Anti-apoptotic protein which acts by inhibiting the activities of CASP3, CASP7 and CASP9. Can inhibit the autocleavage of pro-CASP9 and cleavage of pro-CASP3 by CASP9. Capable of inhibiting CASP9 autoproteolysis at 'Asp-315' and decreasing the rate of auto proteolysis at 'Asp-330'. Acts as a mediator of neuronal survival in pathological conditions. Prevents motor-neuron apoptosis induced by a variety of signals. Possible role in the prevention of spinal muscular atrophy that seems to be caused by inappropriate persistence of motor-neuron apoptosis: mutated or deleted forms of NAIP have been found in individuals with severe spinal muscular atrophy. Acts as a sensor component of the NLRC4 inflammasome that specifically recognizes and binds needle protein CprI from pathogenic bacteria C.violaceum. Association of pathogenic bacteria proteins drives in turn drive assembly and activation of the NLRC4 inflammasome, promoting caspase-1 activation, cytokine production and macrophage pyroptosis. The NLRC4 inflammasome is activated as part of the innate immune response to a range of intracellular bacteria such as C.violaceum and L.pneumophila. Interacts (via NACHT domain) with APAF1 (via CARD and NACHT domains). Interacts with C.violaceum needle protein CprI. Expressed in motor neurons, but not in sensory neurons. Found in liver and placenta, and to a lesser extent in spinal cord. Both the BIR and NACHT domains are essential for effective inhibition of pro-CASP9 cleavage. BIR3 domain binds to procaspase-9 and the NACHT domain interacts with the NACHT domain of APAF1 forming a bridge between pro-CASP9 and APAF1.