Backbone and side-chain resonance assignments of (Ca 2+ ) 4 -calmodulin bound to beta calcineurin A CaMBD peptide.

Calcineurin (CaN) is a heterodimeric and highly conserved serine/threonine phosphatase (PP2B) that plays a critical role in coupling calcium signals to physiological processes including embryonic cardiac development, NF-AT-regulated gene expression in immune responses, and apoptosis. The catalytic subunit (CaN _A ) has three isoforms (α, β, and γ,) in humans and seven isoforms in Paramecium. In all eukaryotes, the EF-hand protein calmodulin (CaM) regulates CaN activity in a calcium-dependent manner. The N- and C-domains of CaM (CaM _N and CaM _C ) recognize a CaM-binding domain (CaMBD) within an intrinsically disordered region of CaN _A that precedes the auto-inhibitory domain (AID) of CaN _A . Here we present nearly complete ¹ H, ¹³ C, and ¹⁵ N resonance assignments of (Ca ²⁺ ) ₄ -CaM bound to a peptide containing the CaMBD sequence in the beta isoform of CaN _A (βCaN _A -CaMBDp). Its secondary structure elements predicted from the assigned chemical shifts were in good agreement with those observed in the high-resolution structures of (Ca ²⁺ ) ₄ -CaM bound to CaMBDs of multiple enzymes. Based on the reported literature, the CaMBD of the α isoform of CaN _A can bind to CaM in two opposing orientations which may influence the regulatory function of CaM. Because a high resolution structure of (Ca ²⁺ ) ₄ -CaM bound to βCaN _A -CaMBDp has not been reported, our studies serve as a starting point for determining the solution structure of this complex. This will demonstrate the preferred orientation of (Ca ²⁺ ) ₄ -CaM on the CaMBD as well as the orientations of CaM _N and CaM _C relative to each other and to the AID of βCaN _A .