Apo and InsP3-bound crystal structures of the ligand-binding domain of an InsP3 receptor

We report the crystal structures of the ligand-binding domain (LBD) of a rat inositol 1,4,5-trisphosphate receptor (InsP(3)R) in its apo and InsP(3)-bound conformations. Comparison of these two conformations reveals that LBD's first beta-trefoil fold (beta-TF1) and armadillo repeat fold (ARF) move together as a unit relative to its second beta-trefoil fold (beta-TF2). Whereas apo LBD may spontaneously transition between gating conformations, InsP(3) binding shifts this equilibrium toward the active state.