Fluorescence analysis of calmodulin mutants containing tryptophan: conformational changes induced by calmodulin-binding peptides from myosin light chain kinase and protein kinase II

An interdomain distance in cardiac troponin C determined by fluorescence spectroscopy

The distance between Ca2+-binding site III in the C-terminal domain and Cys35 in the N-terminal domain in cardiac muscle troponin C (cTnC) was determined with a single-tryptophan mutant using bound Tb3+ as the energy donor and iodoacetamidotetramethylrhodamine linked to the cysteine residue as energy acceptor. The luminescence of bound Tb3+ was generated through sensitization by the tryptophan located in the 12-residue binding loop of site III upon irradiation at 295 nm, and this sensitized luminescence was the donor signal transferred to the acceptor. In the absence of bound cation at site II, the mean interdomain distance was found to be 48-49 A regardless of whether the cTnC was unbound or bound to cardiac troponin I, or reconstituted into cardiac troponin. These results suggest that cTnC retains its overall length in the presence of bound target proteins. The distribution of the distances was wide (half-width >9 A) and suggests considerable interdomain flexibility in isolated cTnC, but the distributions became narrower for cTnC in the complexes with the other subunits. In the presence of bound cation at the regulatory site II, the interdomain distance was shortened by 6 A for cTnC, but without an effect on the half-width. The decrease in the mean distance was much smaller or negligible when cTnC was complexed with cTnI or cTnI and cTnT under the same conditions. Although free cTnC has considerable interdomain flexibility, this dynamics is slightly reduced in troponin. These results indicate that the transition from the relaxed state to an activated state in cardiac muscle is not accompanied by a gross alteration of the cTnC conformation in cardiac troponin.

De novo design of peptides targeted to the EF hands of calmodulin

This report describes the use of the concept of inversion of hydropathy patterns to the de novo design of peptides targeted to a predetermined site on a protein. Eight- and 12-residue peptides were constructed with the EF hands or Ca(2+)-coordinating sites of calmodulin as their anticipated points of interaction. These peptides, but not unrelated peptides nor those with the same amino acid composition but a scrambled sequence, interacted with the two carboxyl-terminal Ca(2+)-binding sites of calmodulin as well as the EF hands of troponin C. The interactions resulted in a conformational change whereby the 8-mer peptide-calmodulin complex could activate phosphodiesterase in the absence of Ca(2+). In contrast, the 12-mer peptide-calmodulin complex did not activate phosphodiesterase but rather inhibited activation by Ca(2+). This inhibition could be overcome by high levels of Ca(2+). Thus, it would appear that the aforementioned concept can be used to make peptide agonists and antagonists that are targeted to predetermined sites on proteins such as calmodulin.

Tyrosine phosphorylation modulates the interaction of calmodulin with its target proteins

The activation of six target enzymes by calmodulin phosphorylated on Tyr99 (PCaM) and the binding affinities of their respective calmodulin binding domains were tested. The six enzymes were: myosin light chain kinase (MLCK), 3'-5'-cyclic nucleotide phosphodiesterase (PDE), plasma membrane (PM) Ca2+-ATPase, Ca2+-CaM dependent protein phosphatase 2B (calcineurin), neuronal nitric oxide synthase (NOS) and type II Ca2+-calmodulin dependent protein kinase (CaM kinase II). In general, tyrosine phosphorylation led to an increase in the activatory properties of calmodulin (CaM). For plasma membrane (PM) Ca2+-ATPase, PDE and CaM kinase II, the primary effect was a decrease in the concentration at which half maximal velocity was attained (Kact). In contrast, for calcineurin and NOS phosphorylation of CaM significantly increased the Vmax. For MLCK, however, neither Vmax nor Kact were affected by tyrosine phosphorylation. Direct determination by fluorescence techniques of the dissociation constants with synthetic peptides corresponding to the CaM-binding domain of the six analysed enzymes revealed that phosphorylation of Tyr99 on CaM generally increased its affinity for the peptides.

Structural studies of the melibiose permease of Escherichia coli by fluorescence resonance energy transfer. II. Identification of the tryptophan residues acting as energy donors

In the accompanying paper, we demonstrated the presence of a fluorescence resonance energy transfer (FRET) between the tryptophans of the melibiose permease (MelB) of Escherichia coli and a fluorescent sugar, 2'-(N-5-dimethylaminonaphthalene-1-sulfonyl)aminoethyl-1-thio-beta-D- galactopyranoside (Dns2-S-Gal) bound at the sugar-binding site (Maehrel, C., Cordat, E., Mus-Veteau, I., and Leblanc, G. (1998) J. Biol. Chem. 273, 33192-33197). To identify the tryptophans that transfer their energy to the fluorescent sugar, we analyzed the FRET properties of MelB mutants carrying the replacement of each of the eight MelB tryptophans by a phenylalanine. The data indicate that Trp64, localized in loop 2-3 from the N-terminal domain, and Trp299, localized in helix IX in the C-terminal domain, are responsible for up to 80% of the FRET signal. Moreover, by assuming that only Trp299 transfers energy to Dns2-S-Gal in mutant W64F, whereas only Trp64 transfers energy to Dns2-S-Gal in mutant W299F, we calculated that Trp299 and Trp64 are about 14 and 20 A away from the probe, respectively. In addition, we observed that mutating Trp342, localized in helix X of the C-terminal domain, produces a significant increase of the polarity of the fluorescent sugar environment, suggesting its proximity to the sugar-binding site. Taken together, these data provide additional support for the suggestion that (i) the sugar-binding site is localized in the C-terminal part of the transporter, probably close to membrane segments IX and X, and (ii) the N-terminal domain, and particularly cytoplasmic loop 2-3, is also close to the sugar-binding site.

Time-resolved fluorescence study of the single tryptophans of engineered skeletal muscle troponin C

The regulatory domain of troponin C (TnC) from chicken skeletal muscle was studied using genetically generated mutants which contained a single tryptophan at positions 22, 52, and 90. The quantum yields of Trp-22 are 0.33 and 0.25 in the presence of Mg2+ (2-Mg state) and Ca2+ (4-Ca state), respectively. The large quantum yield of the 2-Mg state is due to a relatively small nonradiative decay rate and consistent with the emission peak at 331 nm. The intensity decay of this state is monoexponential with a single lifetime of 5.65 ns, independent of wavelength. In the 4-Ca state, the decay is biexponential with the mean of the two lifetimes increasing from 4.54 to 4.92 ns across the emission band. The decay-associated spectrum of the short lifetime is red-shifted by 19 nm relative to the steady-state spectrum. The decay of Trp-52 is biexponential in the 2-Mg state and triexponential in the 4-Ca state. The decay of Trp-90 requires three exponential terms for a satisfactory fit, but can be fitted with two exponential terms in the 4-Ca state. The lower quantum yields (< 0.15) of these two tryptophans are due to a combination of smaller radiative and larger nonradiative decay rates. The results from Trp-22 suggest a homogeneous ground-state indole ring in the absence of bound Ca2+ at the regulatory sites and a ground-state heterogeneity induced by activator Ca2+. The Ca(2+)-induced environmental changes of Trp-52 and Trp-90 deviate from those predicted by a modeled structure of the 4-Ca state. The anisotropy decays of all three tryptophans show two rotational correlation times. The long correlation times (phi 1 = 8.1-8.3 ns) derived from Trp-22 and Trp-90 suggest an asymmetric hydrodynamic shape. TnC becomes more asymmetric upon binding activator Ca2+ (phi 1 = 10.1-11.6 ns). The values of phi 1 obtained from Trp-52 are 3-4 ns shorter than those from Trp-22 and Trp-90, and these reduced correlation times may be related to the mobility of the residue and/or local segmental flexibility.

Kinetic control of the dissociation pathway of calmodulin-peptide complexes

The mechanism of dissociation reactions induced by calcium chelators has been studied for complexes of Drosophila calmodulin with target peptides, including four derived from the skeletal muscle myosin light chain kinase target sequence. Reactions were monitored by fluorescence stopped-flow techniques using a variety of intrinsic probes and the indicator Quin2. For most of the complexes, apparently biphasic kinetics were observed in several fluorescence parameters. The absence of any obvious relationship between dissociation rates and peptide affinities implies kinetic control of the dissociation pathway. A general mechanism for calcium and peptide dissociation was formulated and used in numerical simulation of the experimental data. Unexpectedly, the rate of the slowest step decreases with increasing [peptide]/[calmodulin] ratio. Numerical simulation shows this step could contain a substantial contribution from a reversible relaxation process (involving the species Ca2-calmodulin-peptide), convolved with the following step (loss of C-terminal calcium ions). The results indicate the potentially key kinetic role of the partially calcium-saturated intermediate species. They show that subtle changes in the peptide sequence can have significant effects on both the dissociation rates and also the dissociation pathway. Both effects could contribute to the variety of regulatory behavior shown by calmodulin with different target enzymes.

Characterization of novel calmodulin-binding peptides with distinct inhibitory effects on calmodulin-dependent enzymes

We describe the isolation and interaction with calmodulin (CaM) of two 10-amino-acid peptides (termed peptides 1 and 2; AWDTVRISFG and AWPSLQAIRG respectively) derived from a phage random peptide display library. Both peptides are shorter than previously described CaM-binding peptides and lack certain features found in the sequences of CaM-binding domains present in CaM-activated enzymes. However, 1H NMR spectroscopy and fluorimetry indicate that both peptides interact with CaM in the presence of Ca2+. The two peptides differentially inhibited CaM-dependent kinases I and II (CaM kinases I and II) but did not affect CaM-dependent phosphodiesterase. Peptide 1 inhibited CaM kinase I but not CaM kinase II, whereas peptide 2 inhibited CaM kinase II, but only partially inhibited CaM kinase I at a more than 10-fold higher concentration. Peptide 1 also inhibited a plant calcium-dependent protein kinase, whereas peptide 2 did not. The ability of peptides 1 and 2 to differentially inhibit CaM-dependent kinases and CaM-dependent phosphodiesterase suggests that they may bind to distinct regions of CaM that are specifically responsible for activation of different CaM-dependent enzymes.

Conformational transitions within the calmodulin-binding site of Bordetella pertussis adenylate cyclase studied by time-resolved fluorescence of Trp242 and circular dichroism

The sequence situated around Trp242 in Bordetella pertussis adenylate cyclase, a bifunctional protein of 1706 amino acid residues, forms the core of the calmodulin-binding site. Peptides varying in size and in affinity for calmodulin, and preserving the same sequence around Trp242 were analyzed by time-resolved fluorescence spectroscopy. Their dynamic properties were compared to those of the catalytic domain of B. pertussis adenylate cyclase corresponding to the first 400 amino acid residues of the protein and in which the Trp69 residue was replaced by Phe. The heterogeneity of the fluorescence intensity decays of Trp242 is likely due to the existence of conformers in equilibrium as is suggested by the effect of trifluoroethanol both on the secondary structure content and the lifetime distributions. Binding to calmodulin leads to striking effects on the lifetime distribution profiles by selecting a major excited state population and therefore one major conformer. Trp242 still presents some degree of rotational freedom in the complexes. The reduction of rotational freedom is more important for the shorter peptides than for the longest one. A similar selection of one major conformer with the same lifetime was also observed for the Trp242 in the mutant protein when bound to calmodulin, as in the complexes with the peptides. We conclude that the site of interaction of B. pertussis adenylate cyclase with calmodulin has similar conformational flexibility as that evidenced in the isolated peptides. This property of the molecule allows a better adjustment of the enzyme upon interaction with calmodulin.

Time-resolved fluorescence study of a calcium-induced conformational change in prothrombin fragment 1

The wavelength dependent fluorescence decay properties of bovine prothrombin fragment 1 have been investigated employing a picosecond time-correlated single photon counting technique. All observations are discussed with using the crystal structure (Soriano-Garcia et al., Biochemistry 31:2554-2566, 1992). Fluorescence lifetimes distribution and conventional multiexponential analysis, as well as acrylamide quenching studies lead to the identification of six distinguishable tryptophan excited-states. Accessibility to the quencher and the known structure are used to associate a fluorescence decay of the tryptophan present in the Gla domain (Trp42) with two red shifted components (2.3 and 4.9 ns). The two kringle domain tryptophans (Trp90 and Trp126) exhibit four decay times (0.06, 0.24, 0.68, and 2.3 ns), which are blue shifted. The calcium-induced fluorescence quenching is a result of static quenching: the five decay times remain unchanged, whereas the fluorescence intensity of Trp42 is decreased. The static quenching process is a consequence of a ground state interaction between the Cys18-Cys23 disulfide bridge and Trp42. The monomolecular equilibrium constant for this disulfide-pi-electron interaction is found as 4.8.

The activity of calmodulin is altered by phosphorylation: modulation of calmodulin function by the site of phosphate incorporation

Calmodulin transduces Ca2+ signals by binding to and activating essential regulatory enzymes. The large number of intracellular targets for calmodulin raises the possibility that mechanisms in addition to Ca2+ may modulate calmodulin activity. Phosphocalmodulin is found in cells and tissues, and calmodulin phosphorylation is enhanced by several mitogens. Phosphorylation of calmodulin on serine/threonine residues by casein kinase II decreased its ability to activate Ca2+/calmodulin-dependent protein kinase II (CaM-kinase II). The major effect was a 2.5-fold increase in the concentration at which half-maximal velocity (K0.5) was attained, with no apparent alteration in the Vmax, or the K0.5 for Ca2+. In contrast, calmodulin phosphorylated on tyrosine residues by the insulin receptor kinase produced an increase in the Vmax, with no alteration in the affinity for CaM-kinase II or the K0.5 for Ca2+. Direct determination by surface plasmon resonance of the dissociation constants with a synthetic peptide corresponding to the calmodulin-binding domain of CaM-kinase II revealed that phosphorylation on serine/threonine residues of calmodulin significantly decreased its affinity for the peptide, while tyrosine phosphorylation had no effect on binding. In contrast to CaM-kinase II, neither serine/threonine nor tyrosine phosphorylation of calmodulin altered its ability to activate calcineurin. These data indicate that phosphorylation of calmodulin differentially modifies its interaction with individual target enzymes. Moreover, the amino acid residues phosphorylated provide an additional level of control. These results demonstrate that phosphorylation is an in vitro regulatory mechanism in the targeting of calmodulin responses and, coupled with the stoichiometric phosphorylation of calmodulin in rat hepatocytes, suggest that it may be relevant in intact cells.

Gain of function mutations for yeast calmodulin and calcium dependent regulation of protein kinase activity

Yeast calmodulin binds only three calcium ions in the presence of millimolar concentrations of magnesium due to a defective calcium-binding sequence in its carboxyl terminal domain. Yeast calmodulin's diminished calcium-binding activity can be restored to that of other calmodulins by the use of site-directed mutagenesis to substitute its fourth calcium-binding domain with that of a vertebrate calmodulin sequence. However, the repair of yeast calmodulin's calcium-binding activity is not sufficient to repair quantitatively yeast calmodulin's defective protein kinase activator activity. Yeast calmodulin's activator activity with smooth muscle and skeletal muscle myosin light chain kinases and brain calmodulin-dependent protein kinase II can be progressively repaired by additional substitutions of vertebrate calmodulin sequences, provided that the four calcium-binding sites remain intact. An unexpected result obtained during the course of these studies was the observation that myosin light chain kinases from smooth and skeletal muscle tissues can respond differently to mutations in calmodulin. These and previous results indicate that the binding of four calcium ions by calmodulin is necessary but not sufficient to bring about quantitative activation of protein kinases, and are consistent with the conformational selection/restriction model of the dynamic equilibrium among calcium, calmodulin and each calmodulin regulated enzyme.

Studies on the interaction of the dye, stains-all, with individual calcium-binding domains of calmodulin

We show that the calcium-mimic dye, Stains-all, is a convenient probe to study the structural features of the individual calcium-binding sites of calmodulin (CaM) and related calcium-binding proteins (CaBP). These peptides bind the dye in their calcium-binding sites, and induce a circular dichroism (CD) band in the bound dye in the 620 nm (J band) region, which is abolished upon the addition of calcium. Replacement of Asp by Asn in the + x position of the weaker calcium-binding site (site I of CaM) abolishes the dye binding, while the same change in the higher affinity site IV attenuates the binding of the dye and does not abolish it. Replacement of Tyr in site IV with Trp does not distort the geometry, although it increases the dye binding affinity.

Fluorescence energy transfer analysis of calmodulin-peptide complexes

The interactions between calmodulin and the tryptophan residues of synthetic peptides corresponding to the calmodulin binding domains of skeletal muscle myosin light-chain kinase and the plasma membrane calcium pump were examined. The single tryptophan residue contained in each peptide became relatively immobilized and inaccessible to iodide ion upon binding to calmodulin, indicating that the indole side chain was inserted into a hydrophobic cleft in the surface of calmodulin. Fluorescence energy transfer from peptidyl tryptophan residues to an AEDANS moiety attached to cysteine-26 of spinach calmodulin was measured. Included in these analyses was a tryptophan-containing peptide analog of the calmodulin binding domain of neuromodulin. These data indicated that the indole ring of each peptide inserted 32-35 A away from cysteine-26 and may therefore interact with the carboxyl-terminal lobe of CaM in its "bent" conformation [Persechini & Kretsinger (1988a) J. Cardiovasc. Pharmacol. 12 (Suppl 5), S1-S12; Ikura et al. (1992) Science 256, 632-638; Vorherr et al. (1992) Eur. J. Biochem. 204, 931-937]. The interchange of tryptophan-3 and phenylalanine-21 of the calcium pump peptide increased the efficiency of energy transfer to the AEDANS-moiety approximately 12-fold, reducing the calculated distance to 20 A. These data suggest that phenylalanine-21 of the calcium pump peptide interacts with the hydrophobic cleft in the amino-terminal lobe of CaM.

The uncoupling protein from brown adipose tissue mitochondria. The environment of the tryptophan residues as revealed by quenching of the intrinsic fluorescence

The uncoupling protein from brown adipose tissue is a member of the family of metabolite carriers of the mitochondrial inner membrane. It contains two tryptophan residues which have been characterized by fluorescence spectroscopy. Application of fluorescence-quenching-resolved spectroscopy (FQRS) allowed the determination of the emission maximum for each residue, both of which occur at 332 nm, thus suggesting that they are both located in a non-polar environment. Fluorescence quenching has demonstrated that both residues are accessible to acrylamide and inaccessible to Cs+, while only one of them is accessible to I-. When FQRS is combined with guanidinium hydrochloride denaturation, the unfolding of the regions containing each tryptophan can be monitored separately as they are transferred to the polar medium where the emission maximum appears at 359 nm, revealing also that the iodide-accessible residue is more sensitive to the denaturant. Secondary structure predictions, together with the data presented here, suggest that the iodide-accessible residue could correspond to Trp173 and the denaturant-resistant iodide-inaccessible one to Trp280, located in the center of the sixth transmembrane alpha-helix. Interaction of the protein with GDP (a transport inhibitor) has been studied and has revealed that it partially shields Trp173 from the interaction with I-, as well as reducing the static component of the acrylamide quenching.

The heterodimer calmodulin: myosin light-chain kinase as a prototype vertebrate calcium signal transduction complex

The heterodimer complex of calmodulin (CaM) and the protein kinase catalytic subunit of myosin light chain kinase from vertebrate smooth muscle and non-muscle tissues (sm/nmMLCK) is one of the most extensively characterized CaM-regulated enzyme complexes and it has an established in vivo role in the transduction of calcium signals into biological responses. We have used a combination of approaches to the study of CaM and sm/nmMLCK in order to derive initial insight into the key features of each protein and of the CaM-MLCK heterodimeric complex that are involved in protein-protein and calcium-protein recognition and regulation of enzyme activity. On-going studies are described here that include site-specific mutagenesis, fluorescence spectroscopy, enzymology and peptide analog analysis. These and previous results indicate that: (1), both electrostatic and hydrophobic features are important in the functionally correct interactions between CaM and MLCK; (2), even the interactions between CaM and peptide analogs of the CaM binding site of MLCK are heterogeneous and non-trivial in nature; (3), amino-acid residues that have been conserved in CaM across millions of years of evolution and that are conserved in CaMs with quantitative MLCK activator activity can be mutated without any detectable effect on activity and (4), structures different from the prototypical EF-hand domain of CaM can have similar calcium-binding activity in the presence of a CaM binding structure.

A new intrinsic fluorescent probe for proteins. Biosynthetic incorporation of 5-hydroxytryptophan into oncomodulin

The tryptophan analog, 5-hydroxytryptophan (5HW), has a significant absorbance between 310-320 nm, which allows it to act as an exclusive fluorescence probe in protein mixtures containing a large number of tryptophan residues. Here for the first time a method is reported for the biosynthetic incorporation of 5HW into an expressed protein, the Y57W mutant of the Ca2+ binding protein, oncomodulin. Fluorescence anisotropy and time-resolved fluorescence decay measurements of the interaction between anti-oncomodulin antibodies and the 5HW-incorporated oncomodulin conveniently provide evidence of complex formation and epitope identification that could not be obtained with the natural amino acid. This report demonstrates the significant potential for the use of 5HW as an intrinsic probe in the study of structure and dynamics of protein-protein interactions.

Time-resolved tryptophan emission study of cardiac troponin I

We have carried out a time-resolved fluorescence study of the single tryptophanyl residue (Trp-192) of bovine cardiac Tnl (CTnl). With excitation at 300 nm, the intensity decay was resolved into three components by a nonlinear least-squares analysis with lifetimes of 0.60, 2.22, and 4.75 ns. The corresponding fractional amplitudes were 0.27, 0.50, and 0.23, respectively. These decay parameters were not sensitive to complexation of CTnl with cardiac troponin C (CTnC), and magnesium and calcium had no significant effect on the decay parameters. After incubation with 3':5'-cyclic AMP-dependent protein kinase, the intensity decay of CTnl required a fourth exponential term for satisfactory fitting with lifetimes of 0.11, 0.81, 1.95, and 6.63 ns and fractional amplitudes of 0.06, 0.37, 0.27, and 0.29, respectively. When bound to CTnC, the intensity decay of phosphorylated CTnl (p-CTnl) also required four exponential terms for satisfactory fitting, but the longest lifetime increased by a factor of 1.7. The decay parameters obtained from the complex formed between p-CTnl and CTnC were not sensitive to either magnesium or calcium. The anisotropy decay was resolved into two components with rotational correlation times of 0.90 and 23.48 ns. Phosphorylation resulted in a decrease of the long correlation time to 14.61 ns. The anisotropy values recovered at zero time suggest that the side chain of the Trp-192 had considerable subnanosecond motional freedom not resolved in these experiments. Within the CTnl.CTnC complex, the unresolved fast motions appeared sensitive to calcium binding to the calcium-specific site of CTnC. The observed emission heterogeneity is discussed in terms of possible excited-state interactions in conjunction with the predicted secondary structure of CTnl. The loss of molecular asymmetry of cardiac troponin I induced by phosphorylation as demonstrated in this work may be related to the known physiological effect of beta-agonists on cardiac contractility.