RESUMO
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.
Assuntos
Apoptose , Caspases/metabolismo , Inibidores Enzimáticos/metabolismo , Proteínas/metabolismo , Proteínas Proto-Oncogênicas c-bcl-2 , Sequência de Aminoácidos , Animais , Proteínas Reguladoras de Apoptose , Fator Apoptótico 1 Ativador de Proteases , Northern Blotting , Caspase 9 , Inibidores de Caspase , Linhagem Celular , Clonagem Molecular , Genes Reporter/genética , Humanos , Immunoblotting , Proteínas Inibidoras de Apoptose , Dados de Sequência Molecular , Plasmídeos/genética , Plasmídeos/metabolismo , Primatas , Proteínas/química , Proteínas/genética , Proteínas Proto-Oncogênicas/genética , Proteínas Proto-Oncogênicas/metabolismo , Proteínas Recombinantes de Fusão/genética , Proteínas Recombinantes de Fusão/metabolismo , Alinhamento de Sequência , Transfecção , Proteínas Inibidoras de Apoptose Ligadas ao Cromossomo X , Proteína X Associada a bcl-2RESUMO
The inhibitor of apoptosis protein (IAP) family consists of a number of evolutionarily conserved proteins that function to inhibit programmed cell death. X-linked IAP (XIAP) was cloned due to its sequence homology with other family members and has previously been shown to prevent apoptosis by binding to active caspases 3, 7, and 9 in vitro. XIAP transcripts can be found in a variety of tissues, and the protein levels are regulated both transcriptionally and posttranscriptionally. To better understand the function of XIAP in normal cells, we generated mice deficient in XIAP through homologous gene targeting. The resulting mice were viable, and histopathological analysis did not reveal any differences between XIAP-deficient and wild-type mice. We were unable to detect any defects in induction of caspase-dependent or -independent apoptosis in cells from the gene-targeted mice. One change was observed in cells derived from XIAP-deficient mice: the levels of c-IAP1 and c-IAP2 protein were increased. This suggests that there exists a compensatory mechanism that leads to upregulation of other family members when XIAP expression is lost. The changes in c-IAP1 and c-IAP2 expression may provide functional compensation for loss of XIAP during development or in the induction of apoptosis.
