Xiao G H, Shoarinejad F, Jin F, Golemis E A, Yeung R S
Divisions of Medical, Fox Chase Cancer Center, Philadelphia, Pennsylvania 19111, USA.
J Biol Chem. 1997 Mar 7;272(10):6097-100. doi: 10.1074/jbc.272.10.6097.
The tuberous sclerosis complex 2 (TSC2) is a tumor suppressor gene that plays a causative role in the autosomal dominant syndrome of tuberous sclerosis. The latter is characterized by the development of hamartomas and occasional malignancies. Expression of the wild-type gene in TSC2 mutant tumor cells inhibits proliferation and tumorigenicity. This "suppressor" activity is encoded by functional domain(s) in the C terminus that contains homology to Rap1GAP. Using a yeast two-hybrid assay to identify proteins that interact with the C-terminal domain of tuberin, the product of TSC2, a cytosolic factor, rabaptin-5, was found to associate with a distinct domain lying adjacent to the TSC2 GAP homology region. Rabaptin-5 also binds the active form of GTPase Rab5. Immune complexes of native tuberin, as well as recombinant protein, possessed activity to stimulate GTP hydrolysis of Rab5. Tuberin GAP activity was specific for Rab5 and showed no cross-reactivity with Rab3a or Rab6. Cells lacking tuberin possessed minimal Rab5GAP activity and were associated with an increased uptake of horseradish peroxidase. Re-expression of tuberin in TSC2 mutant cells reduced the rate of fluid-phase endocytosis. These findings suggest that tuberin functions as a Rab5GAP in vivo to negatively regulate Rab5-GTP activity in endocytosis.
结节性硬化症复合物2（TSC2）是一种肿瘤抑制基因，在结节性硬化症的常染色体显性综合征中起致病作用。后者的特征是错构瘤的发展以及偶尔发生的恶性肿瘤。野生型基因在TSC2突变肿瘤细胞中的表达抑制增殖和致瘤性。这种“抑制”活性由C末端的功能域编码，该功能域与Rap1GAP具有同源性。使用酵母双杂交试验来鉴定与结节蛋白（TSC2的产物）的C末端结构域相互作用的蛋白质，发现一种胞质因子rabaptin-5与位于TSC2 GAP同源区域相邻的一个独特结构域相关联。Rabaptin-5也结合GTP酶Rab5的活性形式。天然结节蛋白以及重组蛋白的免疫复合物具有刺激Rab5的GTP水解的活性。结节蛋白的GAP活性对Rab5具有特异性，与Rab3a或Rab6没有交叉反应。缺乏结节蛋白的细胞具有最小的Rab5GAP活性，并与辣根过氧化物酶的摄取增加有关。TSC2突变细胞中结节蛋白的重新表达降低了液相内吞作用的速率。这些发现表明，结节蛋白在体内作为一种Rab5GAP发挥作用，以负向调节内吞作用中Rab5-GTP的活性。
