Fluorescence probe study of Ca2+-dependent interactions of calmodulin with calmodulin-binding peptides of the ryanodine receptor

Kinetics insight into the roles of the N- and C-lobes of calmodulin in RyR1 channel regulation

Calmodulin (CaM) activates the skeletal muscle Ca2+ release channel (ryanodine receptor, RyR1) at nanomolar Ca2+ and inhibits it at micromolar Ca2+. CaM conversion from RyR1 activator to inhibitor is due to structural changes induced by Ca2+ binding at CaM's two lobes. However, it remains unclear which lobe provides the switch for this conversion. Here, we attached the environment-sensitive fluorophore acrylodan (Acr) at either lobe of intact CaM or lobe-specific Ca2+-sensitive CaM mutants, and monitored the effects of Ca2+ binding via the fluorescence change of free or RyR1-bound AcrCaM. Using steady state measurements, we found that Ca2+ binding to free CaM causes a dramatic structural change in the N-lobe, but only a slight effect on the C-lobe of the Ca2+-sensitive lobe-specific mutants, in addition to the previously known higher Ca2+ affinity at the C-lobe versus the N-lobe. Using stopped-flow measurements, we found ∼30x faster Ca2+ dissociation from the N- versus C-lobe, and ∼20x slower Ca2+ association to the N-lobe versus C-lobe. These Ca2+ binding properties hold for the CaM/RyR1 complex, and Ca2+ affinity is enhanced at the CaM C-lobe but decreased at the N-lobe by RyR1 binding. We propose that fast Ca2+-binding at the C-lobe of CaM initiates its inhibition to RyR1 at high [Ca2+], while slow Ca2+ binding to the N-lobe is necessary for timely enhancement of the inhibitory effect. The dysregulation of RyR1 by M124Q-CaM may be explained by the lower Ca2+ affinity versus WT-CaM, as suggested by both steady-state and transient kinetics results.

Photophysical Properties of BADAN Revealed in the Study of GGBP Structural Transitions

The fluorescent dye BADAN (6-bromoacetyl-2-dimetylaminonaphtalene) is widely used in various fields of life sciences, however, the photophysical properties of BADAN are not fully understood. The study of the spectral properties of BADAN attached to a number of mutant forms of GGBP, as well as changes in its spectral characteristics during structural changes in proteins, allowed to shed light on the photophysical properties of BADAN. It was shown that spectral properties of BADAN are determined by at least one non-fluorescent and two fluorescent isomers with overlapping absorbing bands. It was found that BADAN fluorescence is determined by the unsolvated "PICT" (planar intramolecular charge transfer state) and solvated "TICT" (twisted intramolecular charge transfer state) excited states. While "TICT" state can be formed both as a result of the "PICT" state solvation and as a result of light absorption by the solvated ground state of the dye. BADAN fluorescence linked to GGBP/H152C apoform is quenched by Trp 183, but this effect is inhibited by glucose intercalation. New details of the changes in the spectral characteristics of BADAN during the unfolding of the protein apo and holoforms have been obtained.

Calmodulin-dependent activation and inactivation of anoctamin calcium-gated chloride channels

Calcium-dependent chloride channels serve critical functions in diverse biological systems. Driven by cellular calcium signals, the channels codetermine excitatory processes and promote solute transport. The anoctamin (ANO) family of membrane proteins encodes three calcium-activated chloride channels, named ANO 1 (also TMEM16A), ANO 2 (also TMEM16B), and ANO 6 (also TMEM16F). Here we examined how ANO 1 and ANO 2 interact with Ca²⁺/calmodulin using nonstationary current analysis during channel activation. We identified a putative calmodulin-binding domain in the N-terminal region of the channel proteins that is involved in channel activation. Binding studies with peptides indicated that this domain, a regulatory calmodulin-binding motif (RCBM), provides two distinct modes of interaction with Ca²⁺/calmodulin, one at submicromolar Ca²⁺ concentrations and one in the micromolar Ca²⁺ range. Functional, structural, and pharmacological data support the concept that calmodulin serves as a calcium sensor that is stably associated with the RCBM domain and regulates the activation of ANO 1 and ANO 2 channels. Moreover, the predominant splice variant of ANO 2 in the brain exhibits Ca²⁺/calmodulin-dependent inactivation, a loss of channel activity within 30 s. This property may curtail ANO 2 activity during persistent Ca²⁺ signals in neurons. Mutagenesis data indicated that the RCBM domain is also involved in ANO 2 inactivation, and that inactivation is suppressed in the retinal ANO 2 splice variant. These results advance the understanding of Ca²⁺ regulation in anoctamin Cl⁻ channels and its significance for the physiological function that anoctamin channels subserve in neurons and other cell types.

Reverse micelles as a tool for probing solvent modulation of protein dynamics: Reverse micelle encapsulated hemoglobin

Hydration waters impact protein dynamics. Dissecting the interplay between hydration waters and dynamics requires a protein that manifests a broad range of dynamics. Proteins in reverse micelles (RMs) have promise as tools to achieve this objective because the water content can be manipulated. Hemoglobin is an appropriate tool with which to probe hydration effects. We describe both a protocol for hemoglobin encapsulation in reverse micelles and a facile method using PEG and cosolvents to manipulate water content. Hydration properties are probed using the water-sensitive fluorescence from Hb bound pyranine and covalently attached Badan. Protein dynamics are probed through ligand recombination traces derived from photodissociated carbonmonoxy hemoglobin on a log scale that exposes the potential role of both α and β solvent fluctuations in modulating protein dynamics. The results open the possibility of probing hydration level phenomena in this system using a combination of NMR and optical probes.

Distinct binding properties distinguish LQ-type calmodulin-binding domains in cyclic nucleotide-gated channels

Cyclic nucleotide-gated (CNG) channels operate as transduction channels in photoreceptors and olfactory receptor neurons. Direct binding of cGMP or cAMP opens these channels which conduct a mixture of monovalent cations and Ca(2+). Upon activation, CNG channels generate intracellular Ca(2+) signals that play pivotal roles in the transduction cascades of the visual and olfactory systems. Channel activity is controlled by negative feedback mechanisms that involve Ca(2+)-calmodulin, for which all CNG channels possess binding sites. Here we compare the binding properties of the two LQ-type calmodulin binding sites, both of which are thought to be involved in channel regulation. They reside on the isoforms CNGB1 and CNGA4. The CNGB1 subunit is present in rod photoreceptors and olfactory receptor neurons. The CNGA4 subunit is only expressed in olfactory receptor neurons, and there are conflicting results as to its role in calmodulin-mediated feedback inhibition. We examined the interaction of Ca(2+)-calmodulin with two recombinant proteins that encompass either of the two LQ sites. Comparing binding properties, we found that the LQ site of CNGB1 binds Ca(2+)-calmodulin at 10-fold lower Ca(2+) levels than the LQ site of CNGA4. Our data provide biochemical evidence against a contribution of CNGA4 to feedback inhibition. In accordance with previous work on photoreceptor CNG channels, our results indicate that feedback control is the exclusive role of the B-subunits in photoreceptors and olfactory receptor neurons.

Interaction of the Lys(3614)-Asn(3643) calmodulin-binding domain with the Cys(4114)-Asn(4142) region of the type 1 ryanodine receptor is involved in the mechanism of Ca2+/agonist-induced channel activation

In the present study we show that the interaction of the CaM (calmodulin)-binding domain (Lys(3614)-Asn(3643)) with the Cys(4114)-Asn(4142) region (a region included in the CaM-like domain) serves as an intrinsic regulator of the RyR1 (type-1 ryanodine receptor). We tested the effects of antibodies raised against the two putative key regions of RyR1 [anti-(Lys(3614)-Asn(3643)) and anti-(Cys(4114)-Asn(4142)) antibodies]. Both antibodies produced significant inhibition of [3H]ryanodine-binding activity of RyR1. This suggests that the inter-domain interaction between the two domains, Lys(3614)-Asn(3643) and Cys(4114)-Asn(4142), activates the channel, and that the binding of antibody to either side of the interacting domain pair interfered with the formation of a 'channel-activation link' between the two regions. In order to spectroscopically monitor the mode of interaction of these domains, the site of inter-domain interaction was fluorescently labelled with MCA [(7-methoxycoumarin-4-yl)acetyl] in a site-directed manner. The accessibility of the bound MCA to a large molecular mass fluorescence quencher, BSA-QSY (namely, the size of a gap between the interacting domains) decreased with an increase of [Ca2+] in a range of 0.03-2.0 microM, as determined by Stern-Volmer fluorescence quenching analysis. The Ca2+-dependent decrease in the quencher accessibility was more pronounced in the presence of 150 microM 4-CmC (4-chlorometacresol), and was reversed by 1 mM Mg2+ (a well-known inhibitor of Ca2+/agonist-induced channel activation). These results suggest that the Lys(3614)-Asn(3643) and Cys(4114)-Asn(4142) regions of RyR1 interact with each other in a Ca2+- and agonist-dependent manner, and this serves as a mechanism of Ca2+- and agonist-dependent activation of the RyR1 Ca2+ channel.

Structural modulation of calmodulin and calmodulin-dependent protein kinase II by pea protein hydrolysates

The effects of two fractions of pea protein hydrolysate with high levels of positively charged amino acids on the structural conformations of calmodulin (CaM) and CaM-dependent protein kinase II (CaMKII) were determined using fluorescence and circular dichroism methods. In the presence of Ca2 + , addition of the protein hydrolysates to CaM and CaM/CaMKII complex led to increased exposure of aromatic groups as measured by intrinsic and extrinsic fluorescence spectroscopy. Near-UV circular dichroism data revealed an increase in the tertiary structure of CaM in the presence of pea protein hydrolysates. Effect of the protein hydrolysates on the CaM structure was greater with the fraction that contained higher contents of arginine and lysine when compared with the fraction with lower levels of these two amino acids. We concluded that the presence of the pea protein hydrolysates led to rearrangement of the native protein structure and exposure of buried hydrophobic groups of CaM and/or CaMKII.

Role of the Met3534-Ala4271 region of the ryanodine receptor in the regulation of Ca2+ release induced by calmodulin binding domain peptide

CaMBP, a peptide corresponding to the 3614-3643 calmodulin (CaM) binding region of the ryanodine receptor (RyR1), is known to activate RyR1 Ca2+ channel. To analyze the mechanism of channel regulation by the CaMBP-RyR1 interaction, we investigated a), CaMBP binding to RyR1, b), induced local conformational changes in the CaMBP binding region of RyR1 using the fluorescent conformational probe badan attached to CaMBP (CaMBP-badan), and c), effects of "a" and "b" on SR Ca2+ release. We also monitored the interaction of CaMBP-badan with CaM and a peptide corresponding to the Met3534-Ala4271 region of RyR1 (R3534-4271) as a control. At lower peptide concentrations (< or =15 microM), CaMBP binding to RyR1 increased the intensity of badan fluorescence emission at a shorter wavelength (the state resembling CaMBP-badan/Ca-CaM) and induced Ca2+ release. Further increase in CaMBP concentration (up to approximately 50 microM) produced more binding of CaMBP accompanied by further increase in the badan fluorescence emission but at a longer wavelength (the state resembling CaMBP-badan/apo-CaM) and inhibited Ca2+ release. Binding of CaMBP-badan to R3534-4271 increased the intensity of badan fluorescence, showing the similar concentration-dependent red-shift of the emission maximum. It is proposed that CaMBP interacts with two classes of binding sites located in the Met3534-Ala4271 region of RyR1, which activate and inhibit the Ca2+ channel, respectively.

A Ca2+-binding domain in RyR1 that interacts with the calmodulin binding site and modulates channel activity

A fragment of RyR1 (amino acids 4064-4210) is predicted to fold to at least one lobe of calmodulin and to bind Ca(2+). This fragment of RyR1 (R4064-4210) was subcloned, expressed, refolded, and purified. Consistent with the predicted folding pattern, R4064-4210 was found to bind two molecules of Ca(2+) and undergo a structural change upon binding Ca(2+) that exposes hydrophobic amino acids. R4064-4210 also binds to RyR1, the L-type Ca(2+) channel (Cav(1.1)), and several synthetic calmodulin binding peptides. Both R4064-4210 and a peptide representing the calmodulin-binding region of RyR1 (R3614-3643) alter the Ca(2+) dependence of ((3)H)ryanodine binding to RyR1, suggesting that they may both be interfering with an intramolecular interaction between amino acids 4064-4210 and amino acids 3614-3643 in the native RyR1 to alter or regulate the response of the channel to changes in Ca(2+) concentration. The finding that a domain within RyR1 binds Ca(2+) and interacts with calmodulin-binding motifs may provide insights into the mechanism for calcium- and calmodulin-dependent regulation of this channel and perhaps for its regulation by the L-type Ca(2+) channel.