Dynamic regulation of Src-family tyrosine kinases by CD45

抗原受体连接后，src家族蛋白酪氨酸激酶（Src-PTKs）的激活决定了免疫应答的启动。Src-PTKs的活性受到两个酪氨酸残基（自磷酸化和cooh末端负调控位点）的酪氨酸磷酸化的调控。CD45，原型受体型蛋白酪氨酸磷酸酶，已被证明是Src-PTKs的主要调节因子。然而，CD45发挥其作用的确切方式仍然存在争议。我们早期的研究表明，CD45通过去磷酸化自纤维磷酸化和负调节酪氨酸残基来抑制B细胞系中的Lyn活性，并且B细胞受体（BCR）连接诱导两个调节位点的磷酸化，表明CD45对Lyn的作用在BCR连接后减弱。在本报告中，我们报告了与迄今为止报道的大多数研究相反，CD45的负调控通常在B细胞中起作用，并且一些CD45与糖脂富集微域（GEMs）组成性相关，在那里它抑制Src-PTK活性。然而，在BCR连接后，CD45在1分钟内与GEMs分离，导致Src-PTKs活化，然后在60分钟内与GEMs重新结合。CD45不参与IgM、Src-PTKs和Csk在gem中的运动调节。我们提出gem相关CD45的主要作用是抑制Src-PTK，其CD45的动态行为决定了Src-PTK的激活水平和细胞命运。

Initiation of immune responses is determined by activation of Src-family protein tyrosine kinases (Src-PTKs) upon antigen receptor ligation. Activity of Src-PTKs is regulated, among others, by tyrosine phosphorylation of two tyrosine residues: the autophosphorylation and COOH-terminal negative regulatory sites. CD45, the prototypic receptor-type protein tyrosine phosphatase, has been shown to be a major regulator of Src-PTKs. The precise way in which CD45 exerts its effect is still controversial, however. Our earlier studies showed that CD45 inhibits Lyn activity in a B cell line by dephosphorylating both the autopfibsphorylation and negative regulatory tyrosine residues, and that B cell receptor (BCR) ligation induces phosphorylation of both regulatory sites, suggesting CD45 action on Lyn is diminished upon BCR ligation. In this presentation, we report that in contrast to most studies reported thus far, negative regulation by CD45 is generally operative in B cells, and that some CD45 is constitutively associated with glycolipid-enriched microdomains (GEMs), where it inhibits Src-PTK activity. Upon BCR ligation, however, CD45 dissociates from GEMs within 1 min, leading to activation of Src-PTKs, and then subsequently re-associates with the GEMs within 60 min. CD45 is not involved in the regulation of movement of IgM, Src-PTKs and Csk with respect to GEMs. We propose that the primary role of GEM-associated CD45 is inhibition of Src-PTKs, and that its dynamic behavior of CD45 determines the level of Src-PTK activation and the cell fate.