Crk is required for apoptosis in Xenopus egg extracts

Apoptosis is essential for the development and homeostasis of multicellular organisms. Recently, a cell-free extract prepared from Xenopus eggs was shown to recapitulate intracellular apoptotic pathways in vitro. While many stimuli have been shown to trigger apoptosis in a variety of cell types, the intracellular signaling pathways involved in apoptosis remain largely unknown. Here we show that addition of a recombinant protein containing the phosphotyrosine binding (SH2) domain from the adaptor protein crk, but not those derived from a panel of other signaling proteins, can prevent apoptosis in the Xenopus egg extract system. Furthermore, immunodepletion of endogenous crk protein from the egg extracts, or addition of anti-crk antisera to these extracts, prevents apoptosis. The ability to undergo apoptosis can be restored to these extracts by addition of recombinant crk protein. These results directly demonstrate that crk participates in apoptotic signaling.