The Distal Helix in the Regulatory Domain of Calcineurin Is Important for Domain Stability and Enzyme Function

Calcineurin (CaN) is a calmodulin-activated, serine/threonine phosphatase that is necessary for cardiac, vasculature, and nervous system development, as well as learning and memory, skeletal muscle growth, and immune system activation. CaN is activated in a manner similar to that of the calmodulin (CaM)-activated kinases. CaM binds CaN's regulatory domain (RD) and causes a conformational change that removes CaN's autoinhibitory domain (AID) from its catalytic site, activating CaN. In the CaM-activated kinases, the CaM binding region (CaMBR) is located just C-terminal to the AID, whereas in CaN, the AID is 52 residues C-terminal to the CaMBR. Previously published data have shown that these 52 residues in CaN's RD are disordered but approximately half of them gain structure, likely α-helical, upon CaM binding. In this work, we confirm that this increase in the level of structure is α-helical. We posit that this region forms an amphipathic helix upon CaM binding and folds onto the remainder of the RD:CaM complex, removing the AID. Förster resonance energy transfer data suggest the C-terminal end of this distal helix is relatively close to the N-terminal end of the CaMBR when the RD is bound by CaM. We show by circular dichroism spectroscopy and thermal melts that mutations on the hydrophobic face of the distal helix disrupt the structure gained upon CaM binding. Additionally, kinetic analysis of CaN activity suggests that these mutations affect CaN's ability to bind substrate, likely a result of the AID being able to bind to the active site even when CaM is bound. Our data demonstrate the presence of this distal helix and suggest it folds onto the remainder of the RD:CaM complex, creating a hairpinlike chain reversal that removes the AID from the active site.