Backbone and side-chain resonance assignments of (Ca²⁺)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 ¹H, ¹³C, and ¹⁵N resonance assignments of (Ca²⁺)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 (Ca²⁺)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 (Ca²⁺)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 (Ca²⁺)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.