Proteolytic activation of Etk/Bmx tyrosine kinase by caspases

Etk/Bmx is a member of the Btk/Tec family of kinases, which are characteriz ed 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 caspa ses during apoptosis, in vitro translated [S-35]methionine-labeled Etk was incubated with different apoptotic extracts and recombinant caspases, respe ctively. Results showed that Etk was proteolyzed in all conditions tested w ith identical cleavage patterns. Caspase-mediated cleavage of Etk generated a C-terminal fragment, containing the complete SH2 and tyrosine kinase dom ains, but without intact pleckstrin homology and SH3 domains. This fragment has 4-fold higher kinase activity than that of the full-length Etk, Ectopi c expression of the C-terminal fragment of Etk sensitized the PC3 prostate cancer cells to apoptosis in response to apoptosis-inducing stimuli. The fi nding, together with an earlier report that Etk is potentially anti-apoptot ic, suggests that Etk may serve as an apoptotic switch, depending on the fo rms of Etk existing inside the cells. To our knowledge, this is the first c ase where the activity of a tyrosine kinase is induced by caspase cleavage.