Molecular Cloning of ILP-2, a Novel Member of the Inhibitor of Apoptosis Protein Family

Inhibitor of apoptosis protein (IAP)-like protein-1 (ILP-1) (also known as X-linked IAP [XIAP] and mammalian IAP homolog A [MIHA]) is a potent inhibitor of apoptosis and exerts its effects, at least in part, by the direct association with and inhibition of specific caspases. Here, we describe the molecular cloning and characterization of a human gene related to ILP-1, termed ILP-2. Despite high homology to ILP-1, ILP-2 is encoded by a distinct gene, which in normal tissues is expressed solely in testis. In contrast to ILP-1, overexpression of ILP-2 had no protective effect on apoptosis mediated by Fas (also known as CD95) or tumor necrosis factor. However, ILP-2 potently inhibited apoptosis induced by overexpression of Bax or by coexpression of caspase 9 with Apaf-1, and preincubation of cytosolic extracts with ILP-2 abrogated caspase activation in vitro. A processed form of caspase 9 could be coprecipitated with ILP-2 from cells, suggesting a physical interaction between ILP-2 and caspase 9. Thus, ILP-2 is a novel IAP family member with restricted specificity for caspase 9.