Zhang B, Zheng Y
Department of Biochemistry, University of Tennessee, Memphis, Tennessee 38163, USA.
J Biol Chem. 1998 Oct 2;273(40):25728-33. doi: 10.1074/jbc.273.40.25728.
The Rho family GTPases are tightly regulated between the active GTP-bound state and the inactive GDP-bound state in a variety of signal transduction processes. Here the Rho family members Cdc42, Rac2, and RhoA were found to form reversible homodimers in both the GTP- and the GDP-bound states. The homophilic interaction of Cdc42 and Rac2, but not RhoA, in the GTP-bound state, caused a significant stimulation of the intrinsic GTPase activity, i.e. the activated form of Cdc42 and Rac2 acts as GTPase-activating proteins toward Cdc42-GTP or Rac2-GTP. The dimerization of the GTPases appeared to be mediated by the carboxyl-terminal polybasic domain, and the specific GTPase-activating effects of Cdc42 and Rac2 were also attributed to the structural determinant(s) in the same region of the molecules. Moreover, similar to the case of Cdc42 and Cdc42GAP interaction, Cdc42-GDP interacted with tetrafluoroaluminate and Cdc42-GTPgammaS (guanosine 5'-3-O-(thio)triphosphate) to form a transition state complex of the GTPase-activating reaction in which the carboxyl-terminal determinant(s) of the GTPgammaS-bound Cdc42 plays a critical role. These results provide a rationale for the fast rate of intrinsic GTP hydrolysis by Cdc42 and Rac and suggest that dimerization may play a role in the negative regulation of specific Rho family GTPases mediated by the carboxyl-terminal polybasic domain.
在各种信号转导过程中，Rho家族GTP酶在活性GTP结合状态和非活性GDP结合状态之间受到严格调控。在此研究中发现，Rho家族成员Cdc42、Rac2和RhoA在GTP结合状态和GDP结合状态下均能形成可逆的同二聚体。在GTP结合状态下，Cdc42和Rac2（而非RhoA）的嗜同性相互作用显著刺激了内在GTP酶活性，即Cdc42和Rac2的活化形式作为针对Cdc42-GTP或Rac2-GTP的GTP酶激活蛋白发挥作用。GTP酶的二聚化似乎由羧基末端多碱性结构域介导，Cdc42和Rac2的特异性GTP酶激活作用也归因于分子同一区域的结构决定因素。此外，与Cdc42和Cdc42GAP相互作用的情况类似，Cdc42-GDP与四氟铝酸盐和Cdc42-GTPγS（鸟苷5'-3-O-(硫代)三磷酸）相互作用，形成GTP酶激活反应的过渡态复合物，其中结合GTPγS的Cdc42的羧基末端决定因素起关键作用。这些结果为Cdc42和Rac内在GTP快速水解提供了理论依据，并表明二聚化可能在由羧基末端多碱性结构域介导的特定Rho家族GTP酶的负调控中发挥作用。
