O-glycosylation is an important post-translational modification (PTM) required for correct functioning of many proteins (Van den Steen et al. 1998, Moremen et al. 2012). The O-glycosylation of proteins containing thrombospondin type 1 repeat (TSR) domains and O-glycosylation of mucins are currently described here.
View original pathway at:Reactome.
Crew VK, Singleton BK, Green C, Parsons SF, Daniels G, Anstee DJ.; ''New mutations in C1GALT1C1 in individuals with Tn positive phenotype.''; PubMedEurope PMCScholia
Manya H, Chiba A, Yoshida A, Wang X, Chiba Y, Jigami Y, Margolis RU, Endo T.; ''Demonstration of mammalian protein O-mannosyltransferase activity: coexpression of POMT1 and POMT2 required for enzymatic activity.''; PubMedEurope PMCScholia
Basu SS, Basu M, Li Z, Basu S.; ''Characterization of two glycolipid: alpha 2-3sialyltransferases, SAT-3 (CMP-NeuAc:nLcOse4Cer alpha 2-3sialyltransferase) and SAT-4 (CMP-NeuAc:GgOse4Cer alpha 2-3sialyltransferase), from human colon carcinoma (Colo 205) cell line.''; PubMedEurope PMCScholia
Ishida H, Togayachi A, Sakai T, Iwai T, Hiruma T, Sato T, Okubo R, Inaba N, Kudo T, Gotoh M, Shoda J, Tanaka N, Narimatsu H.; ''A novel beta1,3-N-acetylglucosaminyltransferase (beta3Gn-T8), which synthesizes poly-N-acetyllactosamine, is dramatically upregulated in colon cancer.''; PubMedEurope PMCScholia
Vasudevan D, Takeuchi H, Johar SS, Majerus E, Haltiwanger RS.; ''Peters plus syndrome mutations disrupt a noncanonical ER quality-control mechanism.''; PubMedEurope PMCScholia
Kitagawa H, Paulson JC.; ''Cloning and expression of human Gal beta 1,3(4)GlcNAc alpha 2,3-sialyltransferase.''; PubMedEurope PMCScholia
de Graffenried CL, Bertozzi CR.; ''Golgi localization of carbohydrate sulfotransferases is a determinant of L-selectin ligand biosynthesis.''; PubMedEurope PMCScholia
Togayachi A, Sato T, Narimatsu H.; ''Comprehensive enzymatic characterization of glycosyltransferases with a beta3GT or beta4GT motif.''; PubMedEurope PMCScholia
Yeh JC, Ong E, Fukuda M.; ''Molecular cloning and expression of a novel beta-1, 6-N-acetylglucosaminyltransferase that forms core 2, core 4, and I branches.''; PubMedEurope PMCScholia
Schwientek T, Yeh JC, Levery SB, Keck B, Merkx G, van Kessel AG, Fukuda M, Clausen H.; ''Control of O-glycan branch formation. Molecular cloning and characterization of a novel thymus-associated core 2 beta1, 6-n-acetylglucosaminyltransferase.''; PubMedEurope PMCScholia
Shiraishi N, Natsume A, Togayachi A, Endo T, Akashima T, Yamada Y, Imai N, Nakagawa S, Koizumi S, Sekine S, Narimatsu H, Sasaki K.; ''Identification and characterization of three novel beta 1,3-N-acetylglucosaminyltransferases structurally related to the beta 1,3-galactosyltransferase family.''; PubMedEurope PMCScholia
Yoshida-Moriguchi T, Willer T, Anderson ME, Venzke D, Whyte T, Muntoni F, Lee H, Nelson SF, Yu L, Campbell KP.; ''SGK196 is a glycosylation-specific O-mannose kinase required for dystroglycan function.''; PubMedEurope PMCScholia
Zhang MX, Nakayama J, Hidaka E, Kubota S, Yan J, Ota H, Fukuda M.; ''Immunohistochemical demonstration of alpha1,4-N-acetylglucosaminyltransferase that forms GlcNAcalpha1,4Galbeta residues in human gastrointestinal mucosa.''; PubMedEurope PMCScholia
Wandall HH, Hassan H, Mirgorodskaya E, Kristensen AK, Roepstorff P, Bennett EP, Nielsen PA, Hollingsworth MA, Burchell J, Taylor-Papadimitriou J, Clausen H.; ''Substrate specificities of three members of the human UDP-N-acetyl-alpha-D-galactosamine:Polypeptide N-acetylgalactosaminyltransferase family, GalNAc-T1, -T2, and -T3.''; PubMedEurope PMCScholia
Chen CI, Keusch JJ, Klein D, Hess D, Hofsteenge J, Gut H.; ''Structure of human POFUT2: insights into thrombospondin type 1 repeat fold and O-fucosylation.''; PubMedEurope PMCScholia
Dassie-Ajdid J, Causse A, Poidvin A, Granier M, Kaplan J, Burglen L, Doummar D, Teisseire P, Vigouroux A, Malecaze F, Calvas P, Chassaing N.; ''Novel B3GALTL mutation in Peters-plus Syndrome.''; PubMedEurope PMCScholia
Bertini E, D'Amico A, Gualandi F, Petrini S.; ''Congenital muscular dystrophies: a brief review.''; PubMedEurope PMCScholia
Shang J, Qiu R, Wang J, Liu J, Zhou R, Ding H, Yang S, Zhang S, Jin C.; ''Molecular cloning and expression of Galbeta1,3GalNAc alpha2, 3-sialyltransferase from human fetal liver.''; PubMedEurope PMCScholia
Sato T, Furukawa K, Bakker H, Van den Eijnden DH, Van Die I.; ''Molecular cloning of a human cDNA encoding beta-1,4-galactosyltransferase with 37% identity to mammalian UDP-Gal:GlcNAc beta-1,4-galactosyltransferase.''; PubMedEurope PMCScholia
Hiruma T, Togayachi A, Okamura K, Sato T, Kikuchi N, Kwon YD, Nakamura A, Fujimura K, Gotoh M, Tachibana K, Ishizuka Y, Noce T, Nakanishi H, Narimatsu H.; ''A novel human beta1,3-N-acetylgalactosaminyltransferase that synthesizes a unique carbohydrate structure, GalNAcbeta1-3GlcNAc.''; PubMedEurope PMCScholia
Yoshida A, Kobayashi K, Manya H, Taniguchi K, Kano H, Mizuno M, Inazu T, Mitsuhashi H, Takahashi S, Takeuchi M, Herrmann R, Straub V, Talim B, Voit T, Topaloglu H, Toda T, Endo T.; ''Muscular dystrophy and neuronal migration disorder caused by mutations in a glycosyltransferase, POMGnT1.''; PubMedEurope PMCScholia
Kolbinger F, Streiff MB, Katopodis AG.; ''Cloning of a human UDP-galactose:2-acetamido-2-deoxy-D-glucose 3beta-galactosyltransferase catalyzing the formation of type 1 chains.''; PubMedEurope PMCScholia
Ju T, Aryal RP, Kudelka MR, Wang Y, Cummings RD.; ''The Cosmc connection to the Tn antigen in cancer.''; PubMedEurope PMCScholia
Moremen KW, Tiemeyer M, Nairn AV.; ''Vertebrate protein glycosylation: diversity, synthesis and function.''; PubMedEurope PMCScholia
Samyn-Petit B, Krzewinski-Recchi MA, Steelant WF, Delannoy P, Harduin-Lepers A.; ''Molecular cloning and functional expression of human ST6GalNAc II. Molecular expression in various human cultured cells.''; PubMedEurope PMCScholia
Bennett EP, Mandel U, Clausen H, Gerken TA, Fritz TA, Tabak LA.; ''Control of mucin-type O-glycosylation: a classification of the polypeptide GalNAc-transferase gene family.''; PubMedEurope PMCScholia
Kim YJ, Kim KS, Kim SH, Kim CH, Ko JH, Choe IS, Tsuji S, Lee YC.; ''Molecular cloning and expression of human Gal beta 1,3GalNAc alpha 2,3-sialytransferase (hST3Gal II).''; PubMedEurope PMCScholia
Inamori K, Hara Y, Willer T, Anderson ME, Zhu Z, Yoshida-Moriguchi T, Campbell KP.; ''Xylosyl- and glucuronyltransferase functions of LARGE in α-dystroglycan modification are conserved in LARGE2.''; PubMedEurope PMCScholia
Zheng H, Li Y, Ji C, Li J, Zhang J, Yin G, Xu J, Ye X, Wu M, Zou X, Gu S, Xie Y, Mao Y.; ''Characterization of a cDNA encoding a protein with limited similarity to beta1, 3-N-acetylglucosaminyltransferase.''; PubMedEurope PMCScholia
Manzini MC, Tambunan DE, Hill RS, Yu TW, Maynard TM, Heinzen EL, Shianna KV, Stevens CR, Partlow JN, Barry BJ, Rodriguez J, Gupta VA, Al-Qudah AK, Eyaid WM, Friedman JM, Salih MA, Clark R, Moroni I, Mora M, Beggs AH, Gabriel SB, Walsh CA.; ''Exome sequencing and functional validation in zebrafish identify GTDC2 mutations as a cause of Walker-Warburg syndrome.''; PubMedEurope PMCScholia
Stevens E, Carss KJ, Cirak S, Foley AR, Torelli S, Willer T, Tambunan DE, Yau S, Brodd L, Sewry CA, Feng L, Haliloglu G, Orhan D, Dobyns WB, Enns GM, Manning M, Krause A, Salih MA, Walsh CA, Hurles M, Campbell KP, Manzini MC, UK10K Consortium, Stemple D, Lin YY, Muntoni F.; ''Mutations in B3GALNT2 cause congenital muscular dystrophy and hypoglycosylation of α-dystroglycan.''; PubMedEurope PMCScholia
Stamenkovic I, Asheim HC, Deggerdal A, Blomhoff HK, Smeland EB, Funderud S.; ''The B cell antigen CD75 is a cell surface sialytransferase.''; PubMedEurope PMCScholia
Harduin-Lepers A, Stokes DC, Steelant WF, Samyn-Petit B, Krzewinski-Recchi MA, Vallejo-Ruiz V, Zanetta JP, Augé C, Delannoy P.; ''Cloning, expression and gene organization of a human Neu5Ac alpha 2-3Gal beta 1-3GalNAc alpha 2,6-sialyltransferase: hST6GalNAcIV.''; PubMedEurope PMCScholia
Wells L.; ''The o-mannosylation pathway: glycosyltransferases and proteins implicated in congenital muscular dystrophy.''; PubMedEurope PMCScholia
Inamori K, Yoshida-Moriguchi T, Hara Y, Anderson ME, Yu L, Campbell KP.; ''Dystroglycan function requires xylosyl- and glucuronyltransferase activities of LARGE.''; PubMedEurope PMCScholia
Bistrup A, Bhakta S, Lee JK, Belov YY, Gunn MD, Zuo FR, Huang CC, Kannagi R, Rosen SD, Hemmerich S.; ''Sulfotransferases of two specificities function in the reconstitution of high endothelial cell ligands for L-selectin.''; PubMedEurope PMCScholia
Iwai T, Inaba N, Naundorf A, Zhang Y, Gotoh M, Iwasaki H, Kudo T, Togayachi A, Ishizuka Y, Nakanishi H, Narimatsu H.; ''Molecular cloning and characterization of a novel UDP-GlcNAc:GalNAc-peptide beta1,3-N-acetylglucosaminyltransferase (beta 3Gn-T6), an enzyme synthesizing the core 3 structure of O-glycans.''; PubMedEurope PMCScholia
Weh E, Reis LM, Tyler RC, Bick D, Rhead WJ, Wallace S, McGregor TL, Dills SK, Chao MC, Murray JC, Semina EV.; ''Novel B3GALTL mutations in classic Peters plus syndrome and lack of mutations in a large cohort of patients with similar phenotypes.''; PubMedEurope PMCScholia
Van den Steen P, Rudd PM, Dwek RA, Opdenakker G.; ''Concepts and principles of O-linked glycosylation.''; PubMedEurope PMCScholia
Nakayama J, Yeh JC, Misra AK, Ito S, Katsuyama T, Fukuda M.; ''Expression cloning of a human alpha1, 4-N-acetylglucosaminyltransferase that forms GlcNAcalpha1-->4Galbeta-->R, a glycan specifically expressed in the gastric gland mucous cell-type mucin.''; PubMedEurope PMCScholia
Röttger S, White J, Wandall HH, Olivo JC, Stark A, Bennett EP, Whitehouse C, Berger EG, Clausen H, Nilsson T.; ''Localization of three human polypeptide GalNAc-transferases in HeLa cells suggests initiation of O-linked glycosylation throughout the Golgi apparatus.''; PubMedEurope PMCScholia
Hofsteenge J, Huwiler KG, Macek B, Hess D, Lawler J, Mosher DF, Peter-Katalinic J.; ''C-mannosylation and O-fucosylation of the thrombospondin type 1 module.''; PubMedEurope PMCScholia
Luo Y, Nita-Lazar A, Haltiwanger RS.; ''Two distinct pathways for O-fucosylation of epidermal growth factor-like or thrombospondin type 1 repeats.''; PubMedEurope PMCScholia
Bierhuizen MF, Fukuda M.; ''Expression cloning of a cDNA encoding UDP-GlcNAc:Gal beta 1-3-GalNAc-R (GlcNAc to GalNAc) beta 1-6GlcNAc transferase by gene transfer into CHO cells expressing polyoma large tumor antigen.''; PubMedEurope PMCScholia
Inamori K, Willer T, Hara Y, Venzke D, Anderson ME, Clarke NF, Guicheney P, Bönnemann CG, Moore SA, Campbell KP.; ''Endogenous glucuronyltransferase activity of LARGE or LARGE2 required for functional modification of α-dystroglycan in cells and tissues.''; PubMedEurope PMCScholia
Huang C, Zhou J, Wu S, Shan Y, Teng S, Yu L.; ''Cloning and tissue distribution of the human B3GALT7 gene, a member of the beta1,3-Glycosyltransferase family.''; PubMedEurope PMCScholia
Schwientek T, Nomoto M, Levery SB, Merkx G, van Kessel AG, Bennett EP, Hollingsworth MA, Clausen H.; ''Control of O-glycan branch formation. Molecular cloning of human cDNA encoding a novel beta1,6-N-acetylglucosaminyltransferase forming core 2 and core 4.''; PubMedEurope PMCScholia
Togayachi A, Akashima T, Ookubo R, Kudo T, Nishihara S, Iwasaki H, Natsume A, Mio H, Inokuchi J, Irimura T, Sasaki K, Narimatsu H.; ''Molecular cloning and characterization of UDP-GlcNAc:lactosylceramide beta 1,3-N-acetylglucosaminyltransferase (beta 3Gn-T5), an essential enzyme for the expression of HNK-1 and Lewis X epitopes on glycolipids.''; PubMedEurope PMCScholia
Di Costanzo S, Balasubramanian A, Pond HL, Rozkalne A, Pantaleoni C, Saredi S, Gupta VA, Sunu CM, Yu TW, Kang PB, Salih MA, Mora M, Gussoni E, Walsh CA, Manzini MC.; ''POMK mutations disrupt muscle development leading to a spectrum of neuromuscular presentations.''; PubMedEurope PMCScholia
This CandidateSet contains sequences identified by William Pearson's analysis of Reactome catalyst entities. Catalyst entity sequences were used to identify analagous sequences that shared overall homology and active site homology. Sequences in this Candidate set were identified in an April 24, 2012 analysis.
Glycoprotein N acetylgalactosamine 3 beta galactosyltransferase 1 (C1GALT1; MIM:610555) mediates the transfer of Galactose (Gal) from UDP galactose to single O-linked GalNAc residues (Tn antigens) to form Core 1 structures on glycoproteins. C1GALT1 is active when in complex with the molecular chaperone C1GALT1C1 (aka COSMC; MIM:300611) which assists the folding and/or stability of C1GALT1. Defects in C1GALT1C1 causes somatic Tn polyagglutination syndrome (TNPS; MIM:300622), characterised by the polyagglutination of erythrocytes by naturally occurring anti Tn antibodies following exposure of the Tn antigen on their surface. Defects in C1GALT1C1 render C1GALT1 inactive and results in the accumulation of the incompletely glycosylated Tn antigen. The Tn antigen is tumour associated, found in a majority of human carcinomas, and is not normally expressed in peripheral tissues or blood cells (Crew et al. 2008, Ju et al. 2014). C1GALT1 and C1GALT1C1 belong to the CAZy family GT31 (CAZy.org).
Human beta-1,3-glucosyltransferase-like protein (B3GALTL, HGNC Approved Gene Symbol: B3GLCT; MIM:610308; CAZy family GT31), localised on the ER membrane, glucosylates O-fucosylated proteins. The resultant glc-beta-1,3-fuc disaccharide modification on thrombospondin type 1 repeat (TSR1) domain-containing proteins is thought to assist in the secretion of many of these proteins from the ER lumen, and mediate an ER quality-control mechanism of folded TSRs (Vasudevan et al. 2015). Defects in B3GALTL can cause Peters plus syndrome (PpS; MIM:261540), an autosomal recessive disorder characterised by anterior eye chamber defects, short stature, delay in growth and mental developmental and cleft lip and/or palate (Heinonen & Maki 2009). More than 10 mutations in B3GALTL causing PsP are known (Weh et al. 2014) including the missense mutation G393E (Dassie Ajdid et al. 2009).
The O-fucosylation of proteins containing thrombospondin type 1 repeat (TSR1) domains is carried out by protein fucosyltransferase 2 (POFUT2). Only POFUT2 recognises the consensus sequence CSXS/TCG found in TSR1 domains and the fucosyl residue is attached to the hydroxyl group of conserved serine (S) or threonine (T) residues within the consensus sequence (Chen et al. 2012). The modification was first demonstrated on thrombospondin 1 (THBS1, TSP1), found in platelets and the ECM (Hofsteenge et al. 2001, Luo et al. 2006). In humans, there are more than 60 of these TSR1 domain-containing proteins. Only the proteins with similarity to experimentally confirmed ones are listed as candidates.
Co-expression of both protein O-mannosyl-transferases 1 and 2 (POMT1 and POMT2; CAZy family GT39) is necessary for enzyme activity (Manya et al. 2004), that is mediating the transfer of mannosyl residues to the hydroxyl group of serine or threonine residues of proteins such as alpha-dystroglycan (DAG1; MIM:128239). This process occurs in the ER lumen and both POMT isozymes are ER membrane residents. DAG1 is a cell surface protein that plays an important role in the assembly of the extracellular matrix in muscle, brain, and peripheral nerves by linking the basal lamina to cytoskeletal proteins. Defects in POMT2 (MIM:607439) results in defective glycosylation of DAG1 and can cause severe congenital muscular dystrophy dystroglycanopathies ranging from a severe type A, MDDGA2 (brain and eye abnormalities; MIM:613150), through a less severe type B, MDDGB2 (congenital form with mental retardation; MIM:613156) to a milder type C, MDDGC2 (limb girdle form; MIM:603158) (Bertini et al. 2011, Wells 2013).
Golgi membrane resident protein O-linked-mannose beta-1,2-N-acetylglucosaminyltransferase 1 (POMGNT1; MIM:606822) mediates the transfer of N-acetylglucosaminyl (GlcNAc) residues to mannosylated proteins in the Golgi lumen. It can transfer GlcNAc to mannose-O-serine-dystroglycan (Man-DAG1) with a beta-1,2 linkage. Defects in POMGNT1 result in disrupted glycosylation of DAG1 and can cause congenital muscular dystrophy-dystroglycanopathies of varying severity (Yoshida et al. 2001).
Glycosyltransferase-like proteins LARGE (MIM:603590) and LARGE2 (GYLTL1B; MIM:609709) are bifunctional glycosyltransferases with both xylosyltransferase and beta-1,3-glucuronyltransferase activities involved in the biosynthesis of a phosphorylated O-mannosyl trisaccharide (N-acetylgalactosamine-beta-3-N-acetylglucosamine-beta-4-(phosphate-6-)mannose), a structure present in alpha-dystroglycan (DAG1) which plays a key role in skeletal muscle function and regeneration (Inamori et al. 2012, Inamori et al. 2013, Wells 2013). LARGE and LARGE2 belong to the CAZy glycosyltransferase families GT8 and GT49.
Glycosyltransferase-like proteins LARGE and LARGE2 (GYLTL1B) are bifunctional glycosyltransferases with both xylosyltransferase and beta-1,3-glucuronyltransferase activities which can form polymeric Xyl-GlcA repeats. These enzymes are involved in the physiological function of alpha-dystroglycan (DAG1) which plays a key role in skeletal muscle function and regeneration (Inamori et al. 2013, Wells 2013, Inamori et al. 2014).
Alpha-1,4-N-acetylglucosaminyltransferase (A4GNT) can catalyse the transfer of N-acetylglucosamine (GlcNAc) to core 2 branched mucins, creating an alpha1,4-linkage with beta-Gal residues (arbitrarily named Core 2a mucins) (Nakayama et al. 1999, Zhang et al. 2001).
Carbohydrate sulfotransferase 4 (CHST4) transfers sulfate (SO4(2-)) from the high energy donor 3'-phospho-5'-adenylyl sulfate (PAPS) to position 6 of non-reducing N-acetylglucosamine (GlcNAc) residues of mucin-associated glycans that ultimately serve as SELL ligands which are present in high endothelial cells (HEVs) and play a central role in lymphocyte homing at sites of inflammation. CHST4 preferentially sulfates Core 2 mucins (Bistrup et al. 1999). CHST4 is localised to the Golgi membrane (de Graffenried & Bertozzi 2004).
Three enzymes are involved in the biosynthesis of a phosphorylated O-mannosyl trisaccharide structure (N-acetylgalactosamine-beta-1,3-N-acetylglucosamine-beta-1,4-(phosphate-6-)mannose) present in alpha-dystroglycan (DAG1), which is required for binding laminin G-like domain-containing extracellular proteins with high affinity. Defects in any of these enzymes can lead to congenital muscular dystrophy.
In the first step, protein O-linked-mannose beta-1,4-N-acetylglucosaminyltransferase 2 (POMGNT2) transfers N-acetyl-D-glucosamine (GlcNAc) to the 4-position of mannosylated DAG1 to generate N-acetyl-D-glucosamine-beta-1,4-O-D-mannosyl-DAG1 (Yoshida-Moriguchi et al. 2013). Defects in POMGNT2 can cause muscular dystrophy-dystroglycanopathy congenital with brain and eye anomalies A8 (MDDGA8), a congenital muscular dystrophy that severely affects the development of the brain, eyes, and muscle, profound mental retardation, and death usually in the first years of life. This phenotype is also described as Walker-Warburg syndrome (WWS), which represents the most severe end of a phenotypic spectrum of dystroglycanopathies (Manzini et al. 2012).
Three enzymes are involved in the biosynthesis of a phosphorylated O-mannosyl trisaccharide structure (N-acetylgalactosamine-beta-1,3-N-acetylglucosamine-beta-1,4-(phosphate-6-)mannose) present in alpha-dystroglycan (DAG1), which is required for binding laminin G-like domain-containing extracellular proteins with high affinity. Defects in any of these enzymes can lead to congenital muscular dystrophy.
The second step is catalysed by UDP-GalNAc:beta-1,3-N-acetylgalactosaminyltransferase 2 (B3GALNT2), an ER membrane-associated enzyme that transfers N-acetylgalactosamine (GalNAc) to GlcNAc-Man-DAG1 via a 1-3 glycosidic bond (Hiruma et al. 2004, Yoshida-Moriguchi et al. 2013). Defects in B3GALNT2 can cause muscular dystrophy-dystroglycanopathy congenital with brain and eye anomalies A11 (MDDGA11), a hypoglycosylation defect resulting in a reduced ability of DAG1 to bind laminin and other extracellular matrix ligands. The disorder is characterised by dystroglycanopathy with muscle and brain anomolies (Stevens et al. 2013).
Three enzymes are involved in the biosynthesis of a phosphorylated O-mannosyl trisaccharide structure (N-acetylgalactosamine-beta-1,3-N-acetylglucosamine-beta-1,4-(phosphate-6-)mannose) present in alpha-dystroglycan (DAG1), which is required for binding laminin G-like domain-containing extracellular proteins with high affinity. Defects in any of these enzymes can lead to congenital muscular dystrophy.
Once the mannosyl residue attached to DAG1 has had GlcNAc and GalNAc added to it by POMGNT2 and B3GALNT2 respectively, protein O-mannose kinase (POMK, SGK196) can phosphorylate position 6 of the mannosyl residue (Yoshida-Moriguchi et al. 2013). Defects in POMK can cause muscular dystrophy-dystroglycanopathy congenital with brain and eye anomalies A12 (MDDGA12), a congenital muscular dystrophy that disrupts normal muscle development leading to locomotor dysfuction (Di Costanzo et al. 2014).
The family of UDP GalNAc:polypeptide N acetylgalactosaminyltransferases (GalNAc transferases, GALNTs) carry out the addition of N acetylgalactosamine on serine, threonine or possibly tyrosine residues on a wide variety of proteins, and most commonly associated with mucins (Wandall et al. 1997). This reaction takes place in the Golgi apparatus (Rottger et al. 1998). There are 20 known members of the GALNT family, 15 of which have been characterised and 5 candidate members which are thought to belong to this family based on sequence similarity (Bennett et al. 2012). The GALNT-family is classified as belonging to CAZy family GT27.
An unknown N-acetylglucosaminyltransferase mediates the transfer of GlcNAc to Tn antigens via an alpha-1,3 linkage to create Core 5 mucins (Brockhausen et al. 2009).
An unknown galactosyltransferase mediates the transfer of galactose is transferred to Tn antigens via an alpha-1,3 linkage to create Core 8 mucins (Brockhausen et al. 2009).
An unknown N-acetylglucosaminyltransferase mediates the transfer of GlcNAc to Tn antigens via an beta-1,6 linkage to create Core 6 mucins (Brockhausen et al. 2009).
The UDP-GlcNAc:betaGal beta-1,3-N-acetylglucosaminyltransferase family (B3GNTs) consists of 9 members in humans (Kolbinger et al, 1998; Shiraishi et al, 2001; Togayachi et al, 2001; Iwai et al, 2002; Huang et al, 2004; Ishida et al, 2005; Zheng et al, 2004). They catalyse the addition of N-acetylglucosamine to the T antigen to form the Core 3 glycoprotein (Togayachi et al, 2006). Thie reaction occurs in the Golgi.
The human gene GCNT encodes beta-1,6-N-acetylglucosaminyltransferase which mediates core 2 O-glycan branching by the addition of N-acetylgalactosamine, an important step in mucin-type biosynthesis. There are 3 defined members in humans, 1, 3 and 4 (Bierhuizen and Fukuda, 1992; Yeh et al, 1999; Schwientek et al, 2000). Two members (6 and 7) may be part of the family based on sequence similarity.
An unknown N-acetylgalactosaminyltransferase mediates the transfer of GalNAc is transferred to Tn antigens via an alpha-1,6 linkage to create Core 7 mucins (Brockhausen et al. 2009).
Beta-galactoside alpha-2,6-sialyltransferase 1 (ST6GAL1) transfers sialic acid (Neu5Ac) from the donor CMP-Neu5Ac to galactose-containing acceptor substrates such as the Tn antigen (Stamenkovic et al. 1990).
Beta-galactoside alpha-2,6-sialyltransferase 1 (ST6GAL1) transfers sialic acid (Neu5Ac) from the donor CMP-Neu5Ac to the T antigen (Stamenkovic et al. 1990).
The human genes ST3GAL1-4 encode for sialyltransferases1-4 respectively (Shang et al. 1999, Kim et al. 1996, Kitagawa & Paulson 1993, Basu et al. 1993). They add silaic acid (Neu5Ac) onto the T antigen forming an alpha-3-sialyl O-glycan.
The human genes ST6GALNAC3 and 4 encode GalNAc alpha-2,6-sialyltransferase III and IV respectively which can add sialic acid (Neu5Ac) to sialyl T antigen to produce a disialyl T antigen. ST6GALNAC4 is characterised (Harduin-Lepers et al. 2000) while ST6GALNAC3 is thought to perform a similar function based on sequence similarity.
The huma gene ST6GALNAC2 encodes GalNAc alpha-2,6-sialyltransferase II which mediates the transfer of sialic acid (Neu5Ac) onto the T antigen (Samyn-Petit et al. 2000).
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In the first step, protein O-linked-mannose beta-1,4-N-acetylglucosaminyltransferase 2 (POMGNT2) transfers N-acetyl-D-glucosamine (GlcNAc) to the 4-position of mannosylated DAG1 to generate N-acetyl-D-glucosamine-beta-1,4-O-D-mannosyl-DAG1 (Yoshida-Moriguchi et al. 2013). Defects in POMGNT2 can cause muscular dystrophy-dystroglycanopathy congenital with brain and eye anomalies A8 (MDDGA8), a congenital muscular dystrophy that severely affects the development of the brain, eyes, and muscle, profound mental retardation, and death usually in the first years of life. This phenotype is also described as Walker-Warburg syndrome (WWS), which represents the most severe end of a phenotypic spectrum of dystroglycanopathies (Manzini et al. 2012).
The second step is catalysed by UDP-GalNAc:beta-1,3-N-acetylgalactosaminyltransferase 2 (B3GALNT2), an ER membrane-associated enzyme that transfers N-acetylgalactosamine (GalNAc) to GlcNAc-Man-DAG1 via a 1-3 glycosidic bond (Hiruma et al. 2004, Yoshida-Moriguchi et al. 2013). Defects in B3GALNT2 can cause muscular dystrophy-dystroglycanopathy congenital with brain and eye anomalies A11 (MDDGA11), a hypoglycosylation defect resulting in a reduced ability of DAG1 to bind laminin and other extracellular matrix ligands. The disorder is characterised by dystroglycanopathy with muscle and brain anomolies (Stevens et al. 2013).
Once the mannosyl residue attached to DAG1 has had GlcNAc and GalNAc added to it by POMGNT2 and B3GALNT2 respectively, protein O-mannose kinase (POMK, SGK196) can phosphorylate position 6 of the mannosyl residue (Yoshida-Moriguchi et al. 2013). Defects in POMK can cause muscular dystrophy-dystroglycanopathy congenital with brain and eye anomalies A12 (MDDGA12), a congenital muscular dystrophy that disrupts normal muscle development leading to locomotor dysfuction (Di Costanzo et al. 2014).