Proteolytic fragmentation of inositol 1,4,5-trisphosphate receptors: a novel mechanism regulating channel activity?

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are a family of ubiquitously expressed intracellular Ca2+ release channels. Regulation of channel activity by Ca2+, nucleotides, phosphorylation, protein binding partners and other cellular factors is thought to play a major role in defining the specific spatiotemporal characteristics of intracellular Ca2+ signals. These properties are, in turn, believed pivotal for the selective and specific physiological activation of Ca2+-dependent effectors. IP3Rs are also substrates for the intracellular cysteine proteases, calpain and caspase. Cleavage of the IP3R has been proposed to play a role in apoptotic cell death by uncoupling regions important for IP3 binding from the channel domain, leaving an unregulated leaky Ca2+ pore. Contrary to this hypothesis, we demonstrate following proteolysis that N- and C-termini of IP3R1 remain associated, presumably through non-covalent interactions. Further, we show that complementary fragments of IP3R1 assemble into tetrameric structures and retain their ability to be regulated robustly by IP3. While peptide continuity is clearly not necessary for IP3-gating of the channel, we propose that cleavage of the IP3R peptide chain may alter other important regulatory events to modulate channel activity. In this scenario, stimulation of the cleaved IP3R may support distinct spatiotemporal Ca2+ signals and activation of specific effectors. Notably, in many adaptive physiological events, the non-apoptotic activities of caspase and calpain are demonstrated to be important, but the substrates of the proteases are poorly defined. We speculate that proteolytic fragmentation may represent a novel form of IP3R regulation, which plays a role in varied adaptive physiological processes.