Fang D, Wang H Y, Fang N, Altman Y, Elly C, Liu Y C
Division of Cell Biology, La Jolla Institute for Allergy and Immunology, San Diego, CA 92121, USA.
J Biol Chem. 2001 Feb 16;276(7):4872-8. doi: 10.1074/jbc.M008901200. Epub 2000 Nov 21.
Cbl-b is implicated in setting the threshold of T lymphocyte activation. In Cbl-b-deficient T cells, the activation of Vav, a guanine nucleotide exchange factor, is significantly enhanced. The molecular mechanism underlying Cbl-b-regulated Vav activation was unclear. Here it is shown that Cbl-b interacts with and induces ubiquitin conjugation to the p85 regulatory subunit of phosphatidylinositol 3-kinase, an upstream regulator of Vav. A functional RING finger of Cbl-b was essential for p85 ubiquitination. However, a loss of function mutation at the well-conserved amino-terminal variant src homology (SH) 2 domain of Cbl-b did not affect its ligase activity. A distal carboxyl-terminal proline-rich region in Cbl-b was mapped to contain the primary binding sequences for the SH3 domain of p85. Deletion of either the distal proline-rich region in Cbl-b or the SH3 domain of p85 severely reduced ubiquitin conjugation to p85. The data suggest a molecular link for Cbl-b-mediated negative regulation of Vav, with phosphatidylinositol 3-kinase as a direct target for Cbl-b E3 ubiquitin ligase.
Cbl-b与设定T淋巴细胞激活阈值有关。在Cbl-b缺陷的T细胞中，鸟嘌呤核苷酸交换因子Vav的激活显著增强。Cbl-b调节Vav激活的分子机制尚不清楚。本文表明，Cbl-b与磷脂酰肌醇3激酶的p85调节亚基相互作用并诱导其泛素化，磷脂酰肌醇3激酶是Vav的上游调节因子。Cbl-b的功能性环状结构域对p85泛素化至关重要。然而，Cbl-b保守的氨基末端可变src同源（SH）2结构域的功能丧失突变并不影响其连接酶活性。Cbl-b中一个远端羧基末端富含脯氨酸的区域被定位为包含p85的SH3结构域的主要结合序列。删除Cbl-b中的远端富含脯氨酸区域或p85的SH3结构域会严重降低p85的泛素化。这些数据表明了Cbl-b介导的Vav负调控的分子联系，磷脂酰肌醇3激酶是Cbl-b E3泛素连接酶的直接靶点。
