Conformational dynamics and target-dependent myristoyl switch of calcineurin B homologous protein 3

Calcineurin B homologous protein 3 (CHP3) is an EF-hand Ca²⁺-binding protein involved in regulation of cancerogenesis, cardiac hypertrophy and neuronal development via interactions with sodium/proton exchangers (NHEs) and signalling proteins. CHP3 binds Ca²⁺with micromolar affinity providing the basis to respond to intracellular Ca²⁺signals. Ca²⁺binding and myristoylation are important for CHP3 function but the underlying molecular mechanism remained elusive. Here, we show that Ca²⁺binding and myristoylation independently affect conformational dynamics and functions of human CHP3. Ca²⁺binding increased flexibility and hydrophobicity of CHP3 indicative of an open conformation. CHP3 in open Ca²⁺-bound conformation had higher affinity for NHE1 and associated stronger with lipid membranes compared to the closed Mg²⁺-bound conformation. Myristoylation enhanced flexibility of CHP3 and decreased its affinity to NHE1 independently of the bound ion, but did not affect its binding to lipid membranes. The data exclude the proposed Ca²⁺-myristoyl switch for CHP3. Instead, they document a Ca²⁺-independent exposure of the myristoyl moiety induced by binding of the target peptide to CHP3 enhancing its association to lipid membranes. We name this novel regulatory mechanism “target-dependent myristoyl switch”. Taken together, the interplay of Ca²⁺binding, myristoylation and target binding allows for a context-specific regulation of CHP3 functions.