Wu Y M, Huang C L, Kung H J, Huang C Y
Division of Molecular and Genomic Medicine, National Health Research Institutes, Taipei 115, Taiwan, Republic of China.
J Biol Chem. 2001 May 25;276(21):17672-8. doi: 10.1074/jbc.M010964200. Epub 2001 Mar 2.
Etk/Bmx is a member of the Btk/Tec family of kinases, which are characterized by having a pleckstrin homology domain at the N terminus, in addition to the Src homology 3 (SH3), SH2, and the catalytic domains, shared with the Src family kinases. Etk, or Btk kinases in general, has been implicated in the regulation of apoptosis. To test whether Etk is the substrate for caspases during apoptosis, in vitro translated [(35)S]methionine-labeled Etk was incubated with different apoptotic extracts and recombinant caspases, respectively. Results showed that Etk was proteolyzed in all conditions tested with identical cleavage patterns. Caspase-mediated cleavage of Etk generated a C-terminal fragment, containing the complete SH2 and tyrosine kinase domains, but without intact pleckstrin homology and SH3 domains. This fragment has 4-fold higher kinase activity than that of the full-length Etk. Ectopic expression of the C-terminal fragment of Etk sensitized the PC3 prostate cancer cells to apoptosis in response to apoptosis-inducing stimuli. The finding, together with an earlier report that Etk is potentially antiapoptotic, suggests that Etk may serve as an apoptotic switch, depending on the forms of Etk existing inside the cells. To our knowledge, this is the first case where the activity of a tyrosine kinase is induced by caspase cleavage.
Etk/Bmx是Btk/Tec激酶家族的成员，其特征是在N端具有一个普列克底物蛋白同源结构域，此外还具有与Src家族激酶共有的Src同源结构域3（SH3）、SH2和催化结构域。一般来说，Etk或Btk激酶与细胞凋亡的调控有关。为了检测Etk在细胞凋亡过程中是否是半胱天冬酶的底物，分别将体外翻译的[(35)S]甲硫氨酸标记的Etk与不同的凋亡提取物和重组半胱天冬酶一起孵育。结果表明，在所有测试条件下，Etk都以相同的切割模式被蛋白酶解。半胱天冬酶介导的Etk切割产生了一个C端片段，该片段包含完整的SH2和酪氨酸激酶结构域，但没有完整的普列克底物蛋白同源结构域和SH3结构域。这个片段的激酶活性比全长Etk高4倍。Etk C端片段的异位表达使PC3前列腺癌细胞对凋亡诱导刺激诱导的凋亡敏感。这一发现与早期关于Etk可能具有抗凋亡作用的报道一起表明，Etk可能作为一种凋亡开关，这取决于细胞内存在的Etk形式。据我们所知，这是酪氨酸激酶活性由半胱天冬酶切割诱导的首例。
