Recognition and characterization of calmodulin-binding sequences in peptides and proteins

Calmodulin binds the N-terminus of the functional amyloid Orb2A inhibiting fibril formation

The cytoplasmic polyadenylation element-binding protein Orb2 is a key regulator of long-term memory (LTM) in Drosophila. The N-terminus of the Orb2 isoform A is required for LTM and forms cross-β fibrils on its own. However, this N-terminus is not part of the core found in ex vivo fibrils. We previously showed that besides forming cross-β fibrils, the N-terminus of Orb2A binds anionic lipid membranes as an amphipathic helix. Here, we show that the Orb2A N-terminus can similarly interact with calcium activated calmodulin (CaM) and that this interaction prevents fibril formation. Because CaM is a known regulator of LTM, this interaction could potentially explain the regulatory role of Orb2A in LTM.

A two-step purification strategy using calmodulin as an affinity tag

Calmodulin (CaM) is a Ca²⁺-binding protein that plays an important role in cellular Ca²⁺-signaling. CaM interacts with diverse downstream target proteins and regulates their functions in a Ca²⁺-dependent manner. CaM changes its conformation and hydrophobicity upon [Ca²⁺] change and consequently changes its interaction with CaM-binding domains from the targets. Based on these special properties of CaM, it was used as an affinity tag to develop a novel purification strategy by using it for two sequential orthogonal purification steps: 1) an affinity purification step, in which CaM-tag interacts with an immobilized CaM-binding domain; and 2) a hydrophobic interaction chromatography step, during which CaM binds to a phenyl sepharose column. In both steps, the CaM-tagged protein binds in the presence of Ca²⁺ and unbinds in the presence of ethylenediaminetetraacetic acid (EDTA). An optional third step can be added to remove the CaM-tag if necessary. We used green fluorescent protein (GFP) as a test protein to demonstrate the effectiveness of the method. High yield and high purity of GFP with proper function was obtained using this novel strategy. We believe that this method can be applied to a wide range of protein targets for structural and functional studies.

A new calmodulin-binding motif for inositol 1,4,5-trisphosphate 3-kinase regulation

Inositol 1,4,5-trisphosphate 3-kinase regulation by Ca2+/calmodulin involves multiple protein–protein interactions, which form a highly hydrophobic interface and defines a new calmodulin-binding motif. The structural data support that calmodulin binds to an autoinhibitory segment facilitating the kinase activity.

Structural characterization of the interaction of human lactoferrin with calmodulin

Lactoferrin (Lf) is an 80 kDa, iron (Fe³⁺)-binding immunoregulatory glycoprotein secreted into most exocrine fluids, found in high concentrations in colostrum and milk, and released from neutrophil secondary granules at sites of infection and inflammation. In a number of cell types, Lf is internalized through receptor-mediated endocytosis and targeted to the nucleus where it has been demonstrated to act as a transcriptional trans-activator. Here we characterize human Lf’s interaction with calmodulin (CaM), a ubiquitous, 17 kDa regulatory calcium (Ca²⁺)-binding protein localized in the cytoplasm and nucleus of activated cells. Due to the size of this intermolecular complex (∼100 kDa), TROSY-based NMR techniques were employed to structurally characterize Ca²⁺-CaM when bound to intact apo-Lf. Both CaM’s backbone amides and the ε-methyl group of key methionine residues were used as probes in chemical shift perturbation and cross-saturation experiments to define the binding interface of apo-Lf on Ca²⁺-CaM. Unlike the collapsed conformation through which Ca²⁺-CaM binds the CaM-binding domains of its classical targets, Ca²⁺-CaM assumes an extended structure when bound to apo-Lf. Apo-Lf appears to interact predominantly with the C-terminal lobe of Ca²⁺-CaM, enabling the N-terminal lobe to potentially bind another target. Our use of intact apo-Lf has made possible the identification of a secondary interaction interface, removed from CaM’s primary binding domain. Secondary interfaces play a key role in the target’s response to CaM binding, highlighting the importance of studying intact complexes. This solution-based approach can be applied to study other regulatory calcium-binding EF-hand proteins in intact intermolecular complexes.

The transcriptional repressor activity of ASYMMETRIC LEAVES1 is inhibited by direct interaction with calmodulin in Arabidopsis

Calmodulin (CaM), a key Ca2+ sensor, regulates diverse cellular processes by modulating the activity of a variety of enzymes and proteins. However, little is known about the biological function of CaM in plant development. In this study, an ASYMMETRIC LEAVES1 (AS1) transcription factor was isolated as a CaM-binding protein. AS1 contains two putative CaM-binding domains (CaMBDs) at the N-terminus. Using domain mapping analysis, both predicted domains were identified as authentic Ca2+ -dependent CaMBDs. We identified three hydrophobic amino acid residues for CaM binding, Trp49 in CaMBDI, and Trp81 and Phe103 in CaMBDII. The interactions of AS1 with CaM were verified in yeast and plant cells. Based on electrophoretic mobility shift assays, CaM inhibited the DNA-binding activity of the AS1/AS2 complex to two cis-regulatory motifs in the KNAT1 promoter. Furthermore, CaM relieved the suppression of KNAT1 transcription by AS1 not only in transient expression assays of protoplasts but also by the overexpression of a CaM-binding negative form of AS1 in as1 mutant plant. Our study suggests that CaM, a calcium sensor, can be involved in the transcriptional control of meristem cell-specific genes by the inhibition of AS1 under the condition of higher levels of Ca2+ in plants.

Identification of a calmodulin-binding site within the domain I of Bacillus thuringiensis Cry3Aa toxin

Bacillus thuringiensis Cry3Aa toxin is a coleopteran specific toxin highly active against Colorado Potato Beetle (CPB).We have recently shown that Cry3Aa toxin is proteolytically cleaved by CPB midgut membrane associated metalloproteases and that this cleavage is inhibited by ADAM metalloprotease inhibitors. In the present study, we investigated whether the Cry3Aa toxin is a calmodulin (CaM) binding protein, as it is the case of several different ADAM shedding substrates. In pull-down assays using agarose beads conjugated with CaM, we demonstrated that Cry3Aa toxin specifically binds to CaM in a calcium-independent manner. Furthermore, we used gel shift assays and (1)H NMR spectra to demonstrate that CaM binds to a 16-amino acid synthetic peptide corresponding to residues N256-V271 within the domain I of Cry3Aa toxin. Finally, to investigate whether CaM has any effect on Cry3Aa toxin CPB midgut membrane associated proteolysis, cleavage assays were performed in the presence of the CaM-specific inhibitor trifluoperazine. We showed that trifluoperazine significantly increased Cry3Aa toxin proteolysis and also decreased Cry3Aa larval toxicity.

Calcium/calmodulin inhibition of the Arabidopsis BRASSINOSTEROID-INSENSITIVE 1 receptor kinase provides a possible link between calcium and brassinosteroid signalling

The receptor kinase BRI1 (BRASSINOSTEROID-INSENSITIVE 1) is a key component in BR (brassinosteroid) perception and signal transduction, and has a broad impact on plant growth and development. In the present study, we demonstrate that Arabidopsis CaM (calmodulin) binds to the recombinant cytoplasmic domain of BRI1 in a Ca2+-dependent manner in vitro. In silico analysis predicted binding to Helix E of the BRI1 kinase subdomain VIa and a synthetic peptide based on this sequence interacted with Ca2+/CaM. Co-expression of CaM with the cytoplasmic domain of BRI1 in Escherichia coli strongly reduced autophosphorylation of BRI1, in particular on tyrosine residues, and also reduced the BRI1-mediated transphosphorylation of E. coli proteins on tyrosine, threonine and presumably serine residues. Several isoforms of CaM and CMLs (CaM-like proteins) were more effective (AtCaM6, AtCaM7 and AtCML8, where At is Arabidopsis thaliana) than others (AtCaM2, AtCaM4 and AtCML11) when co-expressed with BRI1 in E. coli. These results establish a novel assay for recombinant BRI1 transphosphorylation activity and collectively uncover a possible new link between Ca2+ and BR signalling.

GsCBRLK, a calcium/calmodulin-binding receptor-like kinase, is a positive regulator of plant tolerance to salt and ABA stress

Calcium/calmodulin-dependent kinases play vital roles in protein phosphorylation in eukaryotes, yet little is known about the phosphorylation process of calcium/calmodulin-dependent protein kinase and its role in stress signal transduction in plants. A novel plant-specific calcium-dependent calmodulin-binding receptor-like kinase (GsCBRLK) has been isolated from Glycine soja. A subcellular localization study using GFP fusion protein indicated that GsCBRLK is localized in the plasma membrane. Binding assays demonstrated that calmodulin binds to GsCBRLK with an affinity of 25.9 nM in a calcium-dependent manner and the binding motif lies between amino acids 147 to169 within subdomain II of the kinase domain. GsCBRLK undergoes autophosphorylation and Myelin Basis Protein phosphorylation in the presence of calcium. It was also found that calcium/calmodulin positively regulates GsCBRLK kinase activity through direct interaction between the calmodulin-binding domain and calmodulin. So, it is likely that GsCBRLK responds to an environmental stimulus in two ways: by increasing the protein expression level and by regulating its kinase activity through the calcium/calmodulin complex. Furthermore, cold, salinity, drought, and ABA stress induce GsCBRLK gene transcripts. Over-expression of GsCBRLK in transgenic Arabidopsis resulted in enhanced plant tolerance to high salinity and ABA and increased the expression pattern of a number of stress gene markers in response to ABA and high salt. These results identify GsCBRLK as a molecular link between the stress- and ABA-induced calcium/calmodulin signal and gene expression in plant cells.

Two synthetic peptides corresponding to the proximal heme-binding domain and CD1 domain of human endothelial nitric-oxide synthase inhibit the oxygenase activity by interacting with CaM

Human endothelial nitric-oxide synthase (eNOS) is a complex enzyme, requiring binding of calmodulin (CaM) for electron transfer. The prevailing view is that calcium-activated CaM binds eNOS at the canonical binding site located at residues 493-510, which induces a conformational change to facilitate electron transfer. Here we demonstrated that the CaM enhances the rate of electron transfer from NADPH to FAD on a truncated eNOS FAD subdomain (residues 682-1204) purified from baculovirus-infected Sf9 cells, suggesting more complicated regulatory mechanism of CaM on eNOS. Metabolically (35)S-labeled CaM overlay on fusion proteins spanning the entire linear sequence of eNOS revealed three positive (35)S-CaM binding fragments: sequence 66-205, sequence 460-592, and sequence 505-759. Synthetic peptides derived from these fragments are tested for their effects on CaM binding and eNOS catalytic activities. Peptides corresponding to the proximal heme-binding site (E1, residues 174-193) and the CD1 linker connecting FAD/FMN subdomains (E4, residues 729-757) bind CaM at both high Ca(2+) (Ca(2+)CaM) and low Ca(2+) (apoCaM) concentrations, whereas peptide of the canonical CaM-binding helix (E2, residues 493-510) binds only Ca(2+)CaM. All three peptides E1, E2 and E4 significantly inhibit oxygenase activity in a concentration-dependent manner, but only E2 effectively inhibits reductase activity. Concurrent experiments with human iNOS showed major differences in the CaM binding properties between eNOS and iNOS. The results suggest that multiple regions of eNOS might interact with CaM with differential Ca(2+) sensitivity in vivo. A possible mechanism in regulating eNOS activation and deactivation is proposed.

The identification of a calmodulin-binding domain within the cytoplasmic tail of angiotensin-converting enzyme-2

Angiotensin-converting enzyme (ACE)-2 is a homolog of the well-characterized plasma membrane-bound angiotensin-converting enzyme. ACE2 is thought to play a critical role in regulating heart function, and in 2003, ACE2 was identified as a functional receptor for severe acute respiratory syndrome coronavirus. We have recently shown that like ACE, ACE2 undergoes ectodomain shedding and that this shedding event is up-regulated by phorbol esters. In the present study, we used gel shift assays to demonstrate that calmodulin, an intracellular calcium-binding protein implicated in the regulation of other ectodomain shedding events, binds a 16-amino acid synthetic peptide corresponding to residues 762-777 within the cytoplasmic domain of human ACE2, forming a calcium-dependent calmodulin-peptide complex. Furthermore, we have demonstrated that ACE2 expressed in Chinese hamster ovary cells specifically binds to glutathione-S-transferase-calmodulin, but not glutathione-S-transferase alone, in pull-down assays using cell lysates. Finally, to investigate whether calmodulin has any effect on ACE2 ectodomain shedding in cells that endogenously express the enzyme, cells from a human liver cell line (Huh-7) expressing ACE2 were incubated with calmodulin-specific inhibitors, trifluoperazine and calmidazolium. Both trifluoperazine (25 micromol/liter) and calmidazolium, (25 micromol/liter) significantly increased the release of ACE2 into the medium (44.1 +/- 10.8%, P < 0.05, Student's t test; unpaired, two-tailed, and 51.1 +/- 7.4% P < 0.05, one-way ANOVA, respectively;), as analyzed by an ACE2-specific quenched fluorescence substrate assay. We also show that the calmodulin-specific inhibitor-stimulated shedding of ACE2 is independent from phorbol ester-induced shedding. In summary, we have demonstrated that calmodulin is able to bind ACE2 and suggest that the ACE2 ectodomain shedding and/or sheddase(s) activation regulated by calmodulin is independent from the phorbol ester-induced shedding.

Fluorescence-based electrophoretic mobility shift assay in the analysis of DNA-binding proteins

Changes in gene expression mediated by DNA-binding protein factors are a crucial part of many signal transduction pathways. Generally, these regulatory proteins are low abundant and thus their purification and characterisation is labour- and time-intensive. Here we describe a workflow for purification, characterisation and identification of DNA-binding proteins. We show the use of a fluorescence-based electrophoretic mobility shift assay (fEMSA) and describe its advantages for a rapid and convenient screening for regulatory cis-elements. This involves a crude enrichment of nucleic acid binding proteins by heparin-Sepharose chromatography and the characterisation of fractions using overlapping fluorescence-labelled DNA probes spanning the promoter region of interest. The determined protein-binding sites can then be used for sequence-specific DNA-affinity chromatography to purify specifically interacting proteins. Finally, the DNA-binding complexes can be characterised and identified using two-dimensional EMSA, UV-cross-linking and mass spectrometry.

An S-locus receptor-like kinase in plasma membrane interacts with calmodulin in Arabidopsis

Calmodulin-regulated protein phosphorylation plays a pivotal role in amplifying and diversifying the action of calcium ion. In this study, we identified a calmodulin-binding receptor-like protein kinase (CBRLK1) that was classified into an S-locus RLK family. The plasma membrane localization was determined by the localization of CBRLK1 tagged with a green fluorescence protein. Calmodulin bound specifically to a Ca(2+)-dependent calmodulin binding domain in the C-terminus of CBRLK1. The bacterially expressed CBRLK1 kinase domain could autophosphorylate and phosphorylates general kinase substrates, such as myelin basic proteins. The autophosphorylation sites of CBRLK1 were identified by mass spectrometric analysis of phosphopeptides.

Regulation of MAPK phosphatase 1 (AtMKP1) by calmodulin in Arabidopsis

The mitogen-activated protein kinases (MAPKs) are key signal transduction molecules, which respond to various external stimuli. The MAPK phosphatases (MKPs) are known to be negative regulators of MAPKs in eukaryotes. We screened an Arabidopsis cDNA library using horseradish peroxidase-conjugated calmodulin (CaM), and isolated AtMKP1 as a CaM-binding protein. Recently, tobacco NtMKP1 and rice OsMKP1, two orthologs of Arabidopsis AtMKP1, were reported to bind CaM via a single putative CaM binding domain (CaMBD). However, little is known about the regulation of phosphatase activity of plant MKP1s by CaM binding. In this study, we identified two Ca(2+)-dependent CaMBDs within AtMKP1. Specific binding of CaM to two different CaMBDs was verified using a gel mobility shift assay, a competition assay with a Ca(2+)/CaM-dependent enzyme, and a split-ubiquitin assay. The peptides for two CaMBDs, CaMBDI and CaMBDII, bound CaM in a Ca(2+)-dependent manner, and the binding affinity of CaMBDII was found to be higher than that of CaMBDI. CaM overlay assays using mutated CaMBDs showed that four amino acids, Trp(453) and Leu(456) in CaMBDI and Trp(678) and Ile(684) in CaMBDII, play a pivotal role in CaM binding. Moreover, the phosphatase activity of AtMKP1 was increased by CaM in a Ca(2+)-dependent manner. Our results suggest that two important signaling pathways, Ca(2+) signaling and the MAPK signaling cascade, are connected in plants via the regulation of AtMKP1 activity. To our knowledge, this is the first report to show that the biochemical activity of MKP1 in plants is regulated by CaM.

Specificity of RGS10A as a key component in the RANKL signaling mechanism for osteoclast differentiation

Significant progress has been made in studies of the mechanisms by which RANKL induces terminal osteoclast differentiation. However, many crucial details in the RANKL-evoked signaling pathway for osteoclast differentiation remain to be defined. We characterized genes specifically expressed in osteoclasts by differential screening of a human osteoclastoma cDNA library, and found that the regulator of G-protein signaling 10A (RGS10A), but not the RGS10B isoform, was specifically expressed in human osteoclasts. The expression of RGS10A is also induced by RANKL in osteoclast precursors and is prominently expressed in mouse osteoclast-like cells. RGS10A silencing by RNA interference blocked intracellular [Ca2+]i oscillations, the expression of NFAT2, and osteoclast terminal differentiation in both bone marrow cells and osteoclast precursor cell lines. Reintroduction of RGS10A rescued the impaired osteoclast differentiation. RGS10A silencing also resulted in premature osteoclast apoptosis. RGS10A silencing affected the RANKL-[Ca2+]i oscillation-NFAT2 signaling pathway but not other RANKL-induced responses. Our data demonstrate that target components of RGS10A are distinct from those of RGS12 in the RANKL signaling mechanism. Our results thus show the specificity of RGS10A as a key component in the RANKL-evoked signaling pathway for osteoclast differentiation, which may present a promising target for therapeutic intervention.

RGS10-null mutation impairs osteoclast differentiation resulting from the loss of [Ca2+]i oscillation regulation

Increased osteoclastic resorption leads to many bone diseases, including osteoporosis and rheumatoid arthritis. While rapid progress has been made in characterizing osteoclast differentiation signaling pathways, how receptor activator of nuclear factor kappaB (NF-kappaB) ligand (RANKL) evokes essential [Ca2+]i oscillation signaling remains unknown. Here, we characterized RANKL-induced signaling proteins and found regulator of G-protein signaling 10 (RGS10) is predominantly expressed in osteoclasts. We generated RGS10-deficient (RGS10-/-) mice that exhibited severe osteopetrosis and impaired osteoclast differentiation. Our data demonstrated that ectopic expression of RGS10 dramatically increased the sensitivity of osteoclast differentiation to RANKL signaling; the deficiency of RGS10 resulted in the absence of [Ca2+]i oscillations and loss of NFATc1; ectopic NFATc1 expression rescues impaired osteoclast differentiation from deletion of RGS10; phosphatidylinositol 3,4,5-trisphosphate (PIP3) is essential to PLCgamma activation; and RGS10 competitively interacts with Ca2+/calmodulin and PIP3 in a [Ca2+]i-dependent manner to mediate PLCgamma activation and [Ca2+]i oscillations. Our results revealed a mechanism through which RGS10 specifically regulates the RANKL-evoked RGS10/calmodulin-[Ca2+]i oscillation-calcineurin-NFATc1 signaling pathway in osteoclast differentiation using an in vivo model. RGS10 provides a potential therapeutic target for the treatment of bone diseases.

Binding and activation of nitric oxide synthase isozymes by calmodulin EF hand pairs

Calmodulin (CaM) is a cytosolic Ca(2+) signal-transducing protein that binds and activates many different cellular enzymes with physiological relevance, including the nitric oxide synthase (NOS) isozymes. CaM consists of two globular domains joined by a central linker; each domain contains an EF hand pair. Four different mutant CaM proteins were used to investigate the role of the two CaM EF hand pairs in the binding and activation of the mammalian inducible NOS (iNOS) and the constitutive NOS (cNOS) enzymes, endothelial NOS (eNOS) and neuronal NOS (nNOS). The role of the CaM EF hand pairs in different aspects of NOS enzymatic function was monitored using three assays that monitor electron transfer within a NOS homodimer. Gel filtration studies were used to determine the effect of Ca(2+) on the dimerization of iNOS when coexpressed with CaM and the mutant CaM proteins. Gel mobility shift assays were performed to determine binding stoichiometries of CaM proteins to synthetic NOS CaM-binding domain peptides. Our results show that the N-terminal EF hand pair of CaM contains important binding and activating elements for iNOS, whereas the N-terminal EF hand pair in conjunction with the central linker region is required for cNOS enzyme binding and activation. The iNOS enzyme must be coexpressed with wild-type CaM in vitro because of its propensity to aggregate when residues of the highly hydrophobic CaM-binding domain are exposed to an aqueous environment. A possible role for iNOS aggregation in vivo is also discussed.

Arabidopsis ubiquitin-specific protease 6 (AtUBP6) interacts with calmodulin

Calmodulin (CaM), a key Ca(2+) sensor in eukaryotes, regulates diverse cellular processes by interacting with many proteins. To identify Ca(2+)/CaM-mediated signaling components, we screened an Arabidopsis expression library with horseradish peroxidase-conjugated Arabidopsis calmodulin2 (AtCaM2) and isolated a homolog of the UBP6 deubiquitinating enzyme family (AtUBP6) containing a Ca(2+)-dependent CaM-binding domain (CaMBD). The CaM-binding activity of the AtUBP6 CaMBD was confirmed by CaM mobility shift assay, phosphodiesterase competition assay and site-directed mutagenesis. Furthermore, expression of AtUBP6 restored canavanine resistance to the Deltaubp6 yeast mutant. This is the first demonstration that Ca(2+) signaling via CaM is involved in ubiquitin-mediated protein degradation and/or stabilization in plants.

Interaction of calmodulin with the serotonin 5-hydroxytryptamine2A receptor. A putative regulator of G protein coupling and receptor phosphorylation by protein kinase C

The 5-hydroxytryptamine2A (5-HT2A) receptor is a G(q/11)-coupled serotonin receptor that activates phospholipase C and increases diacylglycerol formation. In this report, we demonstrated that calmodulin (CaM) co-immunoprecipitates with the 5-HT2A receptor in NIH-3T3 fibroblasts in an agonist-dependent manner and that the receptor contains two putative CaM binding regions. The putative CaM binding regions of the 5-HT2A receptor are localized to the second intracellular loop and carboxyl terminus. In an in vitro binding assay peptides encompassing the putative second intracellular loop (i2) and carboxyl-terminal (ct) CaM binding regions bound CaM in a Ca2+-dependent manner. The i2 peptide bound with apparent higher affinity and shifted the mobility of CaM in a nondenaturing gel shift assay. Fluorescence emission spectral analyses of dansyl-CaM showed apparent K(D) values of 65 +/- 30 nM for the i2 peptide and 168 +/- 38 nM for the ct peptide. The ct CaM-binding domain overlaps with a putative protein kinase C (PKC) site, which was readily phosphorylated by PKC in vitro. CaM binding and phosphorylation of the ct peptide were found to be antagonistic, suggesting a putative role for CaM in the regulation of 5-HT2A receptor phosphorylation and desensitization. Finally, we showed that CaM decreases 5-HT2A receptor-mediated [35S]GTPgammaS binding to NIH-3T3 cell membranes, supporting a possible role for CaM in regulating receptor-G protein coupling. These data indicate that the serotonin 5-HT2A receptor contains two high affinity CaM-binding domains that may play important roles in signaling and function.

WRKY group IId transcription factors interact with calmodulin

Calmodulin (CaM) is a ubiquitous Ca(2+)-binding protein known to regulate diverse cellular functions by modulating the activity of various target proteins. We isolated a cDNA encoding AtWRKY7, a novel CaM-binding transcription factor, from an Arabidopsis expression library with horseradish peroxidase-conjugated CaM. CaM binds specifically to the Ca(2+)-dependent CaM-binding domain (CaMBD) of AtWRKY7, as shown by site-directed mutagenesis, a gel mobility shift assay, a split-ubiquitin assay, and a competition assay using a Ca2+/CaM-dependent enzyme. Furthermore, we show that the CaMBD of AtWRKY7 is a conserved structural motif (C-motif) found in group IId of the WRKY protein family.

Direct interaction of a divergent CaM isoform and the transcription factor, MYB2, enhances salt tolerance in arabidopsis

Calmodulin (CaM), a ubiquitous calcium-binding protein, regulates diverse cellular functions by modulating the activity of a variety of enzymes and proteins. Plants express numerous CaM isoforms that exhibit differential activation and/or inhibition of CaM-dependent enzymes in vitro. However, the specific biological functions of plant CaM are not well known. In this study, we isolated a cDNA encoding a CaM binding transcription factor, MYB2, that regulates the expression of salt- and dehydration-responsive genes in Arabidopsis. This was achieved using a salt-inducible CaM isoform (GmCaM4) as a probe from a salt-treated Arabidopsis expression library. Using domain mapping, we identified a Ca2+-dependent CaM binding domain in MYB2. The specific binding of CaM to CaM binding domain was confirmed by site-directed mutagenesis, a gel mobility shift assay, split ubiquitin assay, and a competition assay using a Ca2+/CaM-dependent enzyme. Interestingly, the specific CaM isoform GmCaM4 enhances the DNA binding activity of AtMYB2, whereas this was inhibited by a closely related CaM isoform (GmCaM1). Overexpression of Gm-CaM4 in Arabidopsis up-regulates the transcription rate of AtMYB2-regulated genes, including the proline-synthesizing enzyme P5CS1 (Delta1-pyrroline-5-carboxylate synthetase-1), which confers salt tolerance by facilitating proline accumulation. Therefore, we suggest that a specific CaM isoform mediates salt-induced Ca2+ signaling through the activation of an MYB transcriptional activator, thereby resulting in salt tolerance in plants.

Allosteric regulation of GAP activity by phospholipids in regulators of G-protein signaling

Regulators of G-protein signaling (RGS) proteins are GTPase-activating proteins (GAPs) for alpha subunits of the Gi and/or Gq class of heterotrimeric G proteins. RGS GAP activity is inhibited by phosphatidic acid (PA), lysophosphatidic acid (LPA), and phosphatidylinositol 3,4,5-trisphosphate (PIP3) but not by other phospholipids, phosphoinositides, or diacylglycerol. Both PA and PIP3 can inhibit RGS4 GAP activity and their inhibition is additive, suggesting that PA and PIP3 interact with different domains of RGS4. The N terminus of RGS4 (1-57 amino acids) is required for PA binding and inhibition. Mutation at Lys20, far from the RGS domain of RGS4, decreases PA-mediated inhibition of RGS4 by more than 85%. Amino acid substitutions in helix 5 within the RGS domain of RGS4, opposite to the RGS/Galpha protein contact face, reduce binding affinity and inhibition by PIP3. Calmodulin binds all RGS proteins tested in a Ca(2+)-dependent manner at two sites, one in the N-terminal 33 amino acids and another in the RGS domain. Ca2+/calmodulin does not directly affect GAP activity of RGS4 but reverses PA and PIP3-mediated inhibition. In summary, these results demonstrate that phospholipids such as PA and PIP3 act as allosteric inhibitors of RGS proteins, and Ca2+/calmodulin competition with PA and PIP3 may provide an intracellular mechanism for feedback regulation of Ca2+ signaling evoked by G-protein-coupled agonists.

Molecular variants of the sodium/hydrogen exchanger type 3 gene and essential hypertension

OBJECTIVES

The objectives of this study were to identify polymorphic variants within the gene coding for the sodium/hydrogen exchanger type 3 (NHE3) and to examine their relationship with hypertension and biochemical indices of sodium balance.

DESIGN AND METHODS

Case-control comparisons on a total of 691 subjects of which 399 (68% with essential hypertension) were of African or Afro-Caribbean origin (blacks) and 292 (50% with essential hypertension) were of Caucasian origin (whites).

RESULTS

Eight exons of the C terminus of the NHE3 gene were screened systematically. A total of six variants were identified: (G1579A, G1709A, G1867A, C1945T, A2041G and C2405T). Further analyses in relation to essential hypertension and phenotypic characteristics were confined to the more frequent A2041G and the C2405T polymorphisms. The genotype frequencies of the A2041G polymorphism were significantly different between the whites and blacks, with the A allele being more frequent in the white population (0.43 for the whites and 0.14 for the blacks, respectively; P < 0.001). In contrast, there was no significant difference in the C2405T polymorphism between whites and blacks (C allele frequency: 0.86 for the whites and 0.88 for the blacks, respectively). In both the white and the black groups, there were no significant associations between these variants and essential hypertension (P > 0.05) or with serum electrolytes, creatinine or plasma renin activity (PRA) (ANOVA P > 0.05).

CONCLUSIONS

These results suggest a high degree of structural conservation of the NHE3 gene; however, the lack of association between these polymorphisms and blood pressure status does not necessarily eliminate the participation of this important sodium/hydrogen exchanger in the pathophysiology of essential hypertension, as we cannot exclude the existence of functionally important genetic variants in other sequences within the NEH3 gene.

Calmodulin Interacts with the Third Intracellular Loop of the Serotonin 5-Hydroxytryptamine1A Receptor at Two Distinct Sites

The serotonin 5-HT(1A) receptor couples to heterotrimeric G proteins and intracellular second messengers, yet no studies have investigated the possible role of additional receptor-interacting proteins in 5-HT(1A) receptor signaling. We have found that the ubiquitous Ca(2+)-sensor calmodulin (CaM) co-immunoprecipitates with the 5-HT(1A) receptor in Chinese hamster ovary fibroblasts. The human 5-HT(1A) receptor contains two putative CaM binding motifs, located in the N- and C-terminal juxtamembrane regions of the third intracellular loop of the receptor. Peptides encompassing both the N-terminal (i3N) and C-terminal (i3C) CaM-binding domains were tested for CaM binding. Using in vitro binding assays in combination with gel shift analysis, we demonstrated Ca(2+)-dependent formation of complexes between CaM and both peptides. We determined kinetic data using a combination of BIAcore surface plasmon resonance (SPR) and dansyl-CaM fluorescence. SPR analysis gave an apparent K(D) of approximately 110 nm for the i3N peptide and approximately 700 nm for the i3C peptide. Both peptides also caused characteristic shifts in the fluorescence emission spectrum of dansyl-CaM, with apparent affinities of 87 +/- 23 nm and 1.70 +/- 0.16 microm. We used bioluminescence resonance energy transfer to show that CaM interacts with the 5-HT(1A) receptor in living cells, representing the first in vivo evidence of a G protein-coupled receptor interacting with CaM. Finally, we showed that CaM binding and phosphorylation of the 5-HT(1A) receptor i3 loop peptides by protein kinase C are antagonistic in vitro, suggesting a possible role for CaM in the regulation of 5-HT(1A) receptor phosphorylation and desensitization. These data suggest that the 5-HT(1A) receptor contains high and moderate affinity CaM binding regions that may play important roles in receptor signaling and function.

Structurally homologous binding of plant calmodulin isoforms to the calmodulin-binding domain of vacuolar calcium-ATPase

The discovery that plants contain multiple calmodulin (CaM) isoforms having variable sequence identity to mammalian CaM has sparked a flurry of new questions regarding the intracellular role of Ca(2+) regulation in plants. To date, the majority of research in this field has focused on the differential enzymatic regulation of various mammalian CaM-dependent enzymes by the different plant CaM isoforms. However, there is comparatively little information on the structural recognition of target enzymes found exclusively in plant cells. Here we have used a variety of spectroscopic techniques, including nuclear magnetic resonance, circular dichroism, and fluorescence spectroscopy, to study the interactions of the most conserved and most divergent CaM isoforms from soybean, SCaM-1, and SCaM-4, respectively, with a synthetic peptide derived from the CaM-binding domain of cauliflower vacuolar calcium-ATPase. Despite their sequence divergence, both SCaM-1 and SCaM-4 interact with the calcium-ATPase peptide in a similar calcium-dependent, stoichiometric manner, adopting an antiparallel binding orientation with an alpha-helical peptide. The single Trp residue is bound in a solvent-inaccessible hydrophobic pocket on the C-terminal domain of either protein. Thermodynamic analysis of these interactions using isothermal titration calorimetry demonstrates that the formation of each calcium-SCaM-calcium-ATPase peptide complex is driven by favorable binding enthalpy and is very similar to the binding of mammalian CaM to the CaM-binding domains of myosin light chain kinases and calmodulin-dependent protein kinase I.

Plant MAPK phosphatase interacts with calmodulins

A mitogen-activated protein kinase (MAPK) phosphatase gene, designated NtMKP1, was isolated as a candidate gene for a calmodulin (CaM)-binding protein from tobacco. NtMKP1 protein has four characteristic domains conserved among plant MAPK phosphatases reported so far, namely a dual specificity protein phosphatase catalytic domain, gelsolin-like domain, putative CaM-binding domain (CaMBD), and serine-rich region, indicating that NtMKP1 is the ortholog of Arabidopsis MKP1. The bacterially expressed NtMKP1 protein physically interacted with three plant-specific types of CaM in an overlay assay with labeled CaMs, showing high affinity to NtCaM1 and NtCaM3 but lower affinity to NtCaM13. The peptide for the putative CaMBD bound both NtCaM1 and NtCaM3 significantly but bound NtCaM13 only slightly. Moreover, CaM overlay assays with mutated CaMBDs revealed that Trp440 and Leu443 in the middle of the basic amphiphilic alpha-helix motif (amino acids 436-453) are critical for binding CaM. In comparison with the transient accumulation of a wound-induced MAPK, WIPK transcript, a prolonged activation of NtMKP1 expression was found in response to wounding and tobacco mosaic virus-induced hypersensitive reaction. In transgenic tobacco plants overexpressing NtMKP1, wound-induced activation of SIPK, salicylic acid-induced MAPK, and WIPK was inhibited. These results suggest that plant CaMs are involved in these stress-activated MAPK cascades via NtMKP1.

Regulation of the Dual Specificity Protein Phosphatase, DsPTP1, through Interactions with Calmodulin

Reversible phosphorylation is a key mechanism for the control of intercellular events in eukaryotic cells. In animal cells, Ca2+/CaM-dependent protein phosphorylation and dephosphorylation are implicated in the regulation of a number of cellular processes. However, little is known on the functions of Ca2+/CaM-dependent protein kinases and phosphatases in Ca2+ signaling in plants. From an Arabidopsis expression library, we isolated cDNA encoding a dual specificity protein phosphatase 1, which is capable of hydrolyzing both phosphoserine/threonine and phosphotyrosine residues of the substrates. Using a gel overlay assay, we identified two Ca2+-dependent CaM binding domains (CaMBDI in the N terminus and CaMBDII in the C terminus). Specific binding of CaM to two CaMBD was confirmed by site-directed mutagenesis, a gel mobility shift assay, and a competition assay using a Ca2+/CaM-dependent enzyme. At increasing concentrations of CaM, the biochemical activity of dual specificity protein phosphatase 1 on the p-nitrophenyl phosphate (pNPP) substrate was increased, whereas activity on the phosphotyrosine of myelin basic protein (MBP) was inhibited. Our results collectively indicate that calmodulin differentially regulates the activity of protein phosphatase, dependent on the substrate. Based on these findings, we propose that the Ca2+ signaling pathway is mediated by CaM cross-talks with a protein phosphorylation signal pathway in plants via protein dephosphorylation.

Indolicidin, a 13-residue basic antimicrobial peptide rich in tryptophan and proline, interacts with Ca(2+)-calmodulin

Indolicidin, ILPWKWPWWPWRR-NH(2), a short 13-residue antimicrobial and cytolytic peptide characterized from bovine neutrophils, has the calmodulin-recognition 1-5-10 hydrophobic pattern (indicated by amino acids in bold), is cationic, and thereby fulfills the requirements to interact with calmodulin. Hence, we have investigated the calmodulin-binding properties of indolicidin. Indolicidin interacted with calmodulin with fairly high affinity in a Ca(2+)-dependent manner. However, when bound, the peptide did not adopt helical conformation. Indolicidin also inhibited calmodulin-stimulated phosphodiesterase activity with IC(50) values in the nanomolar range. Replacement of either the proline residues of indolicidin with alanines or tryptophan residues with phenylalanines did not affect binding to calmodulin. However, these replacements had distinctive effects on the conformations of the bound peptides. While the alanine analog of indolicidin adopted predominantly alpha-helical conformation, the phenylalanine analog remained largely unordered. Differences in the ability of these analogs to inhibit the calmodulin-stimulated phosphodiesterase activity were observed. While the alanine analog was capable of inhibiting the activity with IC(50) values comparable to that of indolicidin, the phenylalanine analog did not inhibit the activity. Our results indicate that ability to adopt amphiphilic alpha-helical structure is not a prerequisite for binding to calmodulin and also binding does not necessarily result in inhibition of calmodulin-stimulated enzyme activities.

Exploring the origins of binding specificity through the computational redesign of calmodulin

Calmodulin (CaM) is a second messenger protein that has evolved to bind tightly to a variety of targets and, as such, exhibits low binding specificity. We redesigned CaM by using a computational protein design algorithm to improve its binding specificity for one of its targets, smooth muscle myosin light chain kinase (smMLCK). Residues in or near the CaM/smMLCK binding interface were optimized; CaM interactions with alternative targets were not directly considered in the optimization. The predicted CaM sequences were constructed and tested for binding to a set of eight targets including smMLCK. The best CaM variant, obtained from a calculation that emphasized intermolecular interactions, showed up to a 155-fold increase in binding specificity. The increase in binding specificity was not due to improved binding to smMLCK, but due to decreased binding to the alternative targets. This finding is consistent with the fact that the sequence of wild-type CaM is nearly optimal for interactions with numerous targets.

Interaction of calmodulin, a sorting nexin and kinase-associated protein phosphatase with the Brassica oleracea S locus receptor kinase

Recognition of self-pollen during the self-incompatibility response in Brassica oleracea is mediated by the binding of a secreted peptide (the S locus cysteine-rich protein) to the S locus receptor kinase (SRK), a member of the plant receptor kinase (PRK) superfamily. Here, we describe the characterization of three proteins that interact with the cytosolic kinase domain of SRK. A B. oleracea homolog of Arabidopsis kinase-associated protein phosphatase was shown to interact with and dephosphorylate SRK and was itself phosphorylated by SRK. Yeast (Saccharomyces cerevisiae) two-hybrid screens identified two additional interactors, calmodulin and a sorting nexin, both of which have been implicated in receptor kinase down-regulation in animals. A calmodulin-binding site was identified in sub-domain VIa of the SRK kinase domain. The binding site is conserved and functional in several other members of the PRK family. The sorting nexin also interacted with diverse members of the PRK family, suggesting that all three of the interacting proteins described here may play a general role in signal transduction by this family of proteins.

Novel aspects of calmodulin target recognition and activation

Several crystal and NMR structures of calmodulin (CaM) in complex with fragments derived from CaM-regulated proteins have been reported recently and reveal novel ways for CaM to interact with its targets. This review will discuss and compare features of the interaction between CaM and its target domains derived from the plasma membrane Ca2+-pump, the Ca2+-activated K+-channel, the Ca2+/CaM-dependent kinase kinase and the anthrax exotoxin. Unexpected aspects of CaM/target interaction observed in these complexes include: (a) binding of the Ca2+-pump domain to only the C-terminal part of CaM (b) dimer formation with fragments of the K+-channel (c) insertion of CaM between two domains of the anthrax exotoxin (d) binding of Ca2+ ions to only one EF-hand pair and (e) binding of CaM in an extended conformation to some of its targets. The mode of interaction between CaM and these targets differs from binding conformations previously observed between CaM and peptides derived from myosin light chain kinase (MLCK) and CaM-dependent kinase IIalpha (CaMKIIalpha). In the latter complexes, CaM engulfs the CaM-binding domain peptide with its two Ca2+-binding lobes and forms a compact, ellipsoid-like complex. In the early 1990s, a model for the activation of CaM-regulated proteins was developed based on this observation and postulated activation through the displacement of an autoinhibitory or regulatory domain from the target protein upon binding of CaM. The novel structures of CaM-target complexes discussed here demonstrate that this mechanism of activation may be less general than previously believed and seems to be not valid for the anthrax exotoxin, the CaM-regulated K+-channel and possibly also not for the Ca2+-pump.

Using nitrile-derivatized amino acids as infrared probes of local environment

It is well-known that the C=N stretching vibration in acetonitrile is sensitive to solvent. Therefore, we proposed in this contribution to use this vibrational mode to report local environment of a particular amino acid in proteins or local environmental changes upon binding or folding. We have studied the solvent-induced frequency shift of two nitrile-derivatized amino acids, which are, AlaCN and PheCN, in H(2)O and tetrahydrofuran (THF), respectively. Here, THF was used to approximate a protein's hydrophobic interior because of its low dielectric constant. As expected, the C=N stretching vibrations of both AlaCN and PheCN shift as much as approximately 10 cm(-1) toward higher frequency when THF was replaced with H2O, indicative of the sensitivity of this vibration to solvation. To further test the utility of nitrile-derivatized amino acids as probes of the environment within a peptide, we have studied the binding between calmodulin (CaM) and a peptide from the CaM binding domain of skeletal muscle myosin light chain kinase (MLCK(579-595)), which contains a single PheCN. MLCK(579-595) binds to CaM in a helical conformation. When the PheCN was substituted on the polar side of the helix, which was partially exposed to water, the C=N stretching vibration is similar to that of PheCN in water. In constrast, when PheCN is introduced at a site that becomes buried in the interior of the protein, the C=N stretch is similar to that of PheCN in THF. Together, these results suggest that the C=N stretching vibration of nitrile-derivatized amino acids can indeed be used as local internal environmental markers, especially for protein conformational studies.

Differential expression and functional analysis of three calmodulin isoforms in germinating pea (Pisum sativum L.) seeds

Implication of the ubiquitous, highly conserved, Ca2+ sensor calmodulin (CaM) in pea seed germination has been investigated. Mass spectrometry analysis of purified CaM revealed the coexistence in seeds of three protein isoforms, diverging from each other by single amino acid substitution in the N-terminal alpha-helix. CaM was shown to be encoded by a small multigenic family, and full-length cDNAs of the three isoforms (PsCaM1, 2 and 3) were isolated to allow the design of specific primers in more divergent 5' and 3' untranslated regions. Expression studies, performed by semiquantitative RT-PCR, demonstrated differential expression patterns of the three transcripts during germination. PsCaM1 and 2 were detected at different levels in dry axes and cotyledons, and they accumulated during imbibition and prior to radicle protrusion. In contrast, PsCaM3 appeared only upon radicle protrusion, then gradually increased in both tissues. To characterise the biochemical properties of the CaM isoforms, functional analyses were conducted in vitro using recombinant Strep-tagged proteins (CaM1-ST, CaM2-ST and CaM3-ST) expressed in Escherichia coli. Gel mobility shift assays revealed that CaM1-ST exhibited a stoichiometric binding of a synthetic amphiphilic CaM kinase II peptide while CaM2-ST and CaM3-ST affinities for the same peptide were reduced. Affinity differences were also observed for CaM isoform binding to Trp-3, an idealised helical CaM-binding peptide. However, the three proteins activated in the same way the CaM-dependent pea NAD kinase. Finally, the significance of the single substitutions upon CaM interaction with its targets is discussed in a structural context.

Identification of calmodulin isoform-specific binding peptides from a phage-displayed random 22-mer peptide library

Plants express numerous calmodulin (CaM) isoforms that exhibit differential activation or inhibition of CaM-dependent enzymes in vitro; however, their specificities toward target enzyme/protein binding are uncertain. A random peptide library displaying a 22-mer peptide on a bacteriophage surface was constructed to screen peptides that specifically bind to plant CaM isoforms (soybean calmodulin (ScaM)-1 and SCaM-4 were used in this study) in a Ca2+-dependent manner. The deduced amino acid sequence analyses of the respective 80 phage clones that were independently isolated via affinity panning revealed that SCaM isoforms require distinct amino acid sequences for optimal binding. SCaM-1-binding peptides conform to a 1-5-10 ((FILVW)XXX(FILV) XXXX(FILVW)) motif (where X denotes any amino acid), whereas SCaM-4-binding peptide sequences conform to a 1-8-14 ((FILVW)XXXXXX(FAILVW)XXXXX(FILVW)) motif. These motifs are classified based on the positions of conserved hydrophobic residues. To examine their binding properties further, two representative peptides from each of the SCaM isoform-binding sequences were synthesized and analyzed via gel mobility shift assays, Trp fluorescent spectra analyses, and phosphodiesterase competitive inhibition experiments. The results of these studies suggest that SCaM isoforms possess different binding sequences for optimal target interaction, which therefore may provide a molecular basis for CaM isoform-specific function in plants. Furthermore, the isolated peptide sequences may serve not only as useful CaM-binding sequence references but also as potential reagents for studying CaM isoform-specific function in vivo.

Mlo, a modulator of plant defense and cell death, is a novel calmodulin-binding protein. Isolation and characterization of a rice Mlo homologue

Transient influx of Ca(2+) constitutes an early event in the signaling cascades that trigger plant defense responses. However, the downstream components of defense-associated Ca(2+) signaling are largely unknown. Because Ca(2+) signals are mediated by Ca(2+)-binding proteins, including calmodulin (CaM), identification and characterization of CaM-binding proteins elicited by pathogens should provide insights into the mechanism by which Ca(2+) regulates defense responses. In this study, we isolated a gene encoding rice Mlo (Oryza sativa Mlo; OsMlo) using a protein-protein interaction-based screening of a cDNA expression library constructed from pathogen-elicited rice suspension cells. OsMlo has a molecular mass of 62 kDa and shares 65% sequence identity and scaffold topology with barley Mlo, a heptahelical transmembrane protein known to function as a negative regulator of broad spectrum disease resistance and leaf cell death. By using gel overlay assays, we showed that OsMlo produced in Escherichia coli binds to soybean CaM isoform-1 (SCaM-1) in a Ca(2+)-dependent manner. We located a 20-amino acid CaM-binding domain (CaMBD) in the OsMlo C-terminal cytoplasmic tail that is necessary and sufficient for Ca(2+)-dependent CaM complex formation. Specific binding of the conserved CaMBD to CaM was corroborated by site-directed mutagenesis, a gel mobility shift assay, and a competition assay with a Ca(2+)/CaM-dependent enzyme. Expression of OsMlo was strongly induced by a fungal pathogen and by plant defense signaling molecules. We propose that binding of Ca(2+)-loaded CaM to the C-terminal tail may be a common feature of Mlo proteins.

Isolation and characterization of a novel calmodulin-binding protein from potato

Tuberization in potato is controlled by hormonal and environmental signals. Ca(2+), an important intracellular messenger, and calmodulin (CaM), one of the primary Ca(2+) sensors, have been implicated in controlling diverse cellular processes in plants including tuberization. The regulation of cellular processes by CaM involves its interaction with other proteins. To understand the role of Ca(2+)/CaM in tuberization, we have screened an expression library prepared from developing tubers with biotinylated CaM. This screening resulted in isolation of a cDNA encoding a novel CaM-binding protein (potato calmodulin-binding protein (PCBP)). Ca(2+)-dependent binding of the cDNA-encoded protein to CaM is confirmed by (35)S-labeled CaM. The full-length cDNA is 5 kb long and encodes a protein of 1309 amino acids. The deduced amino acid sequence showed significant similarity with a hypothetical protein from another plant, Arabidopsis. However, no homologs of PCBP are found in nonplant systems, suggesting that it is likely to be specific to plants. Using truncated versions of the protein and a synthetic peptide in CaM binding assays we mapped the CaM-binding region to a 20-amino acid stretch (residues 1216-1237). The bacterially expressed protein containing the CaM-binding domain interacted with three CaM isoforms (CaM2, CaM4, and CaM6). PCBP is encoded by a single gene and is expressed differentially in the tissues tested. The expression of CaM, PCBP, and another CaM-binding protein is similar in different tissues and organs. The predicted protein contained seven putative nuclear localization signals and several strong PEST motifs. Fusion of the N-terminal region of the protein containing six of the seven nuclear localization signals to the reporter gene beta-glucuronidase targeted the reporter gene to the nucleus, suggesting a nuclear role for PCBP.

Application of different surface plasmon resonance biosensor chips to monitor the interaction of the CaM-binding site of nitric oxide synthase I and calmodulin

Surface plasmon resonance biosensors depend on modified gold surfaces to allow immobilization of proteins or peptides for interaction analysis. We investigated sensor chip surfaces that differ in the geometry of the immobilization matrix: two contain a three-dimensional coupling matrix and two have a surface with immobilization sites on a two-dimensional plane. Properties of sensor chips were compared by studying the interaction of calmodulin with a peptide representing the calmodulin-binding site of nitric oxide synthase I. Apparent K(D) values were determined by three different procedures in order to apply tests for self-consistency. At low surface densities (5-8 fmol/mm(2)) on three of the four tested surfaces, estimated K(D) values were within one order of magnitude and similar to the value found in solution (K(D) = 1-3 nM). When immobilization densities were increased by one to two orders of magnitude, apparent association rate constants were less distorted on a flat carboxymethylated surface than on dextran-coated sensor chips.

Identification of common binding sites for calmodulin and inositol 1,4,5-trisphosphate receptors on the carboxyl termini of trp channels

Homologues of Drosophila Trp (transient receptor potential) form plasma membrane channels that mediate Ca(2+) entry following the activation of phospholipase C by cell surface receptors. Among the seven Trp homologous found in mammals, Trp3 has been shown to interact with and respond to IP(3) receptors (IP(3)Rs) for activation. Here we show that Trp4 and other Trp proteins also interact with IP(3)Rs. The IP(3)R-binding domain also interacts with calmodulin (CaM) in a Ca(2+)-dependent manner with affinities ranging from 10 nm for Trp2 to 290 nm for Trp6. In addition, other binding sites for CaM and IP(3)Rs are present in the alpha but not the beta isoform of Trp4. In the presence of Ca(2+), the Trp-IP(3)R interaction is inhibited by CaM. However, a synthetic peptide representing a Trp-binding domain of IP(3)Rs inhibited the binding of CaM to Trp3, -6, and -7 more effectively than that to Trp1, -2, -4, and -5. In inside-out membrane patches, Trp4 is activated strongly by calmidazolium, an antagonist of CaM, and a high (50 microm) but not a low (5 microm) concentration of the Trp-binding peptide of the IP(3)R. Our data support the view that both CaM and IP(3)Rs play important roles in controlling the gating of Trp-based channels. However, the sensitivity and responses to CaM and IP(3)Rs differ for each Trp.

The transient receptor potential, TRP4, cation channel is a novel member of the family of calmodulin binding proteins

The mammalian gene products, transient receptor potential (trp)1 to trp7, are related to the Drosophila TRP and TRP-like ion channels, and are candidate proteins underlying agonist-activated Ca(2+)-permeable ion channels. Recently, the TRP4 protein has been shown to be part of native store-operated Ca(2+)-permeable channels. These channels, most likely, are composed of other proteins in addition to TRP4. In the present paper we report the direct interaction of TRP4 and calmodulin (CaM) by: (1) retention of in vitro translated TRP4 and of TRP4 protein solubilized from bovine adrenal cortex by CaM-Sepharose in the presence of Ca(2+), and (2) TRP4-glutathione S-transferase pull-down experiments. Two domains of TRP4, amino acid residues 688-759 and 786-848, were identified as being able to interact with CaM. The binding of CaM to both domains occurred only in the presence of Ca(2+) concentrations above 10 microM, with half maximal binding occurring at 16.6 microM (domain 1) and 27.9 microM Ca(2+) (domain 2). Synthetic peptides, encompassing the two putative CaM binding sites within these domains and covering amino acid residues 694-728 and 829-853, interacted directly with dansyl-CaM with apparent K(d) values of 94-189 nM. These results indicate that TRP4/Ca(2+)-CaM are parts of a signalling complex involved in agonist-induced Ca(2+) entry.

Activation of Trp3 by inositol 1,4,5-trisphosphate receptors through displacement of inhibitory calmodulin from a common binding domain

Mammalian homologues of Drosophila Trp form plasma membrane channels that mediate Ca(2+) influx in response to activation of phospholipase C and internal Ca(2+) store depletion. Previous studies showed that human Trp3 is activated by inositol 1,4,5-trisphosphate (IP(3)) receptors (IP(3)Rs) and identified interacting domains, one on Trp and two on IP(3)R. We now find that Trp3 binds Ca(2+)-calmodulin (Ca(2+)/CaM) at a site that overlaps with the IP(3)R binding domain. Using patch-clamp recordings from inside-out patches, we further show that Trp3 has a high intrinsic activity that is suppressed by Ca(2+)/CaM under resting conditions, and that Trp3 is activated by the following: a Trp-binding peptide from IP(3)R that displaces CaM from Trp3, a myosin light chain kinase Ca(2+)/CaM binding peptide that prevents CaM from binding to Trp3, and calmidazolium, an inactivator of Ca(2+)/CaM. We conclude that inhibition of the inhibitory action of CaM is a key step of Trp3 channel activation by IP(3)Rs.

Competitive regulation of CaT-like-mediated Ca2+ entry by protein kinase C and calmodulin

A finely tuned Ca(2+) signaling system is essential for cells to transduce extracellular stimuli, to regulate growth, and to differentiate. We have recently cloned CaT-like (CaT-L), a highly selective Ca(2+) channel closely related to the epithelial calcium channels (ECaC) and the calcium transport protein CaT1. CaT-L is expressed in selected exocrine tissues, and its expression also strikingly correlates with the malignancy of prostate cancer. The expression pattern and selective Ca(2+) permeation properties suggest an important function in Ca(2+) uptake and a role in tumor progression, but not much is known about the regulation of this subfamily of ion channels. We now demonstrate a biochemical and functional mechanism by which cells can control CaT-L activity. CaT-L is regulated by means of a unique calmodulin binding site, which, at the same time, is a target for protein kinase C-dependent phosphorylation. We show that Ca(2+)-dependent calmodulin binding to CaT-L, which facilitates channel inactivation, can be counteracted by protein kinase C-mediated phosphorylation of the calmodulin binding site.

Biological activities of retro and diastereo analogs of a 13-residue peptide with antimicrobial and hemolytic activities

The biological activities of synthetic retro and diastereo analogs of PKLLKTFLSKWIG (SPFK), a 13-residue peptide with antimicrobial and hemolytic activities, have been investigated. Retro peptides with C-terminal acid and amide exhibited antibacterial activities comparable with those of SPFK. Their hemolytic activities were, however, only marginally lower. The diastereo analog with C-terminal acid was not antibacterial and was weakly hemolytic. Amidation of this analog could restore antibacterial activity. Both retro analogs were unordered in aqueous medium but had a propensity for a helical structure in trifluoroethanol. However, diastereo analogs were unordered in both aqueous medium and trifluoroethanol. Thus, reversing the sequence in a short amphiphilic peptide may not always result in the selective loss of biological activity such as hemolytic activity. Also, introduction of enantiomeric amino acids in a short peptide to generate a diastereomer may result in loss of structure as well as antimicrobial and hemolytic activities, unless compensated by an increase in positive charges.

Regulation of a recombinant pea nuclear apyrase by calmodulin and casein kinase II

A cDNA encoding a pea nuclear apyrase was previously cloned. Overexpressions of a full-length and a truncated cDNA have been successfully expressed in Escherichia coli BL21(DE3). The resulting fusion proteins, apyrase and the C-terminus (residues 315-453) of apyrase, were used for calmodulin (CaM) binding and phosphorylation studies. Fusion protein apyrase but not the C-terminus of apyrase can be recognized by polyclonal antibody pc480. This suggested that the motif recognized by pc480 was located in the N-terminal region of apyrase. The recombinant apyrase protein also showed an activity 70 times higher than that of endogenous apyrase using ATP as a substrate. The recombinant apyrase has a preference for ATP more than other nucleoside triphosphate substrates. CaM can bind to recombinant apyrase, but not to the C-terminus of apyrase. This implies that the CaM-binding domain must be in the first 315 amino acids of the N-terminal region of apyrase. We found that one segment from residue 293 to 308 was a good candidate for the CaM-binding domain. This segment 293 FNKCKNTIRKALKLNY 308 has a basic amphiphilic-helical structure, which shows the predominance of basic residues on one side and hydrophobic residues on the other when displayed on a helical wheel plot. Using the gel mobility shift binding assay, this synthetic peptide was shown to bind to CaM, indicating that it is the CaM-binding domain. Both recombinant apyrase and the C-terminus of apyrase can be phosphorylated by a recombinant human protein kinase CKII. Phosphorylation does not affect CaM binding to recombinant apyrase. However, CaM does inhibit CKII phosphorylation of recombinant apyrase and this inhibition can be blocked by 5 mM EGTA.

Striated muscle preferentially expressed genes alpha and beta are two serine/threonine protein kinases derived from the same gene as the aortic preferentially expressed gene-1

Aortic preferentially expressed gene (APEG)-1 is a 1.4-kilobase pair (kb) mRNA expressed in vascular smooth muscle cells and is down-regulated by vascular injury. An APEG-1 5'-end cDNA probe identified three additional isoforms. The 9-kb striated preferentially expressed gene (SPEG)alpha and the 11-kb SPEGbeta were found in skeletal muscle and heart. The 4-kb brain preferentially expressed gene was detected in the brain and aorta. We report here cloning of the 11-kb SPEGbeta cDNA. SPEGbeta encodes a 355-kDa protein that contains two serine/threonine kinase domains and is homologous to proteins of the myosin light chain kinase family. At least one kinase domain is active and capable of autophosphorylation. In the genome, all four isoforms share the middle three of the five exons of APEG-1, and they differ from each other by using different 5'- and 3'-ends and alternative splicing. We show that the expression of SPEGalpha and SPEGbeta is developmentally regulated in the striated muscle during C2C12 myoblast to myotube differentiation in vitro and cardiomyocyte maturation in vivo. This developmental regulation suggests that both SPEGalpha and SPEGbeta can serve as sensitive markers for striated muscle differentiation and that they may be important for adult striated muscle function.

A pollen-specific novel calmodulin-binding protein with tetratricopeptide repeats

Calcium is essential for pollen germination and pollen tube growth. A large body of information has established a link between elevation of cytosolic Ca(2+) at the pollen tube tip and its growth. Since the action of Ca(2+) is primarily mediated by Ca(2+)-binding proteins such as calmodulin (CaM), identification of CaM-binding proteins in pollen should provide insights into the mechanisms by which Ca(2+) regulates pollen germination and tube growth. In this study, a CaM-binding protein from maize pollen (maize pollen calmodulin-binding protein, MPCBP) was isolated in a protein-protein interaction-based screening using (35)S-labeled CaM as a probe. MPCBP has a molecular mass of about 72 kDa and contains three tetratricopeptide repeats (TPR) suggesting that it is a member of the TPR family of proteins. MPCBP protein shares a high sequence identity with two hypothetical TPR-containing proteins from Arabidopsis. Using gel overlay assays and CaM-Sepharose binding, we show that the bacterially expressed MPCBP binds to bovine CaM and three CaM isoforms from Arabidopsis in a Ca(2+)-dependent manner. To map the CaM-binding domain several truncated versions of the MPCBP were expressed in bacteria and tested for their ability to bind CaM. Based on these studies, the CaM-binding domain was mapped to an 18-amino acid stretch between the first and second TPR regions. Gel and fluorescence shift assays performed with CaM and a CaM-binding synthetic peptide further confirmed MPCBP binding to CaM. Western, Northern, and reverse transcriptase-polymerase chain reaction analysis have shown that MPCBP expression is specific to pollen. MPCBP was detected in both soluble and microsomal proteins. Immunoblots showed the presence of MPCBP in mature and germinating pollen. Pollen-specific expression of MPCBP, its CaM-binding properties, and the presence of TPR motifs suggest a role for this protein in Ca(2+)-regulated events during pollen germination and growth.

The binding of myristoylated N-terminal nonapeptide from neuro-specific protein CAP-23/NAP-22 to calmodulin does not induce the globular structure observed for the calmodulin-nonmyristylated peptide complex

CAP-23/NAP-22, a neuron-specific protein kinase C substrate, is Nalpha-myristoylated and interacts with calmodulin (CaM) in the presence of Ca2+ ions. Takasaki et al. (1999, J Biol Chem 274:11848-11853) have recently found that the myristoylated N-terminal nonapeptide of CAP-23/NAP-22 (mC/N9) binds to Ca2+ -bound CaM (Ca2+/CaM). In the present study, small-angle X-ray scattering was used to investigate structural changes of Ca2+/CaM induced by its binding to mC/N9 in solution. The binding of one mC/N9 molecule induced an insignificant structural change in Ca2+/CaM. The 1:1 complex appeared to retain the extended conformation much like that of Ca2+/CaM in isolation. However, it could be seen that the binding of two mC/N9 molecules induced a drastic structural change in Ca2+/CaM, followed by a slight structural change by the binding of more than two but less than four mC/N9 molecules. Under the saturated condition (the molar ratio of 1:4), the radius of gyration (Rg) for the Ca2+/CaM-mC/N9 complex was 19.8 +/- 0.3 A. This value was significantly smaller than that of Ca2+/CaM (21.9 +/- 0.3 A), which adopted a dumbbell structure and was conversely 2-3 A larger than those of the complexes of Ca2+/CaM with the nonmyristoylated target peptides of myosin light chain kinase or CaM kinase II, which adopted a compact globular structure. The pair distance distribution function had no shoulder peak at around 40 A, which was mainly due to the dumbbell structure. These results suggest that Ca2+/CaM interacts with Nalpha-myristoylated CAP-23/NAP-22 differently than it does with other nonmyristoylated target proteins. The N-terminal amino acid sequence alignment of CAP-23/NAP-22 and other myristoylated proteins suggests that the protein myristoylation plays important roles not only in the binding of CAP-23/NAP-22 to Ca2+/CaM, but also in the protein-protein interactions related to other myristoylated proteins.

Maize cap1 encodes a novel SERCA-type calcium-ATPase with a calmodulin-binding domain

A cDNA (CAP1) isolated from maize roots shares sequence identity with genes encoding P-type Ca(2+)-ATPases and restores the growth phenotype of yeast mutants defective in Ca(2+)-pumps. CAP1 was transcribed and translated in the yeast mutant. Furthermore, the membrane-integrated product formed a Ca(2+)-dependent phosphorylated intermediate and supported Ca(2+) transport. Although CAP1 shares greater sequence identity with mammalian "endoplasmic reticulum-type" Ca(2+)-pumps, it differs from these genes by having features of calmodulin (CaM)-regulated Ca(2+)-pumps. CAP1 from yeast microsomes bound CaM, and the CAP1-dependent Ca(2+) transport in yeast was stimulated by CaM. Peptides from the C terminus of CAP1 bound CaM. Anti-CAP1 antibodies specifically recognized a maize microsomal polypeptide that also bound CaM. A similar polypeptide also formed a Ca(2+)-dependent phosphoenzyme. Our results suggest that cap1 encodes a novel form of CaM-regulated Ca(2+)-ATPase in maize. CAP1 appears to be encoded by one or two genes in maize. CAP1 RNA is induced only during early anoxia, indicating that the Ca(2+)-pump may play an important role in O(2)-deprived maize cells.

Ca2+/Calmodulin reverses phosphatidylinositol 3,4, 5-trisphosphate-dependent inhibition of regulators of G protein-signaling GTPase-activating protein activity

Regulators of G protein signaling (RGS proteins) are GTPase-activating proteins (GAPs) for G(i) and/or G(q) class G protein alpha subunits. RGS GAP activity is inhibited by phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) but not by other lipid phosphoinositides or diacylglycerol. Both the negatively charged head group and long chain fatty acids (C16) are required for binding and inhibition of GAP activity. Amino acid substitutions in helix 5 within the RGS domain of RGS4 reduce binding affinity and inhibition by PIP(3) but do not affect inhibition of GAP activity by palmitoylation. Conversely, the GAP activity of a palmitoylation-resistant mutant RGS4 is inhibited by PIP(3). Calmodulin binds all RGS proteins we tested in a Ca(2+)-dependent manner but does not directly affect GAP activity. Indeed, Ca(2+)/calmodulin binds a complex of RGS4 and a transition state analog of Galpha(i1)-GDP-AlF(4)(-). Ca(2+)/calmodulin reverses PIP(3)-mediated but not palmitoylation-mediated inhibition of GAP activity. Ca(2+)/calmodulin competition with PIP(3) may provide an intracellular mechanism for feedback regulation of Ca(2+) signaling evoked by G protein-coupled agonists.

Peptide and metal ion-dependent association of isolated helix-loop-helix calcium binding domains: studies of thrombic fragments of calmodulin

Calmodulin (CaM), the ubiquitous, eukaryotic, bilobal calcium-binding regulatory protein, has been cleaved by thrombin to create two fragments. TM1 (1-106) and TM2 (107-148). NMR and CD results indicate that TMI and TM2 can associate in the presence of Ca2+ to form a complex similar to native CaM, even though the cleavage site is not in the linker region between two helix-loop-helix domains, but rather within an alpha-helix. Cadmium-113 NMR results show that this complex has enhanced metal-ion binding properties when compared to either TM1 or TM2 alone. This complex can bind several CaM-binding target peptides, as shown by gel bandshift assays, circular dichroism spectra, and 13C NMR spectra of biosynthetically methyl-13C-Met-labeled TM1 and TM2; moreover, gel bandshift assays show that the addition of a target peptide strengthens the interactions between TM1 and TM2 and increases the stability of the complex. Cadmium-113 NMR spectra indicate that the TM1:TM2 complex can also bind the antipsychotic drug trifluoperazine. However, in contrast to CaM:peptide complexes, the TM1:TM2:peptide complexes are disrupted by 4 M urea; moreover, TM1 and TM2 in combination are unable to activate CaM-dependent enzymes. This suggests that TM1:TM2 mixtures cannot bind target molecules as tightly as intact CaM, or perhaps that binding occurs but additional interactions with the target enzymes that are necessary for proper activation are perturbed by the proteolytic cleavage. The results presented here reflect the importance of the existence of helix-loop-helix Ca2+-binding domains in pairs in proteins such as CaM, and extend the understanding of the association of such domains in this class of proteins in general.