Human junctophilin-2 undergoes a structural rearrangement upon binding PtdIns(3,4,5)P₃ and the S101R mutation identified in hypertrophic cardiomyopathy obviates this response

JP2 (junctophilin-2) is believed to hold the transverse tubular and jSR (junctional sarcoplasmic reticulum) membranes in a precise geometry that facilitates excitation-contraction coupling in cardiomyocytes. We have expressed and purified human JP2 and shown using electronmicroscopy that the protein forms elongated structures ~15 nm long and 2 nm wide. Employing lipid-binding assays and quartz crystal microbalance with dissipation we have determined that JP2 is selective for PS (phosphatidylserine), with a K_d value of ~0.5 μM, with the N-terminal domain mediating this interaction. JP2 also binds PtdIns(3,4,5)P₃ at a different site than PS, resulting in the protein adopting a more flexible conformation; this interaction is modulated by both Ca²⁺ and Mg²⁺ ions.We showthat the S101R mutation identified in patients with hypertrophic cardiomyopathy leads to modification of the protein secondary structure, forming a more flexible molecule with an increased affinity for PS, but does not undergo a structural transition in response to binding PtdIns(3,4,5)P₃. In conclusion, the present study provides new insights into the structural and lipid-binding properties of JP2 and how the S101R mutation may have an effect upon the stability of the dyad organization with the potential to alter JP2-protein interactions regulatingCa²⁺ cycling.