Able to form cytoplasmic structures termed death effector filaments. Enhances APAF1 and cytochrome c-dependent activation of pro-caspase-9 and consecutive apoptosis. Stimulates apoptosis through activation of caspase-3. Involved in activation of caspase-1 and caspase-5 as part of the NALP1 inflammasome complex which leads to processing and release of IL1B and IL18. Binds ATP. Interacts strongly with caspase-2, weakly with caspase-9 and with APAF1 in a cytochrome c-inducible way, leading to the formation of an apoptosome. This interaction may be ATP-dependent. Part of the NALP1 inflammasome complex which is involved in activation of caspase-1 and caspase-5, leading to processing of IL1B and IL18. The complex is activated by bacterial muramyl dipeptide which triggers ATP-binding and oligomerization of NALP1. Widely expressed. Isoform 1 and isoform 2 are expressed in peripheral blood leukocytes and chronic myelogenous leukemia cell line K-562, followed by thymus, spleen and heart. Also detected in brain, lung, placenta, small intestine, colon, kidney, liver, muscle, testis and epithelial cells. Absent from hematopoietic progenitor cells but expressed upon differentiation of cells into granulocytes and, to a lesser extent, monocytes. In peripheral blood cells, highest levels are found in T-lymphocytes, granulocytes and monocytes. Expression is significantly increased in bone marrow blast cells of some acute leukemia patients but not in solid tumors. Belongs to the NLRP family. 4 isoforms of the human protein are produced by alternative splicing. Note: This description may include information from UniProtKB.