Okkenhaug K, Rottapel R
Department of Immunology, University of Toronto, Toronto, Ontario M5S 1A2, Canada.
J Biol Chem. 1998 Aug 14;273(33):21194-202. doi: 10.1074/jbc.273.33.21194.
CD28 provides a costimulatory signal that results in optimal activation of T cells. The signal transduction pathways necessary for CD28-mediated costimulation are presently unknown. Engagement of CD28 leads to its tyrosine phosphorylation and subsequent binding to Src homology 2 (SH2)-containing proteins including the p85 subunit of phosphatidylinositol 3'-kinase (PI3K); however, the contribution of PI3K to CD28-dependent costimulation remains controversial. Here we show that CD28 is capable of binding the Src homology 3 (SH3) domains of several proteins, including Grb2. The interaction between Grb2 and CD28 is mediated by the binding of Grb2-SH3 domains to the C-terminal diproline motif present in the cytoplasmic domain of CD28. While the affinity of the C-terminal SH3 domain of Grb2 for CD28 is greater than that of the N-terminal SH3 domain, optimal binding requires both SH3 domains. Ligation of CD28, but not tyrosine-phosphorylation, is required for the SH3-mediated binding of Grb2 to CD28. We propose a model whereby the association of Grb2 with CD28 occurs via an inducible SH3-mediated interaction and leads to the recruitment of tyrosine-phosphorylated proteins such as p52(shc) bound to the SH2 domain of Grb2. The inducible interaction of Grb2 to the C-terminal region of CD28 may form the basis for PI3K-independent signaling through CD28.
CD28提供一种共刺激信号，可导致T细胞的最佳激活。目前尚不清楚CD28介导的共刺激所需的信号转导途径。CD28的结合会导致其酪氨酸磷酸化，并随后与含Src同源2（SH2）结构域的蛋白质结合，包括磷脂酰肌醇3'-激酶（PI3K）的p85亚基；然而，PI3K对CD28依赖性共刺激的作用仍存在争议。在这里，我们表明CD28能够结合几种蛋白质的Src同源3（SH3）结构域，包括Grb2。Grb2与CD28之间的相互作用是由Grb2的SH3结构域与CD28胞质结构域中存在的C末端双脯氨酸基序的结合介导的。虽然Grb2的C末端SH3结构域对CD28的亲和力大于N末端SH3结构域，但最佳结合需要两个SH3结构域。Grb2与CD28的SH3介导结合需要CD28的连接，但不需要酪氨酸磷酸化。我们提出了一个模型，其中Grb2与CD28的结合通过诱导性的SH3介导的相互作用发生，并导致募集与Grb2的SH2结构域结合的酪氨酸磷酸化蛋白，如p52（shc）。Grb2与CD28 C末端区域的诱导性相互作用可能构成通过CD28进行PI3K非依赖性信号传导的基础。
