Arabidopsis calmodulin-like protein CML36 is a calcium (Ca2+) sensor that interacts with the plasma membrane Ca2+-ATPase isoform ACA8 and stimulates its activity

Abstract

Calmodulin-like (CML) proteins are major EF-hand-containing, calcium (Ca²⁺)-binding proteins with crucial roles in plant development and in coordinating plant stress tolerance. Given their abundance in plants, the properties of Ca²⁺ sensors and identification of novel target proteins of CMLs deserve special attention. To this end, we recombinantly produced and biochemically characterized CML36 from <i>Arabidopsis thaliana</i> We analyzed Ca²⁺ and Mg²⁺ binding to the individual EF-hands, observed metal-induced conformational changes, and identified a physiologically relevant target. CML36 possesses two high-affinity Ca²⁺/Mg²⁺ mixed binding sites and two low-affinity Ca²⁺-specific sites. Binding of Ca²⁺ induced an increase in the α-helical content and a conformational change that lead to the exposure of hydrophobic regions responsible for target protein recognition. Cation binding, either Ca²⁺ or Mg²⁺, stabilized the secondary and tertiary structures of CML36, guiding a large structural transition from a molten globule apo-state to a compact holoconformation. Importantly, through <i>in vitro</i> binding and activity assays, we showed that CML36 interacts directly with the regulative N terminus of the <i>Arabidopsis</i> plasma membrane Ca²⁺-ATPase isoform 8 (ACA8) and that this interaction stimulates ACA8 activity. Gene expression analysis revealed that <i>CML36</i> and <i>ACA8</i> are co-expressed mainly in inflorescences. Collectively, our results support a role for CML36 as a Ca²⁺ sensor that binds to and modulates ACA8, uncovering a possible involvement of the CML protein family in the modulation of plant-autoinhibited Ca²⁺ pumps.