Section of Molecular Carcinogenesis, Division of Hematology/Oncology, Childrens Hospital Los Angeles and the Saban Research Institute of Childrens Hospital, Los Angeles, California 90027, USA.
J Biol Chem. 2009 Dec 18;284(51):35645-51. doi: 10.1074/jbc.M109.062240.
Transglutaminase 2 (TG2) is a multifunctional protein that has been implicated in numerous pathologies including that of neurodegeneration and celiac disease, but the molecular interactions that mediate its diverse activities are largely unknown. Bcr and the closely related Abr negatively regulate the small G-protein Rac: loss of their combined function in vivo results in increased reactivity of innate immune cells. Bcr and Abr are GTPase-activating proteins that catalyze the hydrolysis of the GTP bound to Rac. However, how the Bcr and Abr GTPase-activating activity is regulated is not precisely understood. We here report a novel mechanism of regulation through direct protein-protein interaction with TG2. TG2 bound to the Rac-binding pocket in the GTPase-activating domains of Bcr and Abr, blocked Bcr activity and, through this mechanism, increased levels of active GTP-bound Rac and EGF-stimulated membrane ruffling. TG2 exists in at least two different conformations. Interestingly, experiments using TG2 mutants showed that Bcr exhibits preferential binding to the non-compacted conformation of TG2, in which its catalytic domain is exposed, but transamidation is not needed for the interaction. Thus, TG2 regulates levels of cellular GTP-bound Rac and actin cytoskeletal reorganization through a new mechanism involving direct inhibition of Bcr GTPase-activating activity.
转谷氨酰胺酶 2（TG2）是一种多功能蛋白，与多种病理学有关，包括神经退行性疾病和乳糜泻，但介导其多种活性的分子相互作用在很大程度上尚不清楚。Bcr 和密切相关的 Abr 负调节小 G 蛋白 Rac：它们在体内联合功能的丧失导致先天免疫细胞的反应性增加。Bcr 和 Abr 是 GTPase 激活蛋白，可催化与 Rac 结合的 GTP 的水解。然而，Bcr 和 Abr GTPase 激活活性的调节方式尚不清楚。我们在这里报告了一种通过与 TG2 的直接蛋白-蛋白相互作用进行调节的新机制。TG2 与 Bcr 和 Abr 的 GTPase 激活结构域中的 Rac 结合口袋结合，阻断了 Bcr 的活性，并通过这种机制增加了活性 GTP 结合 Rac 的水平和 EGF 刺激的膜皱襞。TG2 至少存在两种不同的构象。有趣的是，使用 TG2 突变体的实验表明，Bcr 优先与 TG2 的非紧凑构象结合，在这种构象中其催化结构域暴露，但不需要转酰胺化来进行相互作用。因此，TG2 通过一种新的机制调节细胞内 GTP 结合 Rac 的水平和肌动蛋白细胞骨架的重组，该机制涉及直接抑制 Bcr GTPase 激活活性。
