Identification of a Conserved Calmodulin-Binding Motif &;#x220A; the Sequence of F<Subscript>0</Subscript>F<Subscript>1</Subscript>ATPsynthase Inhibitor Protein

The natural inhibitor proteins IF₁ regulate mitochondrial F₀F₁ATPsynthase in a wide range of species. We characterized the interaction of CaM with purified bovine IF₁, two bovine IF₁ synthetic peptides, as well as two homologous proteins from yeast, namely IF₁ and STF₁. Fluorometric analyses showed that bovine and yeast inhibitors bind CaM with a 1:1 stoichiometry in the pH range between 5 and 8 and that CaM-IF₁ interaction is Ca²⁺-dependent. Bovine and yeast IF₁ have intermediate binding affinity for CaM, while the K_d (dissociation constant) of the STF₁-CaM interaction is slightly higher. Binding studies of CaM with bovine IF₁ synthetic peptides allowed us to identify bovine IF₁ sequence 33–42 as the putative CaM-binding region. Sequence alignment revealed that this region contains a hydrophobic motif for CaM binding, highly conserved in both yeast IF₁ and STF₁ sequences. In addition, the same region in bovine IF₁ has an IQ motif for CaM binding, conserved as an IQ-like motif in yeast IF₁ but not in STF₁. Based on the pH and Ca²⁺ dependence of IF₁ interaction with CaM, we suggest that the complex can be formed outside mitochondria, where CaM could regulate IF₁ trafficking or additional IF₁ roles not yet clarified.     

Homologous and heterologous inhibitory effects of ATPase inhibitor proteins on F-ATPases

In Saccharomyces cerevisiae, at least three proteins (IF(1), STF(1), and STF(2)) appear to be involved in the regulation of ATP synthase. Both IF(1) and STF(1) inhibit F(1), whereas the proposed function for STF(2) is to facilitate the binding of IF(1) and STF(1) to F(1). The oligomerization properties of yeast IF(1) and STF(1) have been investigated by sedimentation equilibrium analytical ultracentrifugation and by covalent cross-linking. Both techniques confirm that IF(1) and STF(1) oligomerize in opposite directions in relation to pH, suggesting that both proteins might regulate yeast F(1)F(0)-ATPase under different conditions. Their effects on bovine F-ATPases are also described. Whereas bovine IF(1) inhibits yeast F(1)-ATPase even better than yeast IF(1) or STF(1), the capability of yeast IF(1) to inhibit the bovine enzyme is very low and decreases with time. Such an effect is also observed in the study of the homologous inhibition of yeast F(1)-ATPase. Yeast inhibitors are not as effective as their bovine counterpart, and the complex seems to dissociate gradually.     

Investigation of the role and mechanism of IF1 and STF1 proteins,twin inhibitory peptides which interact with the yeast mitochondrial ATP synthase

Inhibition of the yeast F(0)F(1)-ATP synthase by the regulatory peptides IF1 and STF1 was studied using intact mitochondria and submitochondrial particles from wild-type cells or from mutants lacking one or both peptides. In intact mitochondria, endogenous IF1 only inhibited uncoupled ATP hydrolysis and endogenous STF1 had no effect. Addition of alamethicin to mitochondria readily made the mitochondrial membranes permeable to nucleotides, and bypassed the kinetic control exerted on ATP hydrolysis by the substrate carriers. In addition, alamethicin made the regulatory peptides able to cross mitochondrial membranes. At pH 7.3, F(0)F(1)-ATPase, initially inactivated by either endogenous IF1 or endogenous STF1, was completely reactivated hours or minutes after alamethicin addition, respectively. Previous application of a membrane potential favored the release of endogenous IF1 and STF1. These observations showed that IF1 and STF1 can fully inhibit ATP hydrolysis at physiological concentrations and are sensitive to the same effectors. However, ATP synthase has a much lower affinity for STF1 than for IF1, as demonstrated by kinetic studies of ATPase inhibition in submitochondrial particles by externally added IF1 and STF1 at pHs ranging from 5.5 to 8.0. Our data do not support previously proposed effects of STF1, like the stabilization of the IF1-F(0)F(1) complex or the replacement of IF1 on its binding site in the presence of the proton-motive force or at high pH, and raise the question of the conditions under which STF1 could regulate ATPase activity in vivo.     

Identification of the Ca(2+)-dependent calmodulin-binding region of chromogranin A.

Chromogranin A (CGA), the most abundant protein in bovine adrenal chromaffin granules, is a high-capacity, low-affinity Ca(2+)-binding protein found in most neuroendocrine cells, and binds calmodulin (CaM) in a Ca(2+)-dependent manner. The binding of chromogranin A to calmodulin was determined by measuring the intrinsic tryptophan fluorescence of chromogranin A in the presence and absence of Ca2+. Binding was specifically Ca(2+)-dependent; neither Mg2+ nor Mn2+ could substitute for Ca2+. Chelation of Ca2+ by EGTA completely eliminated the chromogranin A-calmodulin interaction. CaM binding was demonstrated by a synthetic CGA peptide representing residues 40-65. When the CGA peptide and CaM were mixed in the presence of 15 mM CaCl2, the intrinsic tryptophan fluorescence emission underwent a substantial blue-shift, shifting from 350 to 330 nm. Like the intact CGA, the peptide-CaM binding was specifically Ca(2+)-dependent, and neither Mg2+ nor Mn2+ could induce the binding. Calmodulin bound both to CGA and to the synthetic CGA peptide with a stoichiometry of one to one. The dissociation constants (Kd) determined by fluorometric titration were 13 nM for the peptide-CaM binding and 17 nM for intact CGA-CaM binding. The Kd values are comparable to those (approximately 10(-9) M) of other CaM-binding proteins and peptides, demonstrating a tight binding of CaM by CGA. The CaM-binding CGA residues 40-65 are 100% conserved among all the sequenced CGAs in contrast to 50-60% conservation found in the entire sequence, implying essential roles of this region.     

Acidic/IQ motif regulator of calmodulin

The small IQ motif proteins PEP-19 (62 amino acids) and RC3 (78 amino acids) greatly accelerate the rates of Ca(2+) binding to sites III and IV in the C-domain of calmodulin (CaM). We show here that PEP-19 decreases the degree of cooperativity of Ca(2+) binding to sites III and IV, and we present a model showing that this could increase Ca(2+) binding rate constants. Comparative sequence analysis showed that residues 28 to 58 from PEP-19 are conserved in other proteins. This region includes the IQ motif (amino acids 39-62), and an adjacent acidic cluster of amino acids (amino acids 28-40). A synthetic peptide spanning residues 28-62 faithfully mimics intact PEP-19 with respect to increasing the rates of Ca(2+) association and dissociation, as well as binding preferentially to the C-domain of CaM. In contrast, a peptide encoding only the core IQ motif does not modulate Ca(2+) binding, and binds to multiple sites on CaM. A peptide that includes only the acidic region does not bind to CaM. These results show that PEP-19 has a novel acidic/IQ CaM regulatory motif in which the IQ sequence provides a targeting function that allows binding of PEP-19 to CaM, whereas the acidic residues modify the nature of this interaction, and are essential for modulating Ca(2+) binding to the C-domain of CaM.     

Interactions of calmodulin with metal ions and with its target proteins revealed by conformation-sensitive monoclonal antibodies

Two monoclonal antibodies (mAbs) raised against bovine calmodulin (CaM), CAM1 and CAM4, enable one to monitor conformational changes that occur in the molecule. The interaction of CAM1 with CaM depends on the Ca2+ occupancy of its Ca(2+)-binding sites. CAM4, in contrast, interacts with CaM in a Ca(2+)-independent manner, interacting with both holoCaM and EGTA-treated CaM to a similar extent. Their interaction with various CaMs, CaM tryptic fragments and chemically modified CaM, as well as molecular graphics, led to identification of the CAM1 and CAM4 epitopes on the C- and N-terminal lobes of CAM respectively. The two mAbs were used as macromolecular probes to detect conformational changes occurring in the CaM molecule upon binding of metal ions and target proteins and peptides. MAb CAM1 successfully detected changes associated with Al3+ binding even in the presence of Ca2+, indicating that Al3+ and Ca2+ ions may bind to the protein simultaneously, leading to a new conformation of the molecule. MAbs CAM1 and CAM4 were used to follow the interactions of CaM with its target peptides and proteins. Complexes with melittin, mastoparan, calcineurin and phosphodiesterase showed different immunological properties on an immuno-enzyme electrode, indicating unique structural properties for each complex.     

Calcium-dependent and -independent binding of soybean calmodulin isoforms to the calmodulin binding domain of tobacco MAPK phosphatase-1

The recent finding of an interaction between calmodulin (CaM) and the tobacco mitogen-activated protein kinase phosphatase-1 (NtMKP1) establishes an important connection between Ca(2+) signaling and the MAPK cascade, two of the most important signaling pathways in plant cells. Here we have used different biophysical techniques, including fluorescence and NMR spectroscopy as well as microcalorimetry, to characterize the binding of soybean CaM isoforms, SCaM-1 and -4, to synthetic peptides derived from the CaM binding domain of NtMKP1. We find that the actual CaM binding region is shorter than what had previously been suggested. Moreover, the peptide binds to the SCaM C-terminal domain even in the absence of free Ca(2+) with the single Trp residue of the NtMKP1 peptides buried in a solvent-inaccessible hydrophobic region. In the presence of Ca(2+), the peptides bind first to the C-terminal lobe of the SCaMs with a nanomolar affinity, and at higher peptide concentrations, a second peptide binds to the N-terminal domain with lower affinity. Thermodynamic analysis demonstrates that the formation of the peptide-bound complex with the Ca(2+)-loaded SCaMs is driven by favorable binding enthalpy due to a combination of hydrophobic and electrostatic interactions. Experiments with CaM proteolytic fragments showed that the two domains bind the peptide in an independent manner. To our knowledge, this is the first report providing direct evidence for sequential binding of two identical peptides of a target protein to CaM. Discussion of the potential biological role of this interaction motif is also provided.     

Calmodulin mediates Ca2+ sensitivity of sodium channels

Ca2+ has been proposed to regulate Na+ channels through the action of calmodulin (CaM) bound to an IQ motif or through direct binding to a paired EF hand motif in the Nav1 C terminus. Mutations within these sites cause cardiac arrhythmias or autism, but details about how Ca2+ confers sensitivity are poorly understood. Studies on the homologous Cav1.2 channel revealed non-canonical CaM interactions, providing a framework for exploring Na+ channels. In contrast to previous reports, we found that Ca2+ does not bind directly to Na+ channel C termini. Rather, Ca2+ sensitivity appears to be mediated by CaM bound to the C termini in a manner that differs significantly from CaM regulation of Cav1.2. In Nav1.2 or Nav1.5, CaM bound to a localized region containing the IQ motif and did not support the large Ca(2+)-dependent conformational change seen in the Cav1.2.CaM complex. Furthermore, CaM binding to Nav1 C termini lowered Ca2+ binding affinity and cooperativity among the CaM-binding sites compared with CaM alone. Nonetheless, we found suggestive evidence for Ca2+/CaM-dependent effects upon Nav1 channels. The R1902C autism mutation conferred a Ca(2+)-dependent conformational change in Nav1.2 C terminus.CaM complex that was absent in the wild-type complex. In Nav1.5, CaM modulates the Cterminal interaction with the III-IV linker, which has been suggested as necessary to stabilize the inactivation gate, to minimize sustained channel activity during depolarization, and to prevent cardiac arrhythmias that lead to sudden death. Together, these data offer new biochemical evidence for Ca2+/CaM modulation of Na+ channel function.     

The identification and characterization of a noncontinuous calmodulin-binding site in noninactivating voltage-dependent KCNQ potassium channels

We show here that in a yeast two-hybrid assay calmodulin (CaM) interacts with the intracellular C-terminal region of several members of the KCNQ family of potassium channels. CaM co-immunoprecipitates with KCNQ2, KCNQ3, or KCNQ5 subunits better in the absence than in the presence of Ca2+. Moreover, in two-hybrid assays where it is possible to detect interactions with apo-CaM but not with Ca2+-bound calmodulin, we localized the CaM-binding site to a region that is predicted to contain two alpha-helices (A and B). These two helices encompass approximately 85 amino acids, and in KCNQ2 they are separated by a dispensable stretch of approximately 130 amino acids. Within this CaM-binding domain, we found an IQ-like CaM-binding motif in helix A and two overlapping consensus 1-5-10 CaM-binding motifs in helix B. Point mutations in helix A or B were capable of abolishing CaM binding in the two-hybrid assay. Moreover, glutathione S-transferase fusion proteins containing helices A and B were capable of binding to CaM, indicating that the interaction with KCNQ channels is direct. Full-length CaM (both N and C lobes) and a functional EF-1 hand were required for these interactions to occur. These observations suggest that apo-CaM is bound to neuronal KCNQ channels at low resting Ca2+ levels and that this interaction is disturbed when the [Ca2+] is raised. Thus, we propose that CaM acts as a mediator in the Ca2+-dependent modulation of KCNQ channels.     

Characterization of the Munc13-calmodulin interaction by photoaffinity labeling

Sensing of and response to transient increases in the residual presynaptic Ca2+ levels are important adaptive mechanisms that define the short-term plasticity characteristics of neurons. Due to their essential function in synaptic vesicle priming and in the modulation of synaptic strength, Munc13 proteins have emerged as key regulators of these adaptive mechanisms. Indeed, Munc13-1 and ubMunc13-2 contain a conserved calmodulin (CaM) binding site and the Ca2+ -dependent interaction of these Munc13 isoforms with CaM constitutes a molecular mechanism that transduces residual Ca2+ signaling to the synaptic exocytotic machinery. Here, we used Munc13-derived model peptides in photoaffinity labeling (PAL) experiments to demonstrate the stoichiometric and Ca2+ -dependent CaM binding of the other members of the Munc13 family, bMunc13-2 and Munc13-3, via structurally distinct non-conserved binding sites. A PAL-based Ca2+ titration assay revealed that all Munc13 isoforms can form a complex with CaM already at low Ca2+ concentrations just above resting levels, underscoring the Ca2+ sensor/effector function of this interaction in short-term synaptic plasticity phenomena.     

Calcineurin B- and calmodulin-binding preferences identified with phage-displayed peptide libraries

Calcineurin B (CnB) and calmodulin (CaM) are two structurally similar but functionally distinct 'EF-hand' Ca2+-binding proteins. CnB is the regulatory subunit of the CaM-stimulated protein phosphatase, calcineurin. CaM is a unique multifunctional protein that interacts with and modulates the activity of many target proteins. CnB and CaM are both required for the full activation of the phosphatase activity of calcineurin and are not interchangeable. The two proteins recognize distinct binding sites on calcineurin A subunit (CnA) and perform different functions. Phage-displayed peptide libraries (pIII and pVIII libraries) were screened with CnB and CaM to isolate peptides that could then be compared to determine if there were binding preferences of the two proteins. The Ca2+-dependent binding of phage-displayed peptides to CnB and CaM is specifically blocked by synthetic peptides derived from the CnB-binding domain of CnA and the CaM-binding domain of myosin light chain kinase respectively. Both CnB- and CaM-binding peptides have a high content of tryptophan and leucine, but CnB-binding peptides are more hydrophobic than CaM-binding peptides. CnB-binding peptides are negatively charged with clusters of hydrophobic residues rich in phenylalanine, whereas the CaM-binding peptides are positively charged and often contain an Arg/Lys-Trp motif. The binding preferences identified with peptide libraries are consistent with the features of the CnB-binding domains of all CnA isoforms and the CaM-binding domains of CaM targets.     

Calmodulin, conformational states, and calcium signaling. A single-molecule perspective

Single-molecule fluorescence measurements can provide a new perspective on the conformations, dynamics, and interactions of proteins. Recent examples are described illustrating the application of single-molecule fluorescence spectroscopy to calcium signaling proteins with an emphasis on the new information available in single-molecule fluorescence burst measurements, resonance energy transfer, and polarization modulation methods. Calcium signaling pathways are crucial in many cellular processes. The calcium binding protein calmodulin (CaM) serves as a molecular switch to regulate a network of calcium signaling pathways. Single-molecule spectroscopic methods can yield insights into conformations and dynamics of CaM and CaM-regulated proteins. Examples include studies of the conformations and dynamics of CaM, binding of target peptides, and interaction with the plasma-membrane Ca2+ pump. Single-molecule resonance energy transfer measurements revealed conformational substates of CaM, and single-molecule polarization modulation spectroscopy was used to probe interactions between CaM and the plasma-membrane Ca2+-ATPase.     

Interaction of trifluoperazine with Tetrahymena calmodulin. A 19F NMR study

We have used 19F NMR to study interactions of trifluoperazine (TFP), a potent calmodulin (CaM) antagonist, with Tetrahymena calmodulin (Tet. CaM). Changes in chemical shift and bandwidth of TFP caused by adding Tet. CaM in the presence of excess Ca2+ were much smaller than those by adding porcine CaM. The spectral features of the TFP-Tet. CaM solution in the presence of excess Ca2+ were quite similar to those of the TFP-porcine CaM solution in the absence of Ca2+. The exchange rate of TFP from Tet. CaM was estimated to be nearly 20 s-1. The TFP-Tet. CaM solution in the absence of Ca2+ showed a pronounced pH dependence of the 19F NMR chemical shift, whereas the solution in the presence of excess Ca2+ showed a smaller pH dependence. Thus, it was suggested that TFP is located near a hydrophilic region of the Tet. CaM molecule in the absence of Ca2+, while TFP is located near a hydrophobic region of the Tet. CaM in the presence of excess Ca2+.     

Peptides related to the calcium binding domains II and III of calmodulin. Synthesis and calmodulin-like features

Three hexadecapeptides which correspond to the putative Ca2+ binding domains II and III of calmodulin were synthesized employing solid phase methodology. One of the peptides contained an internal cystine bridge which was formed while the corresponding linear peptide was still attached to the polymeric carrier. The interaction of the synthetic peptides with calcium ions was investigated using Tb3+-mediated fluorescence. Binding was of the order Ca12 greater than Ca13 greater than Ca13C (Fig. 1) with binding constants KTb3+ = 0.68 X 10(-5), 0.54 X 10(-5), and 0.21 X 10(-5) M-1 respectively. Biological activity of the compounds was assessed by measuring their stimulatory effect on erythrocyte membrane (Ca2+ + Mg2+)-ATPase activity. For 50% activity as compared with CaM, the concentration of peptides required was for Ca12, Ca13 and Ca13C, 50, 100 and 167 times higher than CaM, respectively. The results suggest that the three synthetic peptides possess certain calmodulin-like features.     

Molecular basis of calmodulin binding to cardiac muscle Ca(2+) release channel (ryanodine receptor)

Calmodulin (CaM) is a ubiquitous Ca2+-binding protein that regulates the ryanodine receptors (RyRs) by direct binding. CaM inhibits the skeletal muscle ryanodine receptor (RyR1) and cardiac muscle receptor (RyR2) at >1 microm Ca2+ but activates RyR1 and inhibits RyR2 at <1 microm Ca2+. Here we tested whether CaM regulates RyR2 by binding to a highly conserved site identified previously in RyR1. Deletion of RyR2 amino acid residues 3583-3603 resulted in background [35S]CaM binding levels. In single channel measurements, deletion of the putative CaM binding site eliminated CaM inhibition of RyR2 at Ca2+ concentrations below and above 1 microm. Five RyR2 single or double mutants in the CaM binding region (W3587A, L3591D, F3603A, W3587A/L3591D, L3591D/F3603A) eliminated or greatly reduced [35S]CaM binding and inhibition of single channel activities by CaM depending on the Ca2+ concentration. An RyR2 mutant, which assessed the effects of 4 amino acid residues that differ between RyR1 and RyR2 in or flanking the CaM binding domain, bound [35S]CaM and was inhibited by CaM, essentially identical to wild type (WT)-RyR2. Three RyR1 mutants (W3620A, L3624D, F3636A) showed responses to CaM that differed from corresponding mutations in RyR2. The results indicate that CaM regulates RyR1 and RyR2 by binding to a single, highly conserved CaM binding site and that other RyR type-specific sites are likely responsible for the differential functional regulation of RyR1 and RyR2 by CaM.     

Structural determinants of calmodulin binding to the intracellular C-terminal domain of the metabotropic glutamate receptor 7A

Calmodulin (CaM) binds in a Ca2+-dependent manner to the intracellular C-terminal domains of most group III metabotropic glutamate receptors (mGluRs). Here we combined mutational and biophysical approaches to define the structural basis of CaM binding to mGluR 7A. Ca2+/CaM was found to interact with mGluR 7A primarily via its C-lobe at a 1:1 CaM:C-tail stoichiometry. Pulldown experiments with mutant CaM and mGluR 7A C-tail constructs and high resolution NMR with peptides corresponding to the CaM binding region of mGluR 7A allowed us to define hydrophobic and ionic interactions required for Ca2+/CaM binding and identified a 1-8-14 CaM-binding motif. The Ca2+/CaM.mGluR 7A peptide complex displays a classical wraparound structure that closely resembles that formed by Ca2+/CaM upon binding to smooth muscle myosin light chain kinase. Our data provide insight into how Ca2+/CaM regulates group III mGluR signaling via competition with intracellular proteins for receptor-binding sites.     

Identification of a novel binding site for calmodulin in ammodytoxin A, a neurotoxic group IIA phospholipase A2.

The molecular mechanism of the presynaptic neurotoxicity of snake venom phospholipases A2 (PLA2s) is not yet fully elucidated. Recently, new high-affinity binding proteins for PLA2 toxins have been discovered, including the important intracellular Ca2+ sensor, calmodulin (CaM). In the present study, the mode of interaction of group IIA PLA2s with the Ca2+-bound form of CaM was investigated by mutational analysis of ammodytoxin A (AtxA) from the long-nosed viper (Vipera ammodytes ammodytes). Several residues in the C-terminal part of AtxA were found to be important in this interaction, particularly those in the region 115-119. In support of this finding, introduction of Y115, I116, R118 and N119, present in AtxA, into a weakly neurotoxic PLA2 from Russell's viper (Daboia russellii russellii) increased by sevenfold its binding affinity for CaM. Furthermore, two out of four peptides deduced from different regions of AtxA were able to compete with the toxin in binding to CaM. The nonapeptide showing the strongest inhibition was that comprising the AtxA region 115-119. This stretch contributes to a distinct hydrophobic patch within the region 107-125 in the C-terminal part of the molecule. This lacks any substantial helical structure and is surrounded by several basic residues, which may form a novel binding motif for CaM on the molecular surface of the PLA2 toxin.     

Glutamic acid in the inhibitory site of mitochondrial ATPase inhibitor, IF(1), participates in pH sensing in both mammals and yeast

The mitochondrial ATPase inhibitor, IF(1), regulates the activity of F(1)F(o)-ATPase. The inhibitory activity of IF(1) is highly pH-dependent. The effective inhibition by IF(1) requires a low pH. Under basic conditions, its activity markedly declines. The importance of His49 in the pH dependence of bovine IF(1) is well-known. However, the residue is not conserved in yeast IF(1). We previously showed that Glu21 is required for the pH dependence of yeast IF(1), but the function of homologous Glu in mammalian IF(1) is not clear. In this study, we examined the requirement for Glu26 of bovine IF(1) (corresponding to Glu21 of yeast IF(1)) regarding its pH dependence by amino acid replacement. Three mutant proteins, E26A, H49K and the double mutant E26A/H49K, were overexpressed and purified. All mutants retained their inhibitory activity well at pH 8.2, although wild-type IF(1) was approximately 10-fold less active at pH 8.2 than at 6.5. A covalent cross-linking study revealed that both wild-type IF(1) and the E26A mutant formed a tetramer at pH 8.2, although H49K and E26A/H49K mutants did not. These results indicate that, in addition to His49, Glu26 participates in pH sensing in bovine IF(1), and the mechanism of pH sensing mediated by Glu26 is different from the dimer-tetramer model proposed previously.     

果蝇TRP离子通道C端一个新钙调蛋白结合位点的鉴定和分析

瞬时受体电位(TRP)通道是一类钙离子透过性的阳离子通道蛋白家族,参与了视觉、味觉、温度感受等重要的生物学过程。之前的研究表明,钙离子既能够正反馈也能够负反馈地调节瞬时受体电位通道的活性,而这种调节可能是通过钙调蛋白（calmodulin,CaM）与TRP通道的相互作用来进行的。为了阐明这一调控机制,我们首先需要对钙调蛋白与瞬时受体电位通道之间的相互作用进行详细的生化研究。在此项研究中,通过大肠杆菌表达系统,表达和纯化了果蝇瞬时受体电位通道羧基末端不同长短的蛋白片段,并发现了一个新的钙调蛋白结合位点。通过快速蛋白液相色谱、静态光散射以及等温量热滴定技术,鉴定了这一钙调蛋白结合位点与果蝇瞬时受体电位通道之间的相互作用,发现它们在钙离子依赖的条件下,可以形成亲和力非常强的稳定的蛋白复合物（解离常数在01～1微摩尔范围）。此外,通过合成多肽的方法,鉴定了果蝇瞬时受体电位通道913～939片段为该钙调蛋白结合位点的核心区域。最后,通过突变实验,进一步明确了果蝇瞬时受体电位通道922位的酪氨酸以及923位的缬氨酸为其钙调蛋白结合位点的关键氨基酸。总而言之,本研究发现和鉴定了果蝇瞬时受体电位通道上一个新的钙依赖的钙调蛋白结合位点,这一发现将为研究瞬时受体电位通道的体内功能提供生化基础,为阐明钙离子通过钙调蛋白调节瞬时受体电位通道的分子机制做出贡献。     

Calmodulin-Mediated Cell Cycle Regulation: New Mechanisms for Old Observations

The significance of divalent calcium ions (Ca2+) to cell cycle progression has been a subject of study for several decades, with a regulatory role for Ca2+ suggested in distinct cell types and multiple organisms. Our interest in proliferative vascular diseases led us to focus on mammalian vascular smooth muscle cells (VSMC) in particular, in which we and others had shown that a coordinate elevation in the intracellular free Ca2+ concentration is required for G1 to S phase cell cycle progression. However, the molecular basis for this Ca2+-sensitive cell cycle transition was not known.Our recent discovery of a functional protein-protein interaction between the late G1-active cyclin E1 and the major calcium signal-transducing factor Calmodulin (CaM) sheds new light on the mechanism(s) through which Ca2+ concentrations regulate cell cycle. Having identified a CaM-binding site on cyclin E1, our studies support a direct role for CaM in mediating Ca2+-sensitive cyclin E/cdk2 activity and G1 to S phase transitions in VSMC. The CaM binding site identified on cyclin E1 has a Kd for CaM consistent with that of known CaM-binding proteins, and is composed of a 22 amino acids N-terminal sequence that is highly conserved across several mammalian species. Deletion of this binding site abolished CaM binding and Ca2+-sensitive cyclin E/cdk2 activity.Here we provide our perspectives on the literature supporting a role for Ca2+ in cell cycle regulation, focusing on the evidence implicating CaM in this functionality, and discuss the potential for therapeutic modulation of CaM-dependent cell cycle machinery.