Immunoreceptor Tyrosine-Based Inhibitory Motif (ITIM)-Mediated Inhibitory Signaling Is Regulated by Sequential Phosphorylation Mediated by Distinct Nonreceptor Tyrosine Kinases: A Case Study Involving
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- 作者：Benjamin E. Tourdot ; Michelle K. Brenner ; Kathleen C. Keough ; Trudy Holyst ; Peter J. Newman ; Debra K. Newman
- 刊名：Biochemistry
- 出版年：2013
- 出版时间：April 16, 2013
- 年：2013
- 卷：52
- 期：15
- 页码：2597-2608
- 全文大小：490K
- 年卷期：v.52,no.15(April 16, 2013)
- ISSN：1520-4995
文摘
The activation state of many blood and vascular cells is tightly controlled by a delicate balance between receptors that contain immunoreceptor tyrosine-based activation motifs (ITAMs) and those that contain immunoreceptor tyrosine-based inhibitory motifs (ITIMs). Precisely how the timing of cellular activation by ITAM-coupled receptors is regulated by ITIM-containing receptors is, however, poorly understood. Using platelet endothelial cell adhesion molecule 1 (PECAM-1) as a prototypical ITIM-bearing receptor, we demonstrate that initiation of inhibitory signaling occurs via a novel, sequential process in which Src family kinases phosphorylate the C-terminal ITIM, thereby enabling phosphorylation of the N-terminal ITIM of PECAM-1 by other Src homology 2 domain-containing nonreceptor tyrosine kinases (NRTKs). NRTKs capable of mediating the second phosphorylation event include C-terminal Src kinase (Csk) and Bruton鈥檚 tyrosine kinase (Btk). Btk and Csk function downstream of phosphatidylinositol 3-kinase (PI3K) activation during ITAM-dependent platelet activation. In ITAM-activated platelets that were treated with a PI3K inhibitor, PECAM-1 was phosphorylated but did not bind the tandem SH2 domain-containing tyrosine phosphatase SHP-2, indicating that it was not phosphorylated on its N-terminal ITIM. Csk bound to and phosphorylated PECAM-1 more efficiently than did Btk and required its SH2 domain to perform these functions. Additionally, the phosphorylation of the N-terminal ITIM of Siglec-9 by Csk is enhanced by the prior phosphorylation of its C-terminal ITIM, providing evidence that the ITIMs of other dual ITIM-containing receptors are also sequentially phosphorylated. On the basis of these findings, we propose that sequential ITIM phosphorylation provides a general mechanism for precise temporal control over the recruitment and activation of tandem SH2 domain-containing tyrosine phosphatases that dampen ITAM-dependent signals.