Modulates apoptotic signal transduction or effector structures within the mitochondrial matrix. Affect cytochrome C release from the mitochondria and caspase 3 activation, but not caspase 8 activation. Isoform 1 increases apoptosis triggered by both TNF and the DNA-damaging agent mytomycin C; in sharp contrast, isoform 2 suppresses apoptosis. Can modulate IFN-gamma-mediated transcriptional activity (By similarity). Isoform 2 may play a role in neuromuscular junction development as an effector of the MUSK signaling pathway. Interacts with JAK2, HSPA9B and IFN-gammaR2 chain. Interacts with Ras GTPase-activating protein 1 (RASA1). Isoform 2 interacts with MUSK (via the cytoplasmic domain). Recruited to the postsynaptic cell membrane of the neuromuscular junction through interaction with MUSK. Modulation of apoptosis, i.e. proapoptotic activity of isoform 1 and antiapoptotic activity of isoform 2, is J domain-dependent (By similarity). Tyrosine phosphorylated.