Post by Max on Nov 3, 2005 5:36:19 GMT -5
Glycosyltransferases
Carbohydrate structures are mainly determined by glycosyltransferases and glycosidases. Since the synthesis of oligosaccharides requires one enzyme for one glycosidic linkage, more than 100 kinds of glycosyltransferases seem to exist [1]. O-GlcNAc is an ubiquitous post-translational protein modification consisting of a single N-acetlyglucosamine moiety linked to serine or threonine residues on nuclear and cytoplasmic proteins. O-GlcNAc regulates cellular responses to hormones such as insulin, initiates a protective response to stress, modulates a cell's capacity to grow and divide, and regulates gene transcription. O-GlcNAc rapidly cycles on and off proteins on a time scale similar to that for phosphorylation/ dephosphorylation. O-GlcNAc is similar to a protein phosphorylation in that both modifications are found on serine and threonine residues, both are dynamically added and removed from proteins in response to cellular signals, and both alter the functions and associations of the modified protein. Many phosphorylation sites are also known glycosylation sites. However, the view that O-GlcNAc is simply reciprocal to phosphorylation is an overly simplistic model, since several proteins can be concomitantly phosphorylated and O-GlcNAcylated [2].
See picture in separate window
Figure 1. Slawson et al. (2005) O-GlcNAc cycling: How a single sugar post-translational modification is changing the way we think about signaling networks
O-GlcNAc is dynamically added to and removed from serine and threonine residues by O-GlcNAc transferase (OGT) and O-GlcNAcase, respectively. The human OGT trimer consists of tetratricopeptide repeats (TPR) as well as the transferase domain. O-GlcNAcase consists of a -N-acetylglucosaminidase domain and a HAT domain with a caspase-3 cleavage site between them.
Significant effects on GlcNAc in cells exposed to retinoic acid
Normal hepatocytes are characterized by very low level of GlcNAc-transferase-V activity whereas hepatoma cells contained high activities. Hepatoma cells exposed to retinoic acid and 1alpha,2,5-dihydroxyvitamin D(3) (Vit-D(3)) resulted in an increase in GlcNAc-transferase-V activity [1].
Relation between glycosyltransferases and transcription factor Sp1
Transcription factor Sp1 is O-glycosylated and contains N-acetylglucosamine side chains [Genecards database]. Sp1 is partially inhibited by O-linkage to glucosamine [GenAtlas]. O-glycosylation (O-GlcNAcylation), is the addition of single O-linked N-acetylglucosamine (O-GlcNAc) monosaccharides to serine or threonine residues [3]. O-GlcNAcylation of a chimeric transcriptional activator containing the second activation domain of Sp1 decreases its transcriptional activity both in an in vitro transcription system and in living cells, which is in concert with the observation that O-GlcNAcylation of Sp1 activation domain blocks its in vitro and in vivo interactions with other Sp1 molecules and TATA-binding protein-associated factor II 110 [4].
In glomerular mesangial cells, decreasing O-GlcNAcylation by these means inhibited the ability of high glucose (HG) to increase endogenous PAI-1 mRNA and protein levels, the activity of a PAI-1 promoter-luciferase reporter gene, and Sp1 transcriptional activation [3].
Carbohydrate structures are mainly determined by glycosyltransferases and glycosidases. Since the synthesis of oligosaccharides requires one enzyme for one glycosidic linkage, more than 100 kinds of glycosyltransferases seem to exist [1]. O-GlcNAc is an ubiquitous post-translational protein modification consisting of a single N-acetlyglucosamine moiety linked to serine or threonine residues on nuclear and cytoplasmic proteins. O-GlcNAc regulates cellular responses to hormones such as insulin, initiates a protective response to stress, modulates a cell's capacity to grow and divide, and regulates gene transcription. O-GlcNAc rapidly cycles on and off proteins on a time scale similar to that for phosphorylation/ dephosphorylation. O-GlcNAc is similar to a protein phosphorylation in that both modifications are found on serine and threonine residues, both are dynamically added and removed from proteins in response to cellular signals, and both alter the functions and associations of the modified protein. Many phosphorylation sites are also known glycosylation sites. However, the view that O-GlcNAc is simply reciprocal to phosphorylation is an overly simplistic model, since several proteins can be concomitantly phosphorylated and O-GlcNAcylated [2].
See picture in separate window
Figure 1. Slawson et al. (2005) O-GlcNAc cycling: How a single sugar post-translational modification is changing the way we think about signaling networks
O-GlcNAc is dynamically added to and removed from serine and threonine residues by O-GlcNAc transferase (OGT) and O-GlcNAcase, respectively. The human OGT trimer consists of tetratricopeptide repeats (TPR) as well as the transferase domain. O-GlcNAcase consists of a -N-acetylglucosaminidase domain and a HAT domain with a caspase-3 cleavage site between them.
Significant effects on GlcNAc in cells exposed to retinoic acid
Normal hepatocytes are characterized by very low level of GlcNAc-transferase-V activity whereas hepatoma cells contained high activities. Hepatoma cells exposed to retinoic acid and 1alpha,2,5-dihydroxyvitamin D(3) (Vit-D(3)) resulted in an increase in GlcNAc-transferase-V activity [1].
Relation between glycosyltransferases and transcription factor Sp1
Transcription factor Sp1 is O-glycosylated and contains N-acetylglucosamine side chains [Genecards database]. Sp1 is partially inhibited by O-linkage to glucosamine [GenAtlas]. O-glycosylation (O-GlcNAcylation), is the addition of single O-linked N-acetylglucosamine (O-GlcNAc) monosaccharides to serine or threonine residues [3]. O-GlcNAcylation of a chimeric transcriptional activator containing the second activation domain of Sp1 decreases its transcriptional activity both in an in vitro transcription system and in living cells, which is in concert with the observation that O-GlcNAcylation of Sp1 activation domain blocks its in vitro and in vivo interactions with other Sp1 molecules and TATA-binding protein-associated factor II 110 [4].
In glomerular mesangial cells, decreasing O-GlcNAcylation by these means inhibited the ability of high glucose (HG) to increase endogenous PAI-1 mRNA and protein levels, the activity of a PAI-1 promoter-luciferase reporter gene, and Sp1 transcriptional activation [3].