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From Proteopedia

N-acetylglucosamine Phosphotransferase

Elizabeth Schneider 21:01, 9 April 2019 (UTC); Student; Erskine College, Due West, SC, United States

N-acetylglucosamine (GlcNAc) phosphotransferase is a cis-Golgi-localized enzyme that recognizes the specific protein sequence in newly synthesized lysosomal enzymes and transfers phosphorylated GlcNAc groups [1]. This enzyme is hexameric, composed of two alpha, two beta, and two gamma subunits [2].

Genetic Coding for N-acetylglucosamine Phosphotransferase

The alpha and beta subunits result from a single cDNA and formed by proteolysis at a lysine-aspartate bond on the alpha/beta precursor [3]. The alpha and beta subunits are encoded by the GNPTAB gene, but the gamma subunit is encoded independently by the GNPTG gene, making GlcNAc an exception to the Garrod-Beadle principle that one enzyme is encoded by one gene [4,5].

Molecular Role of N-acetylglucosamine Phosphotransferase

Lysosomal enzymes are required for the detection of phosphotransferases in the Golgi apparatus. The structure of many of these lysosomal enzymes is unknown, so the interactions among such enzymes and N-acetylglucosamine phosphotransferase is largely undefined; one known fact is the lysine plays a major role[6.2].

Yaghootfam et al. substituted lysine residues on the lysosomal arylsulfatases A and B to find any correlation between this substitution and phosphorylation. The results showed that the effects differed depending on the specific residue and its location on the arylsulfatases. Substitution of only lysine 457 on arylsulfatase A reduced phosphorylation by seventy-seven percent and increased release of the altered enzyme. The location of lysine 457 is in a helix, unlike typical lysine residues which are typically located in an unstructured loop. This could cause an effect, but lysines 497 and 507 were analyzed in arylsulfatases B with similar locations on the enzyme and there was no effect on the phosphorylation with the substitutions on arylsulfatases A and B. Yaghootfam et al. concluded that even homologous enzymes can vary in phosphorylation rates because of the great fluctuation in phopshotransferase recognition by lysosomal enzymes.

Diseases Related to N-acetylglucosamine Phosphotransferase

Mucolipidosis II is an autosomal recessive hereditary phenotypic disease caused by disruptions of the alpha and beta subunits. This disease presents symptoms of mental retardation and skeletal changes as acid mucopolysaccharides, sphingolipids, and/or glycolipids concentrate in visceral and mesenchymal cells in neural tissue [6]. In a study by Kudo et al, patients with the disease had either absent or reduced alpha/beta transcription; however, all patients had normal gamma subunit levels [7]. This suggests that the alpha and/or the beta subunit contains enzymatic qualities.

Post-translational modifications such as interchain and intramolecular disulfide bonds may occur via GlcNAc-phosphotransferase in the cytoplasm outside of the nucleus, including the protein glycosylation on serine and threonine residues. Any imbalance of phosphorylation can lead to diseases including cancer, diabetes, and neurodegenerative diseases [8]. Management of phosphorylation and other modifications performed by GlcNAc-phosphotransferase can allow for better diagnoses and treatments of such diseases.

References [1]. Bioconjugate Chem., 2014, 25 (6), pp 1025–1030 [2]. Bao, M., Elmendorf, B. J., Booth, J. L., Drake, R. R., Canfield, W. M. Bovine UDP-N-acetylglucosamine:lysosomal-enzyme N-acetylglucosamine-1-phosphotransferase: II. Enzymatic characterization and identification of the catalytic subunit. J. Biol. Chem. 271: 31446-31451, 1996. [3]. Canfield, W. M., Bao, M., Pan, J., Brewer, A. D. K., Pan, H., Roe, B., Raas-Rothschild, A. Mucolipidosis II and mucolipidosis IIIA are caused by mutations in the GlcNAc-phosphotransferase alpha/beta gene on chromosome 12p. Am. J. Hum. Genet. 63: A15 only, 1998. [4]. Hartz, P. A. N-Acetylglucosamine-1-phosphotransferase, Alpha/Beta Subunits; GNPTAB. [5]. Kudo, M., Bao, M., D'Souza, A., Ying, F., Pan, H., Roe, B. A., Canfield, W. M. The alpha- and beta-subunits of the human UDP-N-acetylglucosamine:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase are encoded by a single cDNA. J. Biol. Chem. 280: 36141-36149, 2005. [6]. European Bioinformatics Institute. Protein Information Resource. SIB Swiss Institute of Bioinformatics. [7]. Canfield, W. M., Bao, M., Pan, J., Brewer, A. D. K., Pan, H., Roe, B., Raas-Rothschild, A. Mucolipidosis II and mucolipidosis IIIA are caused by mutations in the GlcNAc-phosphotransferase alpha/beta gene on chromosome 12p. Am. J. Hum. Genet. 63: A15 only, 1998. [8]. Lodish, H. F., et al. Molecular cell biology, 8th ed.; W.H. Freeman-Macmillan Learning: New York, 2016. [9]. Q9UJJ9 N-acetylglucosamine-1-phosphotransferase subunit gamma. Swiss-Model. 2019. [10]. RCSB Protein Data Bank. NMR structure of the 140-315 fragment of the N-acetylglucosamine-1-phosphotransferase, alpha and beta subunits.