Roles ofRacregulatorsinpathogenesisofleukemia

Rho GTPases是G蛋白偶联受体、酪氨酸激酶受体和整合素介导的细胞信号转导的重要调节因子，因此得到了广泛的研究。RhoH是Rho家族成员之一，在造血细胞中特异性表达，包括骨髓祖细胞、分化的髓系细胞和淋巴样细胞。RhoH拮抗Rac，然而，使Rac位于RhoH下游的分子尚未被鉴定。在这里，我们进行GTP-RhoH亲和柱层析。用三重四极杆液相色谱-质谱联用仪鉴定了83种与活性RhoH相关的蛋白质。它们包括Rho GTP酶激活蛋白（RhoGAP）、丝氨酸/苏氨酸激酶、磷脂酰肌醇激酶等。其中，T细胞活化的RhoGTPaseactivatingprotein（TAGAP）是造血细胞特异性表达的蛋白质。TAGAP通过其GAP结构域与RhoH结合。TAGAP的全长和GAP结构域的表达有效地使Rac失活，而GAP结构域缺失的TAGAP则不能。这些结果表明RhoH通过TAGAP负调控Rac。

Rho GTPases have been widely studied because they are critical regulators of cellularsignaling mediated by G-protein-coupled receptors, tyrosine kinase receptors and integrins. RhoH, a member of the Rho family, is specifically expressed in the hematopoietic cells, including bone marrow progenitor, differentiated myeloid andlymphoid cells. RhoH antagonizes Rac, however, the molecules that inactivate Racdownstream of RhoH have not been identified. Here we perform GTP-RhoH affinity column chromatography. Using triple quadrupole liquid chromatography tandemmass spectrometer, 83 different proteins associated with active RhoH were identified. They include Rho GTPase activating proteins(RhoGAP), serine/threonine kinases, phosphatidylinositol kinases, and others. Among them, focused on T-cell activationRhoGTPaseactivatingprotein(TAGAP), whichisspecweifically expressed inthe hematopoietic cells. TAGAP binds to RhoH through its GAP domain. Expression of both full length and GAP domain of TAGAP effectively deactivates Rac, whereas GAP domain deleted TAGAP did not. These results suggest that RhoH negatively regulates Rac through TAGAP.