Biological functions of O-mannosyl glycans

O-甘露糖基聚糖存在于脑、神经和骨骼肌的有限数量的糖蛋白中。α-肌营养不良聚糖是O-甘露糖基化蛋白质之一，是肌营养不良蛋白-糖蛋白复合物的中心组分，已被证明与肌营养不良症的发病有关。我们已经鉴定和表征了参与O-甘露糖基聚糖生物合成的糖基转移酶，蛋白O-甘露糖β 1，2-N-乙酰葡糖胺转移酶（POMGnT1）和蛋白O-甘露糖基转移酶1（POMT1）和POMT2。我们随后发现，POMGnT1基因功能的丧失是导致肌肉-眼-脑疾病（MEB）的原因。也有报道POMT 1/2基因与Walker-Warburg综合征（WWS）有关。我们发现POMT1与POMT2形成复合物，并且该复合物具有蛋白质O-甘露糖基转移酶活性。结果表明，POMT 1和POMT 2在体内物理和功能上相关联。最近，三个突变的POMP基因的患者谁表现出比典型的WWS更温和的表型报告。我们共表达了这些突变体POMT1和POMT2，发现它们都没有任何活性。然而，所有的POMT1突变体，包括以前确定的POMT1突变体与POMT2共沉淀。这些结果表明，突变体POMT1可以与POMT2形成杂合物，但这种复合物不足以产生酶活性。我们还表明，在果蝇中，dPOMT1和dPOMT2都需要功能酶活性，以促进体内正常肌肉发育。最后，我们展示了fukaline和POMGnT1之间的相互作用。fukutin-deficient组织中POMGnT1活性的降低可能表明fukutin-POMGnT1复合物的形成对于α-肌营养不良聚糖的功能性糖基化至关重要。这些结果表明α-肌营养不良蛋白聚糖的O-甘露糖基化在肌肉和脑发育中是重要的。

The O-mannosyl glycan is present in a limited number of glycoproteins of brain, nerve, and skeletal muscle. α-Dystroglycan is one of the O-mannosylated proteins and is a central component of the dystrophin-glycoprotein complex that has been shown to be related to the onset of muscular dystrophy. We have identified and characterized glycosyltransferases, protein O-mannose β1,2-N-acetylglucosaminyltransferase (POMGnT1) and protein O-mannosyltransferase 1 (POMT1) and POMT2, involved in the biosynthesis of O-mannosyl glycans. We subsequently found that loss of function of the POMGnT1 gene is responsible for muscle-eye-brain disease (MEB). It has also been reported that the POMT1/2 genes are responsible for Walker-Warburg syndrome (WWS). We show that POMT1 forms a complex with POMT2, and the complex possesses protein O-mannosyltransferase activity. Results indicate that POMT1 and POMT2 associate physically and functionally in vivo. Recently, three mutations were reported in the POMP gene of patients who showed milder phenotypes than typical WWS. We coexpressed these mutant POMT1s with POMT2 and found that none of them had any activity. However, all POMT1 mutants including previously identified POMT1 mutants coprecipitated with POMT2. These results indicate that the mutant POMTls could form heterocomplexes with POMT2 but that such complexes are insufficient for enzymatic activity. We also show that, in Drosophila, both dPOMT1 and dPOMT2 are required for functional enzymatic activity to contribute to normal muscle development in vivo. Finally, we demonstrate an interaction between fukutin and POMGnT1. Decreases in the POMGnT1 activity in fukutin-deficient tissues may indicate that the formation of the fukutin-POMGnT1 complex is crucial for functional glycosylation of α-dystroglycan. These results indicate that O-mannosylation of α-dystroglycan is important in muscle and brain development.