Identification of an endogenous dominant-negative short isoform of caspase-9 that can regulate apoptosis

Alternatively spliced isoforms of certain Apoptosis regulators, such as Bcl-x, Ced-4, and Ich-1, have been shown to play opposing roles in regulating Apoptosis. Here, we describe the identification of an endogenous alternatively spliced isoform of caspase-9, named caspase-9b, which lacks the central large subunit Caspase domain. Caspase-9b is detectable in many cell lines by PCR and at the mRNA and protein levels. Caspase-9b can interact with the Caspase recruitment domain of Apaf-1, and like the active site mutant of caspase-9, it can inhibit multiple forms of Apoptosis, including those triggered by oligomerization of death receptors. It can also block activation of caspase-9 and -3 by Apaf-1 in an in vitro cytochrome c-dependent Caspase activation assay. These results suggest that caspase-9b functions as an endogenous Apoptosis inhibitory molecule by interfering with the formation of a functional Apaf-1-caspase-9 complex.