important for the balance of phosphotransferase subunits rather for general binding of lysosomal enzymes.