Department of Cell Biology, University of Texas Southwestern Medical Center at Dallas, Dallas, TX 75390, U.S.A.
Biochem J. 2010 Jan 27;426(1):91-8. doi: 10.1042/BJ20090615.
JAK2 (Janus tyrosine kinase 2) is important for signalling through many cytokine receptors, and a gain-of-function JAK2 mutation in its pseudokinase domain, V617F, has been implicated in Philadelphia chromosome-negative myeloproliferative neoplasms. How this mutation hyperactivates JAK2 is poorly understood. In the present paper we report our findings that the V617F mutation has little effect on the Vmax of JAK2 kinase activity, but lowers the Km value for substrates. Therefore under physiological conditions where the concentration level of substrates is presumably below saturation, JAK2(V617F) exhibits hyperactivation compared with wild-type JAK2. This lower Km of JAK2(V617F) towards substrates requires the JAK2 FERM (4.1/ezrin/radixin/moesin) domain, as deletion of the FERM domain abolished this effect. We also show that, in contrast with its positive role in JAK2(V617F) hyperactivation, the FERM domain in wild-type JAK2 is inhibitory. Deletion or mutations of the FERM domain resulted in increased basal JAK2 kinase activity. The results of the present study provide the biochemical basis for how V617F hyperactivates JAK2, and identifies novel regulating roles of the JAK2 FERM domain to control kinase activity at different activation states.
JAK2（Janus 酪氨酸激酶 2）对于许多细胞因子受体的信号传递很重要，其假激酶结构域中的功能获得性 JAK2 突变 V617F 与费城染色体阴性骨髓增殖性肿瘤有关。这种突变如何使 JAK2 过度激活还不太清楚。在本文中，我们报告了我们的发现，即 V617F 突变对 JAK2 激酶活性的 Vmax 影响很小，但降低了底物的 Km 值。因此，在生理条件下，底物的浓度水平可能低于饱和状态，与野生型 JAK2 相比，JAK2(V617F)表现出过度激活。JAK2(V617F)对底物的这种较低 Km 值需要 JAK2 FERM（4.1/ezrin/radixin/moesin）结构域，因为缺失 FERM 结构域会消除这种效应。我们还表明，与 JAK2(V617F)过度激活的积极作用相反，野生型 JAK2 中的 FERM 结构域具有抑制作用。FERM 结构域的缺失或突变导致基础 JAK2 激酶活性增加。本研究的结果为 V617F 如何过度激活 JAK2 提供了生化基础，并确定了 JAK2 FERM 结构域在不同激活状态下控制激酶活性的新调节作用。
