Department of Health and Human Services, Section on Biological Chemistry, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892, USA.
Glycobiology. 2012 Jun;22(6):768-77. doi: 10.1093/glycob/cwr183. Epub 2011 Dec 20.
The formation of mucin-type O-glycans is initiated by an evolutionarily conserved family of enzymes, the UDP-N-acetyl-α-D-galactosamine:polypeptide N-acetylgalactosaminyltransferases (GalNAc-Ts). The human genome encodes 20 transferases; 17 of which have been characterized functionally. The complexity of the GalNAc-T family reflects the differential patterns of expression among the individual enzyme isoforms and the unique substrate specificities which are required to form the dense arrays of glycans that are essential for mucin function. We report the expression patterns and enzymatic activity of the remaining three members of the family and the further characterization of a recently reported isoform, GalNAc-T17. One isoform, GalNAcT-16 that is most homologous to GalNAc-T14, is widely expressed (abundantly in the heart) and has robust polypeptide transferase activity. The second isoform GalNAc-T18, most similar to GalNAc-T8, -T9 and -T19, completes a discrete subfamily of GalNAc-Ts. It is widely expressed and has low, albeit detectable, activity. The final isoform, GalNAc-T20, is most homologous to GalNAc-T11 but lacks a lectin domain and has no detectable transferase activity with the panel of substrates tested. We have also identified and characterized enzymatically active splice variants of GalNAc-T13 that differ in the sequence of their lectin domain. The variants differ in their affinities for glycopeptide substrates. Our findings provide a comprehensive view of the complexities of mucin-type O-glycan formation and provide insight into the underlying mechanisms employed to heavily decorate mucins and mucin-like domains with carbohydrate.
黏蛋白型 O-聚糖的形成是由进化上保守的酶家族——UDP-N-乙酰-α-D-半乳糖胺：多肽 N-乙酰半乳糖胺基转移酶（GalNAc-Ts）启动的。人类基因组编码 20 种转移酶；其中 17 种已被功能表征。GalNAc-T 家族的复杂性反映了各个酶同工型的表达模式的差异，以及形成对黏蛋白功能至关重要的密集聚糖的独特的底物特异性。我们报告了该家族其余三个成员的表达模式和酶活性，以及对最近报道的同工型 GalNAc-T17 的进一步表征。同工型 GalNAcT-16 与 GalNAc-T14 最同源，广泛表达（在心脏中丰富表达），并具有强大的多肽转移酶活性。第二个同工型 GalNAc-T18 与 GalNAc-T8、-T9 和 -T19 最相似，完成了 GalNAc-Ts 的一个离散亚家族。它广泛表达，活性低，但仍可检测到。最后一个同工型 GalNAc-T20 与 GalNAc-T11 最相似，但缺乏凝集素结构域，并且在用测试的底物进行检测时没有可检测到的转移酶活性。我们还鉴定并表征了 GalNAc-T13 的酶活性剪接变体，它们在凝集素结构域的序列上有所不同。变体在其对糖肽底物的亲和力上有所不同。我们的发现提供了对黏蛋白型 O-聚糖形成复杂性的全面了解，并深入了解了用于用碳水化合物对黏蛋白和黏蛋白样结构域进行重度修饰的潜在机制。
