Calcium-binding and its effect on circular dichroism of plant calmodulin

Structural plasticity of calmodulin on the surface of CaF2 nanoparticles preserves its biological function

Nanoparticles are increasingly used in biomedical applications and are especially attractive as biocompatible and biodegradable protein delivery systems. Herein, the interaction between biocompatible 25 nm CaF2 nanoparticles and the ubiquitous calcium sensor calmodulin has been investigated in order to assess the potential of these particles to serve as suitable surface protein carriers. Calmodulin is a multifunctional messenger protein that activates a wide variety of signaling pathways in eukaryotic cells by changing its conformation in a calcium-dependent manner. Isothermal titration calorimetry and circular dichroism studies have shown that the interaction between calmodulin and CaF2 nanoparticles occurs with physiologically relevant affinity and that the binding process is fully reversible, occurring without significant alterations in protein secondary and tertiary structures. Experiments performed with a mutant form of calmodulin having an impaired Ca(2+)-binding ability in the C-terminal lobe suggest that the EF-hand Ca(2+)-binding motifs are directly involved in the binding of calmodulin to the CaF2 matrix. The residual capability of nanoparticle-bound calmodulin to function as a calcium sensor protein, binding to and altering the activity of a target protein, was successfully probed by biochemical assays. Even if efficiently carried by CaF2 nanoparticles, calmodulin may dissociate, thus retaining the ability to bind the peptide encompassing the putative C-terminal calmodulin-binding domain of glutamate decarboxylase and activate the enzyme. We conclude that the high flexibility and structural plasticity of calmodulin are responsible for the preservation of its function when bound in high amounts to a nanoparticle surface.

Solution NMR structure of the Ca2+-bound N-terminal domain of CaBP7: a regulator of golgi trafficking

Calcium-binding protein 7 (CaBP7) is a member of the calmodulin (CaM) superfamily that harbors two high affinity EF-hand motifs and a C-terminal transmembrane domain. CaBP7 has been previously shown to interact with and modulate phosphatidylinositol 4-kinase III-β (PI4KIIIβ) activity in in vitro assays and affects vesicle transport in neurons when overexpressed. Here we show that the N-terminal domain (NTD) of CaBP7 is sufficient to mediate the interaction of CaBP7 with PI4KIIIβ. CaBP7 NTD encompasses the two high affinity Ca(2+) binding sites, and structural characterization through multiangle light scattering, circular dichroism, and NMR reveals unique properties for this domain. CaBP7 NTD binds specifically to Ca(2+) but not Mg(2+) and undergoes significant conformational changes in both secondary and tertiary structure upon Ca(2+) binding. The Ca(2+)-bound form of CaBP7 NTD is monomeric and exhibits an open conformation similar to that of CaM. Ca(2+)-bound CaBP7 NTD has a solvent-exposed hydrophobic surface that is more expansive than observed in CaM or CaBP1. Within this hydrophobic pocket, there is a significant reduction in the number of methionine residues that are conserved in CaM and CaBP1 and shown to be important for target recognition. In CaBP7 NTD, these residues are replaced with isoleucine and leucine residues with branched side chains that are intrinsically more rigid than the flexible methionine side chain. We propose that these differences in surface hydrophobicity, charge, and methionine content may be important in determining highly specific interactions of CaBP7 with target proteins, such as PI4KIIIβ.

Calcium binding by spinach stromal proteins

Calcium binding to spinach (Spinacia oleracea L.) stromal proteins was examined by dual-wavelength spectrophotometry using the metallochromic indicator tetramethylmurexide. The data are consistent with the existence of at least two, probably independent, classes of binding sites. The total number of binding sites varied between 90-155 nmol·mg(-1) protein with "average" binding constants of 1.1-2.7·mM(-1). Both Mg(2+) and La(3+) inhibited calcium binding competitively, with "average" inhibitor constants of 0.26·mM(-1) and 39.4·mM(-1), respectively; an increase in the potassium concentration up to 50 mM had no effect. In a typical experiment a decrease in pH (7.8 to 7.1) resulted in a decrease in the total number of calcium binding sites from 90 to 59 nmol·mg(-1) protein, but in an increase of the "average" affinity from 2.7 to 4.5·mM(-1). Calculations, using these data and those of Gross and Hess (1974, Biochim. Biophys. Acta 339, 334-346) for binding site I of washed thylakoid membranes, showed that the free-Ca(2+) concentration in the stroma under dark conditions, pH 7.1, is higher than under light conditions, pH 7.8. The physiological relevance of the observed calcium binding by stromal proteins is discussed.