Zhang J, Guttmann R P, Johnson G V
Department of Psychiatry and Behavioral Neurobiology, University of Alabama at Birmingham, 35294-0017, USA.
J Neurochem. 1998 Jul;71(1):240-7. doi: 10.1046/j.1471-4159.1998.71010240.x.
Tissue transglutaminase (tTG) is a calcium-dependent enzyme that catalyzes the transamidation of specific polypeptide-bound glutamine residues, a reaction that is inhibited by GTP. There is also preliminary evidence that, in situ, calpain and GTP may regulate tTG indirectly by modulating its turnover by the calcium-activated protease calpain. In the present study, the in vitro and in situ proteolysis of tTG by calpain, and modulation of this process by GTP, was examined. tTG is an excellent substrate for calpain and is rapidly degraded. Previously it has been demonstrated that GTP binding protects tTG from degradation by trypsin. In a similar manner, guanosine-5'-O-(3-thiotriphosphate) protects tTG against proteolysis by calpain. Treatment of SH-SY5Y cells with 1 nM maitotoxin, which increases intracellular calcium levels, resulted in a significant increase in in situ TG activity, with only a slight decrease in tTG protein levels. In contrast, when GTP levels were depleted by pretreating the cells with tiazofurin, maitotoxin treatment resulted in an approximately 50% decrease in tTG protein levels, and a significant decrease in TG activity, compared with maitotoxin treatment alone. Addition of calpain inhibitors inhibited the degradation of tTG in response to the combined treatment of maitotoxin and tiazofurin and resulted in a significant increase in in situ TG activity. These studies indicate that tTG is an endogenous substrate of calpain and that GTP selectively inhibits the degradation of tTG by calpain.
组织转谷氨酰胺酶（tTG）是一种钙依赖性酶，它催化特定多肽结合的谷氨酰胺残基的转酰胺反应，该反应受GTP抑制。也有初步证据表明，在原位，钙蛋白酶和GTP可能通过调节钙激活蛋白酶钙蛋白酶对tTG的周转来间接调节tTG。在本研究中，检测了钙蛋白酶对tTG的体外和原位蛋白水解作用，以及GTP对该过程的调节作用。tTG是钙蛋白酶的优良底物，会迅速降解。此前已证明GTP结合可保护tTG不被胰蛋白酶降解。以类似方式，鸟苷-5'-O-（3-硫代三磷酸）可保护tTG免受钙蛋白酶的蛋白水解作用。用1 nM的 maitotoxin处理SH-SY5Y细胞，可增加细胞内钙水平，导致原位TG活性显著增加，而tTG蛋白水平仅略有下降。相反，当用替拉扎明预处理细胞使GTP水平降低时，与单独用maitotoxin处理相比，maitotoxin处理导致tTG蛋白水平下降约50%，TG活性显著降低。添加钙蛋白酶抑制剂可抑制tTG在maitotoxin和替拉扎明联合处理后的降解，并导致原位TG活性显著增加。这些研究表明，tTG是钙蛋白酶的内源性底物，GTP可选择性抑制钙蛋白酶对tTG的降解。
