A provocative paper by Hong-Gang Wang and colleagues in this issue ( Nature Cell Biol. 2, 1–6; 2000) describes an unexpected pro-apoptotic (cell-death-promoting) function of the human RAD9 protein. This seems surprising given that the fission yeast (Schizosaccharomyces pombe) Rad9 protein does not appear to promote apoptosis in yeast, but in fact these results may not be as contradictory as one might think.

Wang and colleagues show that, when overexpressed in mammalian cells, human RAD9 interacts with the anti-apoptotic proteins BCL-2 and BCL-xL and sensitizes cells to death induced by growth-factor withdrawal. Both effects are inhibited by deleting a short region of RAD9 that shares weak sequence homology with a BCL-2 homology domain known as BH3. The authors obtain similar results when they express S. pombe Rad9 in mammalian cells. Expression of an antisense RAD9 DNA construct protects cells from the genotoxic effects of the DNA-damaging agent methyl methanesulphonate (MMS). MMS induces wild-type RAD9 to move to the nuclear envelope (right panel; the left panel shows RAD9 localization before treatment with MMS), where it localizes with BCL-2, but it does not induce the relocalization of a mutant RAD9 lacking the BH3-like domain. All of this suggests that RAD9 promotes cell death by antagonizing the anti-apoptotic activity of BCL-2-like proteins.

Picture courtesy of H. -G. Wang

But a pro-apoptotic role for RAD9 is surprising in light of the previously described function of mammalian and S. pombe Rad9 in cell-cycle arrest in situations where DNA is damaged or incompletely replicated. In mammalian cells, Rad9 might induce either cell-cycle arrest or cell death, according to the extent of DNA damage. But yeast rad9 mutants, which fail to arrest the cell cycle after irradiation-induced DNA damage, are hypersensitive to irradiation — which is at odds with Wang et al.’s data because one might think that, if S. pombe Rad9 were also pro-apoptotic, rad9 mutations would render cells more, not less, resistant to irradiation.

But actually the fact that Rad9 does not seem to promote apoptosis in yeast may not be that surprising. Although an apoptotic morphology can be induced in yeast by overexpression of mammalian pro-apoptotic proteins such as Bax or by mutating genes such as cdc48, cell death may not be a normal part of the yeast life cycle. The yeast genome does not seem to encode any other components of the mammalian cell-death machinery. Yeasts also seem to use a different machinery to respond to DNA damage: they do not express any homologues of p53, a protein required in most mammalian cells as a critical DNA-damage-checkpoint protein. Finally, although overexpression of mammalian Bax or Bak induces apoptosis in yeast, so-called ‘BH3-domain-only’ proteins of the Bcl-2 family (which, like human RAD9, do not contain the BH1, BH2 or BH4 domains) are non-functional in yeast (Wang et al.’s unpublished data).

It seems that, during evolution, RAD9 has acquired another function: it is involved not only in a DNA-damage checkpoint, but also in the control of apoptosis. It will be interesting to see whether other members of the Rad9 checkpoint family also promote cell death, and whether the interaction of human BCL-2 with RAD9 is relevant to the reported effect of overexpression of mammalian Bcl-2 in delaying entry into S phase.