Structural insights into enzyme regulation for inositol 1,4,5-trisphosphate 3-kinase B

D-Myoinositol 1,4,5-trisphophate 3-kinases (IP3-Ks) play important roles in metazoan cellular signaling. It has been demonstrated that mice without a functional version Of IP3-3K isoform B are deficient in peripheral T-cells, indicating that IP3-3KB is essential to the developing immune system. The recent apo IP3-3KA structure exhibited a helix at the catalytic domain N-terminus exhibited a helix at the N-terminus of the catalytic domain, with a tryptophan indole moiety mimicking the binding mode of the substrate ATP purine ring, suggesting a mechanism of autoinhibition. Here we present the structure of the complete catalytic domain Of IP3-3KB, including the CaM binding domain in complex with Mg2+ and ATP. The crystal structure reveals a homodimeric arrangement Of IP3-3KB catalytic domains, mediated via an intermolecular antiparallel beta-sheet formed from part of the CaM binding region. Residues from the putative autoinhibitory helix are rearranged into a loop configuration, with extensive interactions with the bound ATP. Mutagenesis of residues from this region reveals that substitution of the putative autoinhibitory tryptophan generates a hyperactive enzyme which retains Ca2+/CaM sensitivity. The IP3-3KB structure suggests a mechanism of enzyme activation, and raises the possibility that an interaction between IP3-3KB molecules may occur as part of the catalytic or regulatory cycle.