Department of Physiology, University of Manitoba, Winnipeg, Manitoba, Canada.
J Mol Endocrinol. 2012 Apr 2;48(3):203-16. doi: 10.1530/JME-11-0064. Print 2012 Jun.
Transglutaminase 2 (TG2) is an enzyme with diverse biological functions. TG2 catalyzes transamidation reactions, has intrinsic kinase activity, and acts as a G-protein in intracellular signaling. TG2 (Tgm2)-null mice are glucose intolerant and have impaired glucose-stimulated insulin secretion (GSIS). Furthermore, three naturally occurring missense mutations in the human TGM2 gene, corresponding to amino acid substitutions of Met330Arg, Ile331Asn, and Asn333Ser in the TG2 protein, have been reported and found to be associated with early-onset type 2 diabetes. However, their effect on TG2 function is not fully understood. To determine this, we have reproduced naturally occurring mutations in TG2 using site-directed mutagenesis. Overexpression of Myc-TG2 mutants in INS-1E cells resulted in a reduction of GSIS in comparison with cells overexpressing wild-type Myc-TG2 (WT-TG2). The maximum reduction was found in cells overexpressing Ile331Asn-TG2 (32%) followed by Met330Arg-TG2 (20%), and the least in Asn333Ser-TG2 (7%). Enzymatic analysis revealed that TG2 mutants have impaired transamidation and kinase activities in comparison with WT-TG2. GTP-binding assays showed that TG2 mutants also have altered GTP-binding ability, which is found to be modulated in response to glucose stimulation. Collectively, these data suggest that naturally occurring mutations in TG2 affect transamidation, kinase, and GTP-binding functions of TG2. While reduced insulin secretion, as a result of naturally occurring mutations in TG2, is due to the impairment of more than one biological function of TG2, it is the transamidation function that appears to be impaired during the first phase, whereas the GTP-binding function affects the second phase of insulin secretion.
转谷氨酰胺酶 2（TG2）是一种具有多种生物学功能的酶。TG2 催化转酰胺反应，具有内在的激酶活性，并作为细胞内信号转导中的 G 蛋白发挥作用。TG2（Tgm2）- 敲除小鼠不耐受葡萄糖，且葡萄糖刺激的胰岛素分泌（GSIS）受损。此外，已经报道了人类 TGM2 基因中的三个天然存在的错义突变，对应于 TG2 蛋白中 Met330Arg、Ile331Asn 和 Asn333Ser 氨基酸取代，并且与早发性 2 型糖尿病相关。然而，它们对 TG2 功能的影响尚不完全清楚。为了确定这一点，我们使用定点突变在 TG2 中重现了天然存在的突变。与过表达野生型 Myc-TG2（WT-TG2）的细胞相比，Myc-TG2 突变体在 INS-1E 细胞中的过表达导致 GSIS 降低。在过表达 Ile331Asn-TG2（32%）的细胞中发现最大降低，其次是 Met330Arg-TG2（20%），而在过表达 Asn333Ser-TG2（7%）的细胞中发现最小降低。酶分析显示，与 WT-TG2 相比，TG2 突变体的转酰胺和激酶活性受损。GTP 结合测定表明，TG2 突变体也具有改变的 GTP 结合能力，发现该能力可响应葡萄糖刺激而调节。总之，这些数据表明，TG2 中的天然突变影响 TG2 的转酰胺、激酶和 GTP 结合功能。虽然由于 TG2 中的天然突变导致胰岛素分泌减少，但这是由于 TG2 的多种生物学功能受损所致，似乎在第一阶段，转酰胺功能受损，而 GTP 结合功能影响胰岛素分泌的第二阶段。
