| *601774 | |||||||||||||||||||||
| ALPHA-1,6-MANNOSYL-GLYCOPROTEIN BETA-1,6-N-ACETYLGLUCOSAMINYLTRANSFERASE; MGAT5 | |||||||||||||||||||||
| Alternative titles; symbols | |||||||||||||||||||||
| N-ACETYLGLUCOSAMINYLTRANSFERASE V GNT-V GNT-VA | |||||||||||||||||||||
| HGNC Approved Gene Symbol: MGAT5 | |||||||||||||||||||||
| Cytogenetic location: 2q21.3 Genomic coordinates (GRCh37): 2:135,206,959 - 135,212,191 (from NCBI) | |||||||||||||||||||||
| TEXT | |||||||||||||||||||||
| Description | |||||||||||||||||||||
| The MGAT5 gene encodes beta-1,6-N-acetylglucosaminyltransferase V (EC 2.4.1.155), an enzyme that catalyzes the addition of beta-1,6-GlcNAc to N-glycan intermediates found on newly synthesized glycoproteins transiting the medial Golgi (Saito et al., 1994). | |||||||||||||||||||||
| Cloning | |||||||||||||||||||||
| Saito et al. (1994) cloned the cDNA for human N-acetylglucosaminyltransferase V+ (GNT-V) from a fetal liver cDNA library. This gene encodes a 741-amino acid polypeptide that is 97% identical to rat GnT-V. By Northern blot analysis, Inamori et al. (2003) found highest MGAT5 expression in brain, heart, kidney, and placenta, with lower levels in other tissues. RNA dot blot analysis confirmed widespread MGAT5 expression. Using real-time PCR, Kaneko et al. (2003) detected MGAT5 expression in all tissues examined. Highest expression was in spleen, followed by pancreas, placenta, and testis, and expression was almost undetectable in skeletal muscle. | |||||||||||||||||||||
| Mapping | |||||||||||||||||||||
| Saito et al. (1994) used fluorescence in situ hybridization to map the MGAT5 gene to chromosome 2q21. | |||||||||||||||||||||
| Gene Function | |||||||||||||||||||||
| Using transiently transfected COS-7 cells, Kaneko et al. (2003) showed that both GNT-VA and GNT-VB (MGAT5B; 612441) transferred GlcNAc onto synthetic saccharides and N-linked glycopeptides. Unlike GNT-VA, GNT-VB required Mg(2+) for full activity. Chinese hamster ovary cells expressing either GNT-VA or GNT-VB acquired the ability to bind L-phytohemagglutinin, confirming that both enzymes synthesized N-linked beta-1,6-branched glycans. Pinho et al. (2009) demonstrated that wildtype E-cadherin (see CDH1; 192090) regulated MGAT3 (604621) transcription, resulting in increased GnT-III expression. GnT-III and GnT-V competitively modified E-cadherin N-glycans. RNAi-knockdown of GnT-III in MCF-7/AZ cells revealed membrane delocalization of E-cadherin leading to its cytoplasmic accumulation. Further, GnT-III knockdown in cells also caused modifications of E-cadherin N-glycans catalyzed by GnT-III and GnT-V. Pinho et al. (2009) proposed a bidirectional crosstalk between E-cadherin and GnT-III/GnT-V, which may influence tumor progression and metastasis. | |||||||||||||||||||||
| Animal Model | |||||||||||||||||||||
| Malignant transformation is accompanied by increased beta-1,6-GlcNAc branching of N-glycans attached to Asn-X-Ser/Thr sequences in mature glycoproteins. The amount of MGAT5 products correlates with disease progression. Granovsky et al. (2000) generated Mgat5-deficient mice, which are born healthy but develop various abnormalities as adults. The Polyomavirus middle T antigen (PyMT) viral oncogene activates pathways leading to breast cancer in mouse models. Granovsky et al. (2000) crossed PyMT-transgenic mice with Mgat5 -/- mice and found increased tumor latency, slower tumor growth, and reduced incidence of metastases in PyMT/Mgat5 -/- mice. Microscopic and immunoblot analysis of fibroblasts indicated that focal adhesion turnover and signaling through PI3K/Akt (see 602925 and 164730) are defective in Mgat5 -/- mice. Granovsky et al. (2000) suggested that inhibitors of MGAT5 may be useful in the treatment of malignancies by targeting their dependency on focal adhesion signaling for proliferation. Specific glycan structures regulate lymphocyte adhesion, recirculation, and maturation, as seen in LAD2 (266265) patients, who are GDP-fucose-deficient. Activation of T cells requires the clustering of a threshold number of T cell receptors (TCRs) at the antigen presentation site. This number is reduced by CD28 (186760) coreceptor recruitment of signaling proteins to TCRs. Depletion of beta-1,6-GlcNAc-modified glycans potentiates antigen-dependent T-cell proliferation. Demetriou et al. (2001) noted that Mgat5-deficient mice are normal in terms of surface glycoprotein expression in naive T cells, but develop enlarged spleens, leukocyte colonies in kidneys, and, in some, hematuria with glomerulonephritis after 12 months of age. Type IV delayed-type hypersensitivity responses to oxazolone peaked later and higher and persisted longer than in wildtype mice. In contrast to reports with CD28 -/- mice, low doses of myelin basic protein (MBP; 159430) caused a higher incidence of experimental autoimmune encephalomyelitis (EAE) in Mgat5 -/- mice than in controls. In vitro, splenic T cells hyperproliferated in response to anti-TCR (see 186880) antibody or anti-CD3E (186830). Addition of anti-CD28 enhanced proliferation in both wildtype and mutant mice. T cells from Mgat5 knockout mice are totally unresponsive to the T-cell mitogen leukoagglutinin (L-PHA), which binds specifically to Mgat5-modified glycans. Deconvolution microscopy and FACS analysis demonstrated enhanced clustering, followed by internalization, of TCRs and actin filaments in response to ligands in T cells from Mgat5-deficient mice. This response could be mimicked in Mgat5 +/+ mice by preincubation of T cells with lactose, but not with control disaccharides. Immunoprecipitation analysis indicated an enhanced association of CD3Z (186780) with Zap70 (176947) and, therefore, enhanced signal transduction in Mgat5 -/- compared to Mgat5 +/+ mice. Both incubation with lactose and Mgat5 deficiency disrupt the interaction of the macrophage galactose-specific lectin, galectin-3 (LGALS3; 153619), with TCR complex proteins. Demetriou et al. (2001) concluded that Mgat5 deficiency increases the number of TCRs recruited to the antigen-presenting surface, thereby reducing the requirement for CD28 coreceptor engagement. In addition, Mgat5-dependent glycosylation sequesters receptors in a cell-surface galectin-glycoprotein lattice. Mgat5 and CD28 appear to function as opposing regulators of T-cell activation thresholds. Partridge et al. (2004) reported that expression of Mgat5 sensitized mouse cells to multiple cytokines. Lgals3 crosslinked Mgat5-modified N-glycans on epidermal growth factor and transforming growth factor-beta receptors at the cell surface and delayed their removal by constitutive endocytosis. Mgat5 expression in mammary carcinoma was rate limiting for cytokine signaling and consequently for epithelial-mesenchymal transition, cell motility, and tumor metastasis. Mgat5 also promoted cytokine-mediated leukocyte signaling, phagocytosis, and extravasation in vivo. Partridge et al. (2004) concluded that conditional regulation of N-glycan processing drives synchronous modification of cytokine receptors, which balances their surface retention against loss through endocytosis. | |||||||||||||||||||||
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