Modifying Mg2+ binding and exchange with the N-terminal of calmodulin

The effect of Mg2+ on Ca2+ binding to cardiac troponin C in hypertrophic cardiomyopathy associated TNNC1 variants

Cardiac troponin C (cTnC) is the critical Ca2+ -sensing component of the troponin complex. Binding of Ca2+ to cTnC triggers a cascade of conformational changes within the myofilament that culminate in force production. Hypertrophic cardiomyopathy (HCM)-associated TNNC1 variants generally induce a greater degree and duration of Ca2+ binding, which may underly the hypertrophic phenotype. Regulation of contraction has long been thought to occur exclusively through Ca2+ binding to site II of cTnC. However, work by several groups including ours suggest that Mg2+ , which is several orders of magnitude more abundant in the cell than Ca2+ , may compete for binding to the same cTnC regulatory site. We previously used isothermal titration calorimetry (ITC) to demonstrate that physiological concentrations of Mg2+ may decrease site II Ca2+ -binding in both N-terminal and full-length cTnC. Here, we explore the binding of Ca2+ and Mg2+ to cTnC harbouring a series of TNNC1 variants thought to be causal in HCM. ITC and thermodynamic integration (TI) simulations show that A8V, L29Q and A31S elevate the affinity for both Ca2+ and Mg2+ . Further, L48Q, Q50R and C84Y that are adjacent to the EF hand binding motif of site II have a more significant effect on affinity and the thermodynamics of the binding interaction. To the best of our knowledge, this work is the first to explore the role of Mg2+ in modifying the Ca2+ affinity of cTnC mutations linked to HCM. Our results indicate a physiologically significant role for cellular Mg2+ both at baseline and when elevated on modifying the Ca2+ binding properties of cTnC and the subsequent conformational changes which precede cardiac contraction.

Non-Canonical Interaction between Calmodulin and Calcineurin Contributes to the Differential Regulation of Plant-Derived Calmodulins on Calcineurin

Calmodulin (CaM) serves as an important Ca2+ signaling hub that regulates many protein signaling pathways. Recently, it was demonstrated that plant CaM homologues can regulate mammalian targets, often in a manner that opposes the impact of the mammalian CaM (mCaM). However, the molecular basis of how CaM homologue mutations differentially impact target activation is unclear. To understand these mechanisms, we examined two CaM isoforms found in soybean plants that differentially regulate a mammalian target, calcineurin (CaN). These CaM isoforms, sCaM-1 and sCaM-4, share >90 and ∼78% identity with the mCaM, respectively, and activate CaN with comparable or reduced activity relative to mCaM. We used molecular dynamics (MD) simulations and fluorometric assays of CaN-dependent dephosphorylation of MUF-P to probe whether calcium and protein-protein binding interactions are altered by plant CaMs relative to mCaM as a basis for differential CaN regulation. In the presence of CaN, we found that the two sCaMs' Ca2+ binding properties, such as their predicted coordination of Ca2+ and experimentally measured EC50 [Ca2+] values are comparable to mCaM. Furthermore, the binding of CaM to the CaM binding region (CaMBR) in CaN is comparable among the three CaMs, as evidenced by MD-predicted binding energies and experimentally measured EC50 [CaM] values. However, mCaM and sCaM-1 exhibited binding with a secondary region of CaN's regulatory domain that is weakened for sCaM-4. We speculate that this secondary interaction affects the turnover rate (kcat) of CaN based on our modeling of enzyme activity, which is consistent with our experimental data. Together, our data describe how plant-derived CaM variants alter CaN activity through enlisting interactions other than those directly influencing Ca2+ binding and canonical CaMBR binding, which may additionally play a role in the differential regulation of other mammalian targets.

Binding of calcium and magnesium to human cardiac troponin C

Cardiac muscle thin filaments are composed of actin, tropomyosin, and troponin that change conformation in response to Ca²⁺ binding, triggering muscle contraction. Human cardiac troponin C (cTnC) is the Ca²⁺-sensing component of the thin filament. It contains structural sites (III/IV) that bind both Ca²⁺ and Mg²⁺ and a regulatory site (II) that has been thought to bind only Ca²⁺. Binding of Ca²⁺ at this site initiates a series of conformational changes that culminate in force production. However, the mechanisms that underpin the regulation of binding at site II remain unclear. Here, we have quantified the interaction between site II and Ca²⁺/Mg²⁺ through isothermal titration calorimetry and thermodynamic integration simulations. Direct and competitive binding titrations with WT N-terminal cTnC and full-length cTnC indicate that physiologically relevant concentrations of both Ca²⁺/Mg²⁺ interacted with the same locus. Moreover, the D67A/D73A N-terminal cTnC construct in which two coordinating residues within site II were removed was found to have significantly reduced affinity for both cations. In addition, 1 mM Mg²⁺ caused a 1.4-fold lower affinity for Ca²⁺. These experiments strongly suggest that cytosolic-free Mg²⁺ occupies a significant population of the available site II. Interaction of Mg²⁺ with site II of cTnC likely has important functional consequences for the heart both at baseline as well as in diseased states that decrease or increase the availability of Mg²⁺, such as secondary hyperparathyroidism or ischemia, respectively.

Nickel reduces calcium dependent dimerization in neural cadherin

Cadherins are the transmembrane component in adherens junctions, structures that link the actin cytoskeletons in adjacent cells within solid tissues including neurological synapses, epithelium and endothelium. Cell-cell adhesion by cadherins requires the binding of calcium ions to specific sites in the extracellular region. Given the complexity of the cell adhesion microenvironment, we are investigating whether other divalent cations might affect calcium-dependent dimerization of neural (N) cadherin. The studies reported herein characterize the impact of binding physiological magnesium(ii) or neurotoxic nickel(ii) on calcium-dependent N-cadherin function. Physiological levels of magnesium have only a small effect on the calcium-binding affinity and calcium-induced dimerization of N-cadherin. However, a tenfold lower concentration of nickel decreases the apparent calcium-binding affinity and calcium-induced dimerization of N-cadherin. Competitive binding studies indicate that the apparent dissociation constants for nickel and magnesium are 0.2 mM and 2.5 mM, respectively. These Kd values are consistent with concentrations observed for a range of divalent cations in the extracellular space. Results from these studies indicate that calcium-induced dimerization by N-cadherin is attenuated by natural and non-physiological divalent cations in the extracellular microenvironment.

OsCML16 interacts with a novel CC-NBS-LRR protein OsPi304 in the Ca2+/Mg2+ dependent and independent manner in rice

In plants, many target proteins of calmodulins (CaMs) have been identified in cellular metabolism and responses. However, calmodulin-like proteins (CMLs) and their target proteins have not been discovered in stress responses in rice. In this study, a novel CC-NBS-LRR protein was obtained in screening a cold stress rice seedlings yeast cDNA library with OsCML16 as bait. Furthermore, yeast two-hybrid and BiFC assays demonstrated that the full length, CC region in the N-terminus and LRR in the C-terminus of Pi304 protein could interact with OsCML16. More interestingly, OsCML16 bound to the 1-10 motif rather than 1-14 motif in the Ca²⁺ or Mg²⁺ dependent manner in vitro. In addition, transcript levels of OsCML16 and OsPi304 were induced more markedly in Nipponbare than in 9311 under cold stress. Taken together, these data indicates that they are involved in the cold stress signaling and response in rice.

Divergent Soybean Calmodulins Respond Similarly to Calcium Transients: Insight into Differential Target Regulation

Plants commonly respond to stressors by modulating the expression of a large family of calcium binding proteins including isoforms of the ubiquitous signaling protein calmodulin (CaM). The various plant CaM isoforms are thought to differentially regulate the activity of specific target proteins to modulate cellular stress responses. The mechanism(s) behind differential target activation by the plant CaMs is unknown. In this study, we used steady-state and stopped-flow fluorescence spectroscopy to investigate the strategy by which two soybean CaMs (sCaM1 and sCaM4) have evolved to differentially regulate NAD kinase (NADK), which is activated by sCaM1 but inhibited by sCaM4. Although the isolated proteins have different cation binding properties, in the presence of Mg2+ and the CaM binding domains from proteins that are differentially regulated, the two plant CaMs respond nearly identically to rapid and slow Ca2+ transients. Our data suggest that the plant CaMs have evolved to bind certain targets with comparable affinities, respond similarly to a particular Ca2+ signature, but achieve different structural states, only one of which can activate the enzyme. Understanding the basis for differential enzyme regulation by the plant CaMs is the first step to engineering a vertebrate CaM that will selectively alter the CaM signaling network.

Designing proteins to combat disease: Cardiac troponin C as an example

Throughout history, muscle research has led to numerous scientific breakthroughs that have brought insight to a more general understanding of all biological processes. Potentially one of the most influential discoveries was the role of the second messenger calcium and its myriad of handling and sensing systems that mechanistically control muscle contraction. In this review we will briefly discuss the significance of calcium as a universal second messenger along with some of the most common calcium binding motifs in proteins, focusing on the EF-hand. We will also describe some of our approaches to rationally design calcium binding proteins to palliate, or potentially even cure cardiovascular disease. Considering not all failing hearts have the same etiology, genetic background and co-morbidities, personalized therapies will need to be developed. We predict designer proteins will open doors for unprecedented personalized, and potentially, even generalized medicines as gene therapy or protein delivery techniques come to fruition.

Characterization of the 1st and 2nd EF-hands of NADPH oxidase 5 by fluorescence, isothermal titration calorimetry, and circular dichroism

Background

Superoxide generated by non-phagocytic NADPH oxidases (NOXs) is of growing importance for physiology and pathobiology. The calcium binding domain (CaBD) of NOX5 contains four EF-hands, each binding one calcium ion. To better understand the metal binding properties of the 1^st and 2^nd EF-hands, we characterized the N-terminal half of CaBD (NCaBD) and its calcium-binding knockout mutants.

Results

The isothermal titration calorimetry measurement for NCaBD reveals that the calcium binding of two EF-hands are loosely associated with each other and can be treated as independent binding events. However, the Ca²⁺ binding studies on NCaBD(E31Q) and NCaBD(E63Q) showed their binding constants to be 6.5 × 10⁵ and 5.0 × 10² M^-1 with ΔHs of -14 and -4 kJ/mol, respectively, suggesting that intrinsic calcium binding for the 1^st non-canonical EF-hand is largely enhanced by the binding of Ca²⁺ to the 2^nd canonical EF-hand. The fluorescence quenching and CD spectra support a conformational change upon Ca²⁺ binding, which changes Trp residues toward a more non-polar and exposed environment and also increases its α-helix secondary structure content. All measurements exclude Mg²⁺-binding in NCaBD.

Conclusions

We demonstrated that the 1^st non-canonical EF-hand of NOX5 has very weak Ca²⁺ binding affinity compared with the 2^nd canonical EF-hand. Both EF-hands interact with each other in a cooperative manner to enhance their Ca²⁺ binding affinity. Our characterization reveals that the two EF-hands in the N-terminal NOX5 are Ca²⁺ specific.

Graphical abstract

Engineering Parvalbumin for the Heart: Optimizing the Mg Binding Properties of Rat β-Parvalbumin

Parvalbumin (PV), an EF-hand protein family member, is a delayed calcium buffer that exchanges magnesium for calcium to facilitate fast skeletal muscle relaxation. Genetic approaches that express parvalbumin in the heart also enhance relaxation and show promise of being therapeutic against various cardiac diseases where relaxation is compromised. Unfortunately, skeletal muscle PVs have very slow rates of Ca²⁺ dissociation and are prone to becoming saturated with Ca²⁺, eventually losing their buffering capability within the constantly beating heart. In order for PV to have a more therapeutic potential in the heart, a PV with faster rates of calcium dissociation and high Mg²⁺ affinity is needed. We demonstrate that at 35°C, rat β-PV has an ~30-fold faster rate of Ca²⁺ dissociation compared to rat skeletal muscle α-PV, and still possesses a physiologically relevant Ca²⁺ affinity (~100 nM). However, rat β-PV will not be a delayed Ca²⁺ buffer since its Mg²⁺ affinity is too low (~1 mM). We have engineered two mutations into rat β-PV, S55D and E62D, when observed alone increase Mg²⁺ affinity up to fivefold, but when combined increase Mg²⁺ affinity ~13-fold, well within a physiologically relevant affinity. Furthermore, the Mg²⁺ dissociation rate (172/s) from the engineered S55D, E62D PV is slow enough for delayed Ca²⁺ buffering. Additionally, the engineered PV retains a high Ca²⁺ affinity (132 nM) and fast rate of Ca²⁺ dissociation (64/s). These PV design strategies hold promise for the development of new therapies to remediate relaxation abnormalities in different heart diseases and heart failure.

Sequential binding of calcium leads to dimerization in neural cadherin

Neural cadherin (N-cadherin) is a calcium-dependent homophilic cell-adhesive molecule and critical for synaptogenesis and synapse maintenance. The extracellular region plays an important role in cadherin-mediated cell adhesion and has five tandemly repeated ectodomains (EC1-EC5) with three calcium-binding sites situated between each of these domains. Adhesive dimer formation is significantly dependent on binding of calcium such that mutations in the calcium-binding sites adversely affect cell adhesion. To investigate the relative significance of the calcium-binding sites at the EC1-EC2 interface in calcium-induced dimerization, we mutated three important amino acids, D134, D136, and D103, in NCAD12, a construct containing EC1 and EC2. Spectroscopic and chromatographic experiments showed that all three mutations affected calcium binding and dimerization. Mutation of D134, a bidentate chelator in site 3, severely impaired the binding of calcium to all three sites. These findings confirm that binding to site 3 is required for binding to occur at site 2 and site 1. Interestingly, while the D103A mutation diminished only the affinity for calcium, it completely eliminated dimerization. Equilibrium dialysis experiments showed a stoichiometry of 3 at 2 mM calcium for D103A, but no dimerization was apparent even at 10 mM calcium. These results indicate that calcium binding alone is not sufficient for dimerization but requires cooperativity between calcium-binding sites. In summary, our findings confirm that the calcium-binding sites are occupied sequentially in the order of site 3, then site 2 and site 1, and that cooperativity between site 2 and site 1 is essential for formation of adhesive dimers by N-cadherin.

Structures and metal-ion-binding properties of the Ca2+-binding helix–loop–helix EF-hand motifs

The ‘EF-hand’ Ca2+-binding motif plays an essential role in eukaryotic cellular signalling, and the proteins containing this motif constitute a large and functionally diverse family. The EF-hand is defined by its helix–loop–helix secondary structure as well as the ligands presented by the loop to bind the Ca2+ ion. The identity of these ligands is semi-conserved in the most common (the ‘canonical’) EF-hand; however, several non-canonical EF-hands exist that bind Ca2+ by a different co-ordination mechanism. EF-hands tend to occur in pairs, which form a discrete domain so that most family members have two, four or six EF-hands. This pairing also enables communication, and many EF-hands display positive co-operativity, thereby minimizing the Ca2+ signal required to reach protein saturation. The conformational effects of Ca2+ binding are varied, function-dependent and, in some cases, minimal, but can lead to the creation of a protein target interaction site or structure formation from a molten-globule apo state. EF-hand proteins exhibit various sensitivities to Ca2+, reflecting the intrinsic binding ability of the EF-hand as well as the degree of co-operativity in Ca2+ binding to paired EF-hands. Two additional factors can influence the ability of an EF-hand to bind Ca2+: selectivity over Mg2+ (a cation with very similar chemical properties to Ca2+ and with a cytoplasmic concentration several orders of magnitude higher) and interaction with a protein target. A structural approach is used in this review to examine the diversity of family members, and a biophysical perspective provides insight into the ability of the EF-hand motif to bind Ca2+ with a wide range of affinities.

Calcium activation of ryanodine receptor channels--reconciling RyR gating models with tetrameric channel structure

Despite its importance and abundance of experimental data, the molecular mechanism of RyR2 activation by calcium is poorly understood. Recent experimental studies involving coexpression of wild-type (WT) RyR2 together with a RyR2 mutant deficient in calcium-dependent activation (Li, P., and S.R. Chen. 2001. J. Gen. Physiol. 118:33–44) revealed large variations of calcium sensitivity of the RyR tetramers with their monomer composition. Together with previous results on kinetics of Ca activation (Zahradníková, A., I. Zahradník, I. Györke, and S. Györke. 1999. J. Gen. Physiol. 114:787–798), these data represent benchmarks for construction and testing of RyR models that would reproduce RyR behavior and be structurally realistic as well. Here we present a theoretical study of the effects of RyR monomer substitution by a calcium-insensitive mutant on the calcium dependence of RyR activation. Three published models of tetrameric RyR channels were used either directly or after adaptation to provide allosteric regulation. Additionally, two alternative RyR models with Ca binding sites created jointly by the monomers were developed. The models were modified for description of channels composed of WT and mutant monomers. The parameters of the models were optimized to provide the best approximation of published experimental data. For reproducing the observed calcium dependence of RyR tetramers containing mutant monomers (a) single, independent Ca binding sites on each monomer were preferable to shared binding sites; (b) allosteric models were preferable to linear models; (c) in the WT channel, probability of opening to states containing a Ca²⁺-free monomer had to be extremely low; and (d) models with fully Ca-bound closed states, additional to those of an Monod-Wyman-Changeaux model, were preferable to models without such states. These results provide support for the concept that RyR activation is possible (albeit vanishingly small in WT channels) in the absence of Ca²⁺ binding. They also suggest further avenues toward understanding RyR gating.

A FRET-based calcium biosensor with fast signal kinetics and high fluorescence change

Genetically encoded calcium biosensors have become valuable tools in cell biology and neuroscience, but some aspects such as signal strength and response kinetics still need improvement. Here we report the generation of a FRET-based calcium biosensor employing troponin C as calcium-binding moiety that is fast, is stable in imaging experiments, and shows a significantly enhanced fluorescence change. These improvements were achieved by engineering magnesium and calcium-binding properties within the C-terminal lobe of troponin C and by the incorporation of circularly permuted variants of the green fluorescent protein. This sensor named TN-XL shows a maximum fractional fluorescence change of 400% in its emission ratio and linear response properties over an expanded calcium regime. When imaged in vivo at presynaptic motoneuron terminals of transgenic fruit flies, TN-XL exhibits highly reproducible fluorescence signals with the fastest rise and decay times of all calcium biosensors known so far.

Designing calcium-sensitizing mutations in the regulatory domain of cardiac troponin C

Cardiac troponin C belongs to the EF-hand superfamily of calcium-binding proteins and plays an essential role in the regulation of muscle contraction and relaxation. To follow calcium binding and exchange with the regulatory N-terminal domain (N-domain) of human cardiac troponin C, we substituted Phe at position 27 with Trp, making a fluorescent cardiac troponin C(F27W). Trp(27) accurately reported the kinetics of calcium association and dissociation of the N-domain of cardiac troponin C(F27W). To sensitize the N-domain of cardiac troponin C(F27W) to calcium, we individually substituted the hydrophobic residues Phe(20), Val(44), Met(45), Leu(48), and Met(81) with polar Gln. These mutations were designed to increase the calcium affinity of the N-domain of cardiac troponin C by facilitating the movement of helices B and C (BC unit) away from helices N, A, and D (NAD unit). As anticipated, these selected hydrophobic residue substitutions increased the calcium affinity of the regulatory domain of cardiac troponin C(F27W) approximately 2.1-15.2-fold. Surprisingly, the increased calcium affinity caused by the hydrophobic residue substitutions was largely due to faster calcium association rates (2.6-8.7-fold faster) rather than to slower calcium dissociation rates (1.2-2.9-fold slower). The regulatory N-domains of cardiac troponin C(F27W) and its mutants were also able to bind magnesium competitively and with physiologically relevant affinities (1.2-2.7 mm). The design of calcium-sensitizing cardiac troponin C mutants presented in this work enhances the understanding of how to control cation binding properties of EF-hand proteins and ultimately their structure and physiological function.

DC3, the smallest subunit of the Chlamydomonas flagellar outer dynein arm-docking complex, is a redox-sensitive calcium-binding protein

The outer dynein arm-docking complex (ODA-DC) targets the outer dynein arm to its correct binding site on the flagellar axoneme. The Chlamydomonas ODA-DC contains three proteins; loss of any one prevents normal assembly of the outer arm, leading to a slow, jerky swimming phenotype. We showed previously that the smallest ODA-DC subunit, DC3, has four EF-hands (Casey, D. M., Inaba, K., Pazour, G. J., Takada, S., Wakabayashi, K., Wilkerson, C. G., Kamiya, R., and Witman, G. B. (2003) Mol. Biol. Cell 14, 3650-3663). Two of the EF-hands fit the consensus pattern for calcium binding, and one of these contains two cysteine residues within its binding loop. To determine whether the predicted EF-hands are functional, we purified bacterially expressed wild-type DC3 and analyzed its calcium-binding potential in the presence and absence of dithiothreitol and Mg2+. The protein bound one calcium ion with an affinity (Kd) of approximately 1 x 10-5 m. Calcium binding was observed only in the presence of dithiothreitol and thus is redox-sensitive. DC3 also bound Mg2+ at physiological concentrations but with a much lower affinity. Changing the essential glutamate to glutamine in both EF-hands eliminated the calcium binding activity of the bacterially expressed protein. To investigate the role of the EF-hands in vivo, we transformed the modified DC3 gene into a Chlamydomonas insertional mutant lacking DC3. The transformed strain swam normally, assembled a normal number of outer arms, and had a normal photoshock response, indicating that the Glu to Gln mutations did not affect ODA-DC assembly, outer arm assembly, or Ca2+-mediated outer arm activity. Thus, DC3 is a true calcium-binding protein, but the function of this activity remains unknown.

Engineering competitive magnesium binding into the first EF-hand of skeletal troponin C

The goal of this study was to examine the mechanism of magnesium binding to the regulatory domain of skeletal troponin C (TnC). The fluorescence of Trp(29), immediately preceding the first calcium-binding loop in TnC(F29W), was unchanged by addition of magnesium, but increased upon calcium binding with an affinity of 3.3 microm. However, the calcium-dependent increase in TnC(F29W) fluorescence could be reversed by addition of magnesium, with a calculated competitive magnesium affinity of 2.2 mm. When a Z acid pair was introduced into the first EF-hand of TnC(F29W), the fluorescence of G34DTnC(F29W) increased upon addition of magnesium or calcium with affinities of 295 and 1.9 microm, respectively. Addition of 3 mm magnesium decreased the calcium sensitivity of TnC(F29W) and G34DTnC(F29W) approximately 2- and 6-fold, respectively. Exchange of G34DTnC(F29W) into skinned psoas muscle fibers decreased fiber calcium sensitivity approximately 1.7-fold compared with TnC(F29W) at 1 mm [magnesium](free) and approximately 3.2-fold at 3 mm [magnesium](free). Thus, incorporation of a Z acid pair into the first EF-hand allows it to bind magnesium with high affinity. Furthermore, the data suggests that the second EF-hand, but not the first, of TnC is responsible for the competitive magnesium binding to the regulatory domain.

Cation binding mode of fully oxidised calmodulin explained by the unfolding of the apostate

Calmodulin is the most ubiquitous calcium binding protein. The protein is very sensitive to oxidation and this modification has pronounced effects on calmodulin function. In this work, we decided to fully oxidise calmodulin in order to study the consequences on cation binding, domain stability, and alpha helicity. Oxidation of methionines unfolds completely the apostate of the protein, which upon calcium binding recovers the major part of its secondary and tertiary structure. However, the unstructuring of the apostate results in a protein that binds calcium to any site in an independent manner, does not bind magnesium and does not possess auxiliary sites anymore.

Calcium binding to calmodulin mutants monitored by domain-specific intrinsic phenylalanine and tyrosine fluorescence

Cooperative calcium binding to the two homologous domains of calmodulin (CaM) induces conformational changes that regulate its association with and activation of numerous cellular target proteins. Calcium binding to the pair of high-affinity sites (III and IV in the C-domain) can be monitored by observing calcium-dependent changes in intrinsic tyrosine fluorescence intensity (lambda(ex)/lambda(em) of 277/320 nm). However, calcium binding to the low-affinity sites (I and II in the N-domain) is more difficult to measure with optical spectroscopy because that domain of CaM does not contain tryptophan or tyrosine. We recently demonstrated that calcium-dependent changes in intrinsic phenylalanine fluorescence (lambda(ex)/lambda(em) of 250/280 nm) of an N-domain fragment of CaM reflect occupancy of sites I and II (VanScyoc, W. S., and M. A. Shea, 2001, Protein Sci. 10:1758-1768). Using steady-state and time-resolved fluorescence methods, we now show that these excitation and emission wavelength pairs for phenylalanine and tyrosine fluorescence can be used to monitor equilibrium calcium titrations of the individual domains in full-length CaM. Calcium-dependent changes in phenylalanine fluorescence specifically indicate ion occupancy of sites I and II in the N-domain because phenylalanine residues in the C-domain are nonemissive. Tyrosine emission from the C-domain does not interfere with phenylalanine fluorescence signals from the N-domain. This is the first demonstration that intrinsic fluorescence may be used to monitor calcium binding to each domain of CaM. In this way, we also evaluated how mutations of two residues (Arg74 and Arg90) located between sites II and III can alter the calcium-binding properties of each of the domains. The mutation R74A caused an increase in the calcium affinity of sites I and II in the N-domain. The mutation R90A caused an increase in calcium affinity of sites III and IV in the C-domain whereas R90G caused an increase in calcium affinity of sites in both domains. This approach holds promise for exploring the linked energetics of calcium binding and target recognition.