The γ-Subunit of Skeletal Muscle Phosphorylase Kinase Contains Two Noncontiguous Domains That Act in Concert to Bind Calmodulin

A recombinant form of the catalytic subunit of phosphorylase kinase that is soluble, monomeric, and includes key C-terminal residues

Residues 302-326 of the catalytic (gamma) subunit of phosphorylase kinase (PhK) may comprise an autoinhibitory, pseudosubstrate domain that binds calmodulin. To study this, the cDNA corresponding to rabbit muscle PhKgamma was expressed using Escherichia coli. This yielded two stable, high-activity PhKgamma forms (35 and 42 kDa by SDS-PAGE) that were smaller than an authentic sample of rabbit muscle PhKgamma (45 kDa by SDS-PAGE). Each recombinant form was purified to homogeneity. The N-terminal sequence of the larger, 42-kDa form (pk42) matched that of the rabbit muscle enzyme. This suggested that pk42 consisted of PhKgamma residues 1-362, including the putative calmodulin-binding, autoinhibitory domain. Kinetic parameters obtained for pk42 were like those previously reported for the intact gamma subunit. This implied that the lack of 25 PhKgamma C-terminal residues did not affect phosphorylase kinase activity, but greatly improved enzyme stability. An additional 60 residues were removed from the C-terminus of pk42 using the protease m-calpain. This increased the kinase activity 1.5-fold. Consistent with this, the activity of a mutant PhKgamma that consisted of residues 1-300, denoted gamma1-300, was like that of the m-calpain-treated enzyme. Therefore, although the effect was small, some influence by the C-terminus of pk42 was noted. Moreover, when pk42 was incubated with ATP alone, a C-terminal threonine residue became phosphorylated. Although the influence of this autophosphorylation cannot be inferred from this data, it was evidence that the C-terminus accessed the enzyme's active site. Taken together, these data imply that pk42 will be useful to study phosphorylase kinase structure/activity relationships.

Mapping subdomains in the C-terminal region of troponin I involved in its binding to troponin C and to thin filament

Troponin I (TnI) is the inhibitory component of troponin, the ternary complex that regulates skeletal and cardiac muscle contraction. Previous work showed that the C-terminal region of TnI, when linked to the "inhibitory region" (residues 98-116), possesses the major regulatory functions of the molecule (Farah, C. S., Miyamoto, C. A., Ramos, C. H. I., Silva, A. C. R., Quaggio, R. B., Fujimori, K., Smillie, L. B., and Reinach, F. C. (1994) J. Biol. Chem. 269, 5230-5240). To investigate these functions in more detail, serial deletion mutants of the C-terminal region of TnI were constructed. These experiments showed that longer C-terminal deletions result in lower inhibition of the actomyosin ATPase activity and weaken the interaction with the N-terminal domain of troponin C (TnC), consistent with the antiparallel model for the interaction between these two proteins. The conclusion is that the whole C-terminal region of TnI is necessary for its full regulatory activity. The region between residues 137 and 144, which was shown to have homology with residues 108-115 in the inhibitory region (Farah, C. S., and Reinach, F. C. (1995) FASEB J. 9, 755-767), is involved in the binding to TnC. The region between residues 98 and 129 is involved in modulating the affinity of TnC for calcium. The C-terminal residues 166-182 are involved in the binding of TnI to thin filament. A model for the function of TnI is discussed.

Calmodulin is the Ca2+ sensor for Ca2+ -dependent inactivation of L-type calcium channels

Elevated intracellular Ca2+ triggers inactivation of L-type calcium channels, providing negative Ca2+ feedback in many cells. Ca2+ binding to the main alpha1c channel subunit has been widely proposed to initiate such Ca2+ -dependent inactivation. Here, we find that overexpression of mutant, Ca2+ -insensitive calmodulin (CaM) ablates Ca2+ -dependent inactivation in a "dominant-negative" manner. This result demonstrates that CaM is the actual Ca2+ sensor for inactivation and suggests that CaM is constitutively tethered to the channel complex. Inactivation is likely to occur via Ca2+ -dependent interaction of tethered CaM with an IQ-like motif on the carboxyl tail of alpha1c. CaM also binds to analogous IQ regions of N-, P/Q-, and R-type calcium channels, suggesting that CaM-mediated effects may be widespread in the calcium channel family.

Oxidation of the skeletal muscle Ca2+ release channel alters calmodulin binding

This study presents evidence for a close relationship between the oxidation state of the skeletal muscle Ca2+ release channel (RyR1) and its ability to bind calmodulin (CaM). CaM enhances the activity of RyR1 in low Ca2+ and inhibits its activity in high Ca2+. Oxidation, which activates the channel, blocks the binding of 125I-labeled CaM at both micromolar and nanomolar Ca2+ concentrations. Conversely, bound CaM slows oxidation-induced cross-linking between subunits of the RyR1 tetramer. Alkylation of hyperreactive sulfhydryls (<3% of the total sulfhydryls) on RyR1 with N-ethylmaleimide completely blocks oxidant-induced intersubunit cross-linking and inhibits Ca2+-free 125I-CaM but not Ca2+/125I-CaM binding. These studies suggest that 1) the sites on RyR1 for binding apocalmodulin have features distinct from those of the Ca2+/CaM site, 2) oxidation may alter the activity of RyR1 in part by altering its interaction with CaM, and 3) CaM may protect RyR1 from oxidative modifications during periods of oxidative stress.

Calcium-calmodulin-induced dimerization of the carboxyl-terminal domain from petunia glutamate decarboxylase. A novel calmodulin-peptide interaction motif

The acidic, bilobed protein calmodulin (CaM; molecular mass of 16.7 kDa) can activate some 40 distinct proteins in a calcium-dependent manner. The majority of the CaM-binding domain regions of the target proteins are basic and hydrophobic in nature, are devoid of multiple negatively charged residues, and have a propensity to form an alpha-helix. The CaM-binding domain in the C-terminal region of petunia glutamate decarboxylase (PGD) is atypical because it contains five negatively charged residues. Therefore, we chose to study the binding of calcium-CaM to a 26-residue synthetic peptide encompassing the C-terminal region of PGD. Gel band shift assays, fluorescence spectroscopy, and NMR titration studies showed that a single unique complex of calcium-CaM with two PGD peptides is formed. The formation of a 1:2 protein-peptide complex is unusual; normally, calcium-CaM forms 1:1 complexes with the majority of its target proteins. Circular dichroism spectroscopy showed that the bound PGD peptides have an alpha-helical structure. NMR studies of biosynthetically [methyl-13C]methionine-labeled CaM revealed that all the Met side chains in CaM are involved in the binding of the PGD peptides. Analysis of fluorescence spectra showed that the single Trp residue of the two peptides becomes bound to the N- and C-terminal lobes of CaM. These results predict that binding of calcium-CaM to PGD will give rise to dimerization of the protein, which may be necessary for activation. Possible models for the structure of the protein-peptide complex, such as a dimeric peptide structure, are discussed.

Calmodulin-dependent regulation of inducible and neuronal nitric-oxide synthase

Neuronal and endothelial nitric-oxide synthases depend upon Ca2+/calmodulin for activation, whereas the activity of the inducible nitric-oxide synthase is Ca2+-independent, presumably due to tightly bound calmodulin. To study these different mechanisms, a series of chimeras derived from neuronal and inducible nitric- oxide synthases were analyzed. Chimeras containing only the oxygenase domain, calmodulin-binding region, or reductase domain of inducible nitric-oxide synthase did not confer significant Ca2+-independent activity. However, each chimera was more sensitive to Ca2+ than the neuronal isoform. The calmodulin-binding region of inducible nitric-oxide synthase with either its oxygenase or reductase domains resulted in significant, but not total, Ca2+-independent activity. Co-immunoprecipitation experiments showed no calmodulin associated with the former chimera in the absence of Ca2+. Trifluoperazine also inhibited this chimera in the absence of Ca2+. The combined interactions of calmodulin bound to inducible nitric-oxide synthase calmodulin-binding region with the oxygenase domain may be weaker than with the reductase domain. Thus, Ca2+-independent activity of inducible nitric-oxide synthase appears to result from the concerted interactions of calmodulin with both the oxygenase and reductase domains in addition to the canonical calmodulin-binding region. The neuronal isoform is not regulated by a unique autoinhibitory element in its reductase domain.

Characterization of the Ca2+ -dependent and -independent interactions between calmodulin and its binding domain of inducible nitric oxide synthase

Most interactions of calmodulin (CaM) with its target proteins are Ca2+-dependent, but a few Ca2+-independent CaM-target protein interactions have been identified. One example is the inducible isoform of nitric oxide synthase (iNOS) expressed in macrophages. We describe here the characterization of the Ca2+-independent interaction between CaM and a synthetic peptide corresponding to the CaM-binding domain of murine macrophage iNOS using circular dichroism (CD) spectroscopy. The CD spectrum of free iNOS peptide indicated a beta-sheet conformation. The interaction of iNOS peptide with apo-CaM in the absence of Ca2+ resulted in the peptide acquiring a type II beta-turn structure. This is in contrast to the situation in the presence of Ca2+ in which case the peptide acquired an alpha-helical conformation upon interaction with CaM, i.e. similar to the Ca2+-dependent interactions of CaM with numerous targets such as myosin light chain kinase (MLCK). Consistent with this similar structural change, iNOS peptide inhibited the Ca2+-CaM-dependent activation of smooth muscle MLCK by competing with MLCK for binding to Ca2+-CaM. The Kd of Ca2+-CaM for iNOS peptide was calculated from competition assays to be 0.3 nM. These results indicate that the structure of the CaM-binding domain of iNOS is quite different when bound to apo-CaM than Ca2+-CaM.

Identification of a domain on the beta-subunit of the rod cGMP-gated cation channel that mediates inhibition by calcium-calmodulin

The cGMP-gated cation channel mediating phototransduction in retinal rods has recently been shown to be inhibited by calcium-calmodulin, through direct binding of the latter to the beta-subunit of the heterotetrameric channel complex. Here, we report the characterization of this inhibition and the identification of a domain crucial for this modulation. Heterologous expression of the alpha- and beta-subunits of the human rod channel in HEK 293 cells produced a cGMP-gated current that was highly sensitive to calcium-calmodulin, with half-maximal inhibition at approximately 4 nM. In biochemical and electrophysiological experiments on deletion mutants of the beta-subunit, we have identified a region on its cytoplasmic N terminus that binds calmodulin and is necessary for the calmodulin-mediated inhibition of the channel. However, in gel shift assays and fluorescence emission experiments, peptides derived from this region indicated a low calmodulin affinity, with dissociation constants of approximately 3-10 microM. On the C terminus, a region was also found to bind calmodulin, but it was likewise of low affinity, and its deletion did not abolish the calmodulin-mediated inhibition. We suggest that although the identified region on the N terminus of the beta-subunit is crucial for the calmodulin effect, other regions are likely to be involved as well. In this respect, the rod channel appears to differ from the olfactory cyclic nucleotide-gated channel, which is also modulated by calcium-calmodulin.

The structural effects of endogenous and exogenous Ca2+/calmodulin on phosphorylase kinase

The activity of phosphorylase b kinase (PbK) is stimulated by Ca2+ ions, which act through its endogenous calmodulin subunit (delta), and further stimulated by the Ca2+-dependent binding of exogenous calmodulin (delta'). In contrast to their highly characterized effects on activity, little is known regarding the structural effects on the (alphabetagammadelta)4 PbK holoenzyme induced by Ca2+ and delta'/Ca2+. We have used mono- and bifunctional chemical modifiers as conformational probes to compare how the two effectors influence the structure of the catalytic gamma subunit and the interactions among all of the subunits. As determined by reductive methylation and carboxymethylation, Ca2+ increased the accessibility of the gamma subunit; it also increased the formation by phenylenedimaleimide of an alphagammagamma conjugate that is characteristic of activated conformations of PbK (Nadeau, O. W., Sacks, D. M., and Carlson, G. M. (1997) J. Biol. Chem. 272, 26196-26201); however, Ca2+ also had structural effects that were clearly distinct from other activators. Moreover, similar structural effects of Ca2+ were observed with PbK that had been activated by phosphorylation, consistent with the fact that such activation does not eliminate the catalytic dependence of the enzyme on Ca2+. Our results suggest tiers of conformational transitions in the activation of PbK, with the most fundamental being induced by Ca2+. Analysis of the various cross-linked conjugates formed in the presence of Ca2+ by o-phenylenedimaleimide or m-maleimidobenzoyl-N-hydroxysuccinimide ester showed that the binding of Ca2+ to the delta subunit triggers changes in the interactions among all subunits, including between protomers, indicating an extensive communication network throughout the PbK complex. Most of the structural effects of delta'/Ca2+ were qualitatively similar to, but quantitatively greater than, the effects of Ca2+ alone; but delta'/Ca2+ also had distinct effects, especially involving cross-linking of the delta subunit.

RC3/neurogranin, a postsynaptic calpacitin for setting the response threshold to calcium influxes

In this review, we attempt to cover the descriptive, biochemical and molecular biological work that has contributed to our current knowledge about RC3/neurogranin function and its role in dendritic spine development, long-term potentiation, long-term depression, learning, and memory. Based on the data reviewed here, we propose that RC3, GAP-43, and the small cerebellum-enriched peptide, PEP-19, belong to a protein family that we have named the calpacitins. Membership in this family is based on sequence homology and, we believe, a common biochemical function. We propose a model wherein RC3 and GAP-43 regulate calmodulin availability in dendritic spines and axons, respectively, and calmodulin regulates their ability to amplify the mobilization of Ca2+ in response to metabotropic glutamate receptor stimulation. PEP-19 may serve a similar function in the cerebellum, although biochemical characterization of this molecule has lagged behind that of RC3 and GAP-43. We suggest that these molecules release CaM rapidly in response to large influxes of Ca2+ and slowly in response to small increases. This nonlinear response is analogous to the behavior of a capacitor, hence the name calpacitin. Since CaM regulates the ability of RC3 to amplify the effects of metabotropic glutamate receptor agonists, this activity must, necessarily, exhibit nonlinear kinetics as well. The capacitance of the system is regulated by phosphorylation by protein kinase C, which abrogates interactions between calmodulin and RC3 or GAP-43. We further propose that the ratio of phosphorylated to unphosphorylated RC3 determines the sliding LTP/LTD threshold in concept with Ca2+/ calmodulin-dependent kinase II. Finally, we suggest that the close association between RC3 and a subset of mitochondria serves to couple energy production with the synthetic events that accompany dendritic spine development and remodeling.

Intermolecular tuning of calmodulin by target peptides and proteins: differential effects on Ca2+ binding and implications for kinase activation

Ca(2+)-activated calmodulin (CaM) regulates many target enzymes by docking to an amphiphilic target helix of variable sequence. This study compares the equilibrium Ca2+ binding and Ca2+ dissociation kinetics of CaM complexed to target peptides derived from five different CaM-regulated proteins: phosphorylase kinase. CaM-dependent protein kinase II, skeletal and smooth myosin light chain kinases, and the plasma membrane Ca(2+)-ATPase. The results reveal that different target peptides can tune the Ca2+ binding affinities and kinetics of the two CaM domains over a wide range of Ca2+ concentrations and time scales. The five peptides increase the Ca2+ affinity of the N-terminal regulatory domain from 14- to 350-fold and slow its Ca2+ dissociation kinetics from 60- to 140-fold. Smaller effects are observed for the C-terminal domain, where peptides increase the apparent Ca2+ affinity 8- to 100-fold and slow dissociation kinetics 13- to 132-fold. In full-length skeletal myosin light chain kinase the inter-molecular tuning provided by the isolated target peptide is further modulated by other tuning interactions, resulting in a CaM-protein complex that has a 10-fold lower Ca2+ affinity than the analogous CaM-peptide complex. Unlike the CaM-peptide complexes, Ca2+ dissociation from the protein complex follows monoexponential kinetics in which all four Ca2+ ions dissociate at a rate comparable to the slow rate observed in the peptide complex. The two Ca2+ ions bound to the CaM N-terminal domain are substantially occluded in the CaM-protein complex. Overall, the results indicate that the cellular activation of myosin light chain kinase is likely to be triggered by the binding of free Ca2(2+)-CaM or Ca4(2+)-CaM after a Ca2+ signal has begun and that inactivation of the complex is initiated by a single rate-limiting event, which is proposed to be either the direct dissociation of Ca2+ ions from the bound C-terminal domain or the dissociation of Ca2+ loaded C-terminal domain from skMLCK. The observed target-induced variations in Ca2+ affinities and dissociation rates could serve to tune CaM activation and inactivation for different cellular pathways, and also must counterbalance the variable energetic costs of driving the activating conformational change in different target enzymes.

Proximal regions of the catalytic gamma and regulatory beta subunits on the interior lobe face of phosphorylase kinase are structurally coupled to each other and with enzyme activation

Phosphorylase kinase from skeletal muscle is a hexadecameric enzyme with the subunit composition (alphabeta gammadelta)4 and a mass of 1.3 x 10(6) Da. The catalytic gamma subunit and the remaining regulatory subunits are packed as a tetrahedral structure composed of two elongated, opposing (alphabeta gammadelta)2 octameric lobes. We show by immunoelectron microscopy with subunit-specific monoclonal antibodies that a portion of the beta subunit occurs on the interior face of the lobes at a region of inter-lobal interactions, and that at a proximal position slightly more central and distal on the interior lobe face lies the base (residues 277 to 290) of the helical domain of the catalytic core of the gamma subunit. Activation of the kinase by a variety of means caused similar increases in the binding to the holoenzyme of the monoclonal antibodies against these two regions of the beta and gamma subunits. Moreover, monovalent fragments of the antibodies against both regions stimulated the activity of the non-activated holoenzyme. Thus, the epitopes of the beta and gamma subunits recognized by the monoclonal antibodies are structurally coupled to each other and with the activation of phosphorylase kinase. Activation of the holoenzyme apparently involves the repositioning of the base of the catalytic domain of the gamma subunit and a proximal region of the beta subunit within the identified area on the interior face of the lobes of the tetrahedral phosphorylase kinase molecule.

Antipeptide antibodies as probes of subunit-dependent structural changes in the regulatory domain of the gamma-subunit of phosphorylase kinase

The gamma-subunit of phosphorylase kinase contains a protein kinase catalytic domain (residues 20-276) and a regulatory domain (residues 276-386). The purpose of the present investigation was to develop monospecific antibodies against four synthetic gamma-subunit regulatory domain peptides (PhK1: 362-386; PhK5: 342-366; PhK9: 322-346; PhK13: 302-326) to use as probes to study the structure of the regulatory domain. Each affinity-purified antibody was characterized with regard to its ability to bind three different structural forms of the gamma-subunit: the isolated gamma-subunit, the gamma-delta complex, and the holoenzyme complex (alpha beta delta gamma)4. Of the four antibodies, binding of affinity-purified anti-PhK13 was most affected by alterations in gamma-subunit interactions. Taken together, the data from this investigation indicate that the regulatory domain of the gamma-subunit can assume different immunochemically distinguishable conformations as the result of interactions among the alpha-, beta-, gamma-, and delta-subunits of phosphorylase kinase.

Phosphorylation of the IQ domain regulates the interaction between Ca2+-vector protein and its target in Amphioxus

Calcium vector protein target (CaVPT), a 26-kDa endogenous target of calcium vector protein from Amphioxus (CaVP), contains three distinct regions: a N-terminal Pro-Ala-Lys-rich motif, segment 36-50 displaying sequence similarity to the calmodulin-binding site in neuromodulin and neurogranin where they are designated as the IQ domain; and two immunoglobulin-like folds. The phosphorylation by protein kinase C of Ser-43 in the IQ domain drastically decreases the affinity of CaVPT for CaVP and CaVP protects CaVPT from phosphorylation. Phosphorylation by the catalytic subunit of cyclic AMP-dependent protein kinase has a similar effect, but in addition to Ser-43 four other phosphorylated sites were identified. Removal of the Pro-Ala-Lys-rich region and the IQ domain in CaVPT by trypsin leads to the loss of binding to CaVP, whereas the chymotryptic fragment, containing these regions and first immunoglobulin-like domain, retained the ability to interact with CaVP. A synthetic IQ domain alone interacts strongly with calmodulin, but not with CaVP. Two main conclusions can be drawn from this study: 1) the regulation of interaction between CaVP and CaVPT is very similar to the mechanism observed in the complex between neuromodulin or neurogranin and calmodulin; 2) in spite of this similarity the entire CaVP-binding site is not restricted to the IQ domain; in addition the Pro-Ala-Lys-rich motif may be necessary for high affinity binding to CaVP.

Inducible nitric oxide synthase requires both the canonical calmodulin-binding domain and additional sequences in order to bind calmodulin and produce nitric oxide in the absence of free Ca2+

All three mammalian isoforms of nitric oxide synthase (NOS) must bind calmodulin (CaM) for enzymatic activity. Only NOS2 (the inducible isoform, iNOS) does so at the low levels of free Ca2+ in resting cells and when almost all Ca2+ is chelated in cell-free preparations. To test directly whether the predicted CaM-binding region of mouse NOS2 accounts for its Ca2+ independence, we prepared chimeric NOS's in which mouse NOS2 residues 503-532 were reciprocally exchanged with the corresponding residues 725-754 of rat NOS1 (neuronal NOS). Unlike either parent, both chimeras required an intermediate level of free Ca2+ to bind CaM and generate NO. In cell lysates, the concentration of Ca2+ necessary for half-maximal activity (EC50) was approximately 0 for NOS2, 200-300 n for NOS1, and 7-10 n for the chimeras. Results were similar when the region exchanged was enlarged by 7-8 residues toward the amino terminus. In contrast, when the carboxyl-terminal half of NOS2 (residues 454-1144) was replaced with that of NOS1 (residues 675-1429), the resulting chimera resembled NOS1 (EC50, 200-300 n free Ca2+). Truncation analysis suggested that NOS2 residues within the sequence 484-726 were required for Ca2+-independent CaM-binding. Thus, both the canonical CaM-binding domain and additional residues within the region 484-726 are necessary for NOS2's ability to bind CaM and produce NO when Ca2+ levels approach zero.

Activation and inhibition of phosphorylase kinase by monospecific antibodies raised against peptides from the regulatory domain of the gamma-subunit

The C terminus of the catalytic gamma-subunit of phosphorylase kinase comprises a regulatory domain that contains regions important for subunit interactions and autoinhibitory functions. Monospecific antibodies raised against four synthetic peptides from this region, PhK1 (362-386), PhK5 (342-366), PhK9 (322-346), and PhK13 (302-326), were found to have significant effects on the catalytic activities of phosphorylase kinase holoenzyme and the gamma delta complex. Antibodies raised against the very C terminus of the gamma-subunit, anti-PhK1 and anti-PhK5, markedly activated both holoenzyme and the gamma delta complex, in the presence and absence of Ca2+. In the presence of Ca2+ at pH 8.2, anti-PhK1 activated the holoenzyme more than 11-fold and activated the gamma delta complex 2.5-fold. Activation of the holoenzyme and the gamma delta complex by anti-PhK5 was 50-70% of that observed with anti-PhK1. Prior phosphorylation of the holoenzyme by the cAMP-dependent protein kinase blocked activation by both anti-PhK1 and anti-PhK5. Antibodies raised against the peptides from the N terminus of the regulatory domain, anti-PhK9 and anti-PhK13, were inhibitory, with their greatest effects on the gamma delta complex. These data demonstrate that the binding of antibodies to specific regions within the regulatory domain of the gamma-subunit can augment or inhibit structural changes and subunit interactions important in regulating phosphorylase kinase activity.

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.

A comparison of the properties of the binary and ternary complexes formed by calmodulin and troponin C with two regulatory peptides of phosphorylase kinase

The regulatory peptides Phk13 (301-327) and a modified form of Phk5 (342-367) from the gamma-subunit of glycogen phosphorylase kinase form binary and ternary complexes with both calmodulin and the related muscle protein troponin C. Neither peptide appears to affect to a major extent a fluorescent probe linked to Cys-27 of wheat germ calmodulin. Phk13, but not Phk5, significantly modifies the properties of a probe joined to Cys-98 of troponin C. A comparison by means of radiationless energy transfer of the average separations of Trp-16 of Phk5 from specific groups in the N- and C-terminal halves of calmodulin and troponin C indicate significant changes upon going from the 1:1 binary complex to the 1:1:1 ternary complex with Phk13. A comparison of the effects of addition of Phk13 to calmodulin, troponin C, and their binary complexes with Phk5 suggests that the conformation of Phk13 is similar in the binary and ternary complexes.

A novel plant calmodulin-binding protein with a kinesin heavy chain motor domain

Calmodulin, a ubiquitous calcium-binding protein, regulates many diverse cellular functions by modulating the activity of the proteins that interact with it. Here, we report isolation of a cDNA encoding a novel kinesin-like calmodulin-binding protein (KCBP) from Arabidopsis using biotinylated calmodulin as a probe. Calcium-dependent binding of the cDNA-encoded protein to calmodulin is confirmed by 35S-labeled calmodulin. Sequence analysis of a full-length cDNA indicates that it codes for a protein of 1261 amino acids. The predicted amino acid sequence of the KCBP has a domain of about 340 amino acids in the COOH terminus that shows significant sequence similarity with the motor domain of kinesin heavy chains and kinesin-like proteins and contains ATP and microtubule binding sites typical of these proteins. Outside the motor domain, the KCBP has no sequence similarity with any of the known kinesins, but contains a globular domain in the NH2 terminus and a putative coiled-coil region in the middle. By analyzing the calmodulin binding activity of truncated proteins expressed in Escherichia coli, the calmodulin binding region is mapped to a stretch of about 50 amino acid residues in the COOH terminus region of the protein. Using a synthetic peptide, the calmodulin binding domain is further narrowed down to a 23-amino acid stretch. The synthetic peptide binds to calmodulin with high affinity in a calcium-dependent manner as judged by electrophoretic mobility shift assay of calmodulin-peptide complex. The KCBP is coded by a single gene and is highly expressed in developing flowers and suspension cultured cells. Although many kinesin heavy chains and kinesin-like proteins have been extensively characterized at the biochemical and molecular level in evolutionarily distant organisms, none of them is known to bind calmodulin. The plant kinesin-like protein with a calmodulin binding domain and a unique amino-terminal region is a new member of the kinesin superfamily. The presence of a calmodulin-binding motif in a kinesin heavy chain-like protein suggests a role for calcium and calmodulin in kinesin-driven motor function(s) in plants.

Identification, characterization, and comparison of the calmodulin-binding domains of the endothelial and inducible nitric oxide synthases

The calmodulin (CaM)-binding regions in bovine endothelial nitric oxide synthase (eNOS) and murine inducible nitric oxide synthase (iNOS) are identified in this study as eNOS residues 493-512 and iNOS residues 501-532. Peptides corresponding to eNOS 493-512 and NOS 501-532 produce a (Ca2+)-dependent, electrophoretic mobility shift of CaM on 4 M urea gels. The two peptides are also potent inhibitors of the CaM-mediated activation of neuronal nitric oxide synthase and have dissociation constants for CaM binding of 4.0 and 1.5 nM respectively. Substitution of eNOS and iNOS CaM-binding domains in eNOS/iNOS chimeric proteins produces major alterations in the Ca2+ and CaM dependence of the intact enzymes expressed and purified from a baculovirus/Sf9 insect cell system. Replacement of aligned NOS sequence with eNOS 493-512 creates a functional, chimeric iNOS that is both (Ca2+)- and CaM-dependent. Replacement of aligned eNOS sequence with NOS 501-532 creates a functional, chimeric eNOS that is CaM-independent but that remains (Ca2+)-dependent. Specific amino acid residues critical for CaM binding by eNOS are also identified in this study as Phe-498, Lys-499, and Leu-511 in the bovine eNOS sequence.

Identification of inhibitory and calmodulin-binding domains of the PDE1A1 and PDE1A2 calmodulin-stimulated cyclic nucleotide phosphodiesterases

Using a bovine 61-kDa (PDE1A2) calmodulin-stimulated phosphodiesterase (CaM-PDE) cDNA and a bovine lung 59-kDa (PDE1A1) CaM-PDE cDNA reported here, we have identified two new regions within the primary structure of these two related isozymes that are important for regulation by Ca2+/CaM. PDE1A1 is identical to the PDE1A2 isozyme except for the amino-terminal 18 residues. In agreement with earlier studies, the CaM concentration required for half-maximal activation (KCaM) of recombinant PDE1A1 (0.3 nM) was approximately 10-fold less than the KCaM for recombinant PDE1A2 (4 nM). A series of deletion mutations of the PDE1A2 cDNA removing nucleotide sequence encoding the first 46-106 aminoterminal residues were constructed and expressed using the baculovirus system. Deletion of the amino acids encompassing a previously identified, putative CaM-binding domain (residues 4-46) produced a polypeptide that was still activated 3-fold by CaM (KCaM approximately 3 nM). However, complete CaM-independent activation occurred when residues 4-98 were deleted. To determine the location of the additional CaM-binding domain(s), the inhibitory potency of seven overlapping, synthetic peptides spanning amino acids 76-149 of PDE1A2 was tested using the CaM-activated enzyme. One peptide spanning amino acids 114-137 of PDE1A2 appeared to be the most potent inhibitor of CaM-stimulated activity. These results reveal the existence of a CaM-binding domain located approximately 90 residues carboxyl-terminal to the putative CaM-binding domains previously identified within the PDE1A1 and PDE1A2 isozymes. Moreover, a discrete segment important for holding these CaM-PDEs in a less active state at low Ca2+ concentrations is located between the two CaM-binding domains.

Characterization of the regulatory domain of the gamma-subunit of phosphorylase kinase. The two noncontiguous calmodulin-binding subdomains are also autoinhibitory

Phosphorylase kinase is a multimeric protein kinase (alpha 4 beta 4 gamma 4 delta 4) whose enzymatic activity is conferred by its gamma-subunit. A library of 18 overlapping synthetic peptides spanning residues 277-386 of the gamma-subunit has been prepared to use in identifying important regulatory structures in the protein. In the present study, the library was screened to identify regions that might function as autoinhibitory domains. Peptides from two distinct regions were found to inhibit the Ca2(+)-activated holoenzyme. The same regions were previously found to bind calmodulin (i.e. the delta-subunit; Dasgupta, M. Honeycutt, T., and Blumenthal, D. K. (1989) J. Biol. Chem. 264, 17156-17163). The most potent substrate antagonist peptides were PhK13 (residues 302-326; Ki = 300 nM) and PhK5 (residues 342-366; Ki = 20 microM). Both peptides inhibited the holoenzyme competitively with respect to phosphorylase b and noncompetitively with respect to Mg.ATP. When the pattern of inhibition with both peptides present was analyzed, inhibition was observed to be synergistic and modestly cooperative indicating that the two peptides can simultaneously occupy the protein substrate-binding site(s). These data are consistent with a model in which the regions of the gamma-subunit represented by PhK5 and PhK13 work in concert as regulatory subdomains that transduce Ca2(+)-induced conformational changes in the delta-subunit to the catalytic gamma-subunit through a pseudosubstrate autoinhibitory mechanism.

Protein engineering and NMR studies of calmodulin

The calcium regulatory protein calmodulin (CaM) plays a role as an on-off switch in the activation of many enzymes and proteins. CaM has a dumbbell shaped structure with two folded domains, which are connected by a flexible linker in solution. The calmodulin-binding domains of the target proteins are contained in 20 residue long amino acid sequences, that share no obvious amino acid sequence homology. In this contribution, we discuss the features of CaM, which allow it to be rather promiscuous, and bind effectively to all these distinct domains. In particular, we describe the role of the methionine-rich hydrophobic surfaces of the protein in providing a malleable and sticky surface for binding many hydrophobic peptides. The enzyme activation properties of various Met --> Leu mutants of CaM are discussed. In addition, the role of the flexible linker region that connects the two domains is also analyzed. Finally, we describe various NMR and spectroscopic experiments that aid in determining the CaM-bound structures of synthetic peptides containing various CaM-binding domains. All structures analyzed to date are alpha-helical when bound to CaM, and they interact with CaM only through amino acid sidechains. This form of protein-protein interaction is rather unique, and may contribute to CaM's capacity to bind effectively to such a wide range of distinct partners.

The gamma subunit of phosphorylase kinase contains a pseudosubstrate sequence

The catalytic subunit, gamma, of phosphorylase kinase is regulated by a complex set of interactions involving the calcium-binding protein calmodulin and two other subunits designated alpha and beta. These interactions regulate gamma activity that, at least for the calmodulin interactions, involves the regulatory domain in gamma spanning residues 302-366. Within this regulatory domain, we report the identification of a sequence (residues 326-334) that resembles the phosphorylation site in gamma substrates with the exception that a V residue (V332) occurs at the analogous position of the phosphorylated S/T residue. The inhibitory properties of the sequence were assayed with a 10-amino-acid peptide of the sequence. This peptide inhibits a truncated version of gamma, residues 1-300, which is missing the regulatory domain, more potently than it inhibits full-length gamma, and it is a better inhibitor of the full-length gamma at pH 8.2 than at pH 6.8. A similar peptide of the same sequence, except for a S substitution of the V residue, is a good substrate with a comparable Km and better Vmax than peptides of similar length that represent the phosphorylation site in the substrate of the enzyme, glycogen phosphorylase. A mutant gamma protein, with a S for V332 substitution ([V332S]gamma), was prepared using the baculovirus expression system. [V332S]gamma autophosphorylates by an intramolecular mechanism. This demonstrates that this sequence can occupy the catalytic site in the protein. Development of [V332S]gamma affords an experimental model in which the effects of the regulatory factors on autophosphorylation can be determined.

Two structures of the catalytic domain of phosphorylase kinase: an active protein kinase complexed with substrate analogue and product

BACKGROUND

Control of intracellular events by protein phosphorylation is promoted by specific protein kinases. All the known protein kinase possess a common structure that defines a catalytically competent entity termed the 'kinase catalytic core'. Within this common structural framework each kinase displays its own unique substrate specificity, and a regulatory mechanism that may be modulated by association with other proteins. Structural studies of phosphorylase kinase (Phk), the major substrate of which is glycogen phosphorylase, may be expected to shed light on its regulation.

RESULTS

We report two crystal structures of the catalytic core (residues 1-298; Phk gamma trnc) of the gamma-subunit of rabbit muscle phosphorylase kinase: the binary complex with Mn2+/beta-gamma-imidoadenosine 5'-triphosphate (AMPPNP) to a resolution of 2.6 A and the binary complex with Mg2+/ADP to a resolution of 3.0 A. The structures were solved by molecular replacement using the cAMP-dependent protein kinase (cAPK) as a model.

CONCLUSIONS

The overall structure of Phk gamma trnc is similar to that of the catalytic core of other protein kinases. It consists of two domians joined on one edge by a 'hinge', with the catalytic site located in the cleft between the domains. Phk gamma trnc is constitutively active, and lacks the need for an activatory phosphorylation event that is essential for many kinases. The structure exhibits an essentially 'closed' conformation of the domains which is similar to that of cAPK complexed with substrates. The phosphorylated residue that is located at the domain interface in many protein kinases and that is believed to stabilize an active conformation is substituted by a glutamate in Phk gamma trnc. The glutamate, in a similar manner to the phosphorylated residue in other protein kinases, interacts with an arginine adjacent to the catalytic aspartate but does not participate in interdomain contacts. The interactions between the enzyme and the nucleotide product of its activity, Mg2+/ADP, explain the inhibitory properties of the nucleotides that are observed in kinetic studies.

Identification of the substrate and pseudosubstrate binding sites of phosphorylase kinase gamma-subunit

Using site-directed mutagenesis, we proposed that an autoinhibitory domain(s) is located at the C-terminal region (301-386) of the phosphorylase kinase gamma-subunit (Huang, C.-Y.F., Yuan C.-J., Livanova, N.B., and Graves, D.J. (1993) Mol. Cell. Biochem. 127/128, 7-18). Removal of the putative inhibitory domain(s) by truncation results in the generation of a constitutively active and calmodulin-independent form, gamma 1-300. To probe the structural basis of autoinhibition of gamma-subunit activity, two synthetic peptides, PhK13 (gamma 303-327) and PhK5 (gamma 343-367), corresponding to the two calmodulin-binding regions, were assayed for their ability to inhibit gamma 1-300. Competitive inhibition of gamma 1-300 by PhK13 was found versus phosphorylase b (Ki = 1.8 microM) and noncompetitive inhibition versus ATP. PhK5 showed noncompetitive inhibition with respect to both phosphorylase b and ATP. Calmodulin released the inhibition caused by both peptides. These results indicate that there are two distinct auto-inhibitory domains within the C terminus of the gamma-subunit and that these two domains overlap with the calmodulin-binding regions. Two mutant forms of gamma 1-300, E111K and E154R, were used to probe the enzyme-substrate-binding region using peptide substrate analogs corresponding to residues 9-18 of phosphorylase b (KRK11Q12ISVRGL). The data suggest that Glu111 interacts with the P-3 position of the substrate (Lys11) and Glu154 interacts with the P-2 site (Gln12). Both E111K and E154R were competitively inhibited with respect to phosphorylase b by PhK13, with 14- and 8-fold higher Ki values, respectively, than that observed with the wild-type enzyme. These data are consistent with a model for the regulation of the gamma-subunit of phosphorylase kinase in which PhK13 acts as a competitive pseudosubstrate that directly binds the substrate binding site of the gamma-subunit (Glu111 and Glu154).

Calmodulin binding of a peptide derived from the regulatory domain of Bordetella pertussis adenylate cyclase

This paper reports the solution conformation and calmodulin binding of a 43-residue peptide from the calmodulin-binding domain of Bordetella pertussis adenylate cyclase. The peptide (P225-267) was synthesized and 15N-labeled at specific amino acids. It binds calmodulin with an equilibrium dissociation constant of 25 nM. Assignment of the NMR spectrum of the free peptide and analysis of the NOE connectivities and secondary shifts of C alpha protons allowed us to identify a 10-amino acid fragment (Arg237 to Arg246) which is in rapid equilibrium between alpha-helical and irregular structures. Titration experiments showed that at substoichiometric molar ratios the two molecules are in intermediate exchange between free and bound conformations. Using 15N-edited methods we assigned a large part of resonances of the labeled residues in the bound peptide. Analysis of the chemical shift differences between free and bound states shows that the fragment Leu240-Ala257 is the most affected by the interaction. The proton spectra of the calmodulin, in the free and complexed states were extensively assigned using homonuclear experiments. Medium- and long-range NOE patterns are consistent with a largely conserved secondary and tertiary structure. The main changes in chemical shift of calmodulin resonances are grouped in six structural regions both in NH2- and COOH-terminal domains. Intermolecular NOE connectivities indicate that the NH2-terminal of the bound peptide fragment is engulfed in the COOH-terminal domain of calmodulin. The interaction geometry appears to be similar to those previously described for myosin light chain kinase or calmodulin kinase II fragments.

Calmodulin stabilizes an amphiphilic alpha-helix within RC3/neurogranin and GAP-43/neuromodulin only when Ca2+ is absent

Two neuronal protein kinase C substrates, RC3/neurogranin and GAP-43/neuromodulin, preferentially bind to calmodulin (CaM) when Ca2+ is absent. We examine RC3.CaM and GAP-43.CaM interactions by circular dichroism spectroscopy using purified, recombinant RC3 and GAP-43, sequence variants of RC3 displaying qualitative and quantitative differences in CaM binding affinities, and overlapping peptides that cumulatively span the entire amino acid sequence of RC3. We conclude that CaM stabilizes a basic, amphiphilic alpha-helix within RC3 and GAP-43 under physiological salt concentrations only when Ca2+ is absent. This provides structural confirmation for two binding modes and suggests that CaM regulates the biological activities of RC3 and GAP-43 through an allosteric, Ca(2+)-sensitive mechanism that can be uncoupled by protein kinase C-mediated phosphorylation. More generally, our observations imply an alternative allosteric regulatory role for the Ca(2+)-free form of CaM.

The interaction of calmodulin with clathrin-coated vesicles, triskelions, and light chains. Localization of a binding site

The binding of clathrin-coated vesicles, clathrin triskelions, and free clathrin light chains to calmodulin-Sepharose was compared. When isolated from bovine brain, all three components bound to calmodulin-Sepharose in the presence of calcium and could be eluted by its removal. In contrast, coated vesicles and triskelions isolated from bovine adrenal gland did not bind to calmodulin-Sepharose, although the free light chains from adrenal gland bound as effectively as those from brain. As distinct isoforms of the clathrin light chains are expressed by brain and adrenal gland, these results implicate the clathrin light chains as the calmodulin-binding component of coated vesicles and triskelions. Furthermore, the insertion sequences found in the neuron-specific isoforms, although not necessary for the binding of free clathrin light chains to calmodulin, must facilitate the interaction of heavy chain-associated light chains with calmodulin. Recombinant mutants of LCa, with deletions spanning the entire sequence, were tested for binding to calmodulin-Sepharose. Those mutants retaining structural integrity, as assessed by the binding of a panel of monoclonal antibodies, exhibited varying amounts of calmodulin binding activity. However, deletion of the carboxyl-terminal 20 residues abolished calmodulin interaction. Thus, the carboxyl terminus of LCa appears to constitute a calmodulin-binding site. Peptides corresponding to the carboxyl terminus of LCa or LCb inhibited the interaction of the light chains with calmodulin, suggesting that this region forms the calmodulin-binding site of both LCa and LCb. The carboxyl-terminal peptides of LCa and LCb inhibited the interaction of light chains with calmodulin approximately 10-fold less effectively than a calmodulin-binding peptide derived from smooth muscle myosin light chain kinase, but much more effectively than a calmodulin-binding peptide derived from adenylate cyclase. This comparison places the clathrin light chain-calmodulin interaction within the physiological range seen for other calmodulin-binding proteins.

Identification and characterization of three calmodulin binding sites of the skeletal muscle ryanodine receptor

In the present study, we have identified calmodulin binding sequences in the skeletal muscle ryanodine receptor Ca2+ release channel. Ligand overlays on RYR fusion proteins indicate that the skeletal muscle RYR contains three calmodulin binding regions defined by residues 2937-3225, 3546-3655, and 4425-4621. The RYR fusion protein PC28 (residues 2937-3225) bound calmodulin in the presence of EGTA and Ca2+, while RYR fusion protein PC26 (residues 3546-3655) exhibited strong calmodulin binding at 10 microM Ca2+. The RYR fusion protein PC15 (residues 4425-4621) did not bind calmodulin in the presence of either EGTA or 10-50 microM Ca2+. In the presence of 100-500 microM Ca2+, the RYR fusion protein PC15 exhibited an affinity for calmodulin of approximately 50 nM. Peptides RYR1 PM2 (residues 3610-3629) and RYR1 PM3 (4534-4552) encompassing putative RYR-calmodulin binding sites were synthesized. The synthetic peptides interacted directly with dansylcalmodulin as demonstrated by their capacity to affect the fluorescence emission of dansylcalmodulin. Missense mutation analysis indicates that the Lys and Arg residues are essential for calmodulin binding to the synthetic peptide RYR1 PM3. The RYR calmodulin binding site defined by peptide PM3 lies in the myoplasmic loop 2, a few residues upstream of the putative transmembrane segment M5; the other two calmodulin binding sites are next to the putative transmembrane segments M' and M''. Thus, the effect of calmodulin on Ca2+ release might involve the regulation of the putative transmembrane segments M5, M', and M''.

Molecular tuning of ion binding to calcium signaling proteins

Intracellular calcium plays an essential role in the transduction of most hormonal, neuronal, visual, and muscle stimuli. (Recent reviews include Putney, 1993; Berridge, 1993a,b; Tsunoda, 1993; Gnegy, 1993; Bachset al.1992; Hanson & Schulman, 1992; Villereal & Byron, 1992; Premack & Gardner, 1992; Meanset al.1991).

Mutational analyses of the metal ion and substrate binding sites of phosphorylase kinase gamma subunit

Phosphorylase kinase (PhK) and truncated gamma subunit, denoted gamma 1-300, can phosphorylate seryl and tyrosyl residues dependent on the metal ion [Yuan, C.-J., Huang, C. F., & Graves, D. J. (1993) J. Biol. Chem. 268, 17683-17686]. Recombinant gamma 1-300 was used to explore its dual specificity and the location of the metal ion binding sites by using site-directed mutagenesis. Two approaches were taken to generate 26 mutants. First, on the basis of the crystal structure of cAMP-dependent protein kinase (cAPK), the invariant Asn155 and highly conserved Asp168-Phe169-Gly170 residues were mutated. Changes included production of N155H, D168E, D168N, F169R, G170V, G170I, G170L (less than 1% of enzymatic activities were found in these mutants), F169W, and G170A mutants. Second, charge to alanine and charge reversal scanning mutations were used to probe the metal ion binding sites. Two mutants, E111K and E154R, showed very different metal ion response compared to wild-type gamma and were further characterized. The mutants F169W, G170A, E111K, and E154R had 15%, 5%, 8%, and 25% specific activity relative to wild-type gamma, respectively. The folding pattern of wild-type and mutated enzyme forms of gamma was determined by photoacoustic infrared spectroscopy. Conformational disruptions were found in G170V, G170I, and G170L mutants, but the conformation of the rest of the mutants was similar to that of wild-type gamma, suggesting that the loss of enzymatic activities of these mutants was not because of incorrect refolding.(ABSTRACT TRUNCATED AT 250 WORDS)

Young Investigator Award Lecture. Structures of larger proteins, protein-ligand and protein-DNA complexes by multidimensional heteronuclear NMR

The recent development of a whole panoply of multidimensional heteronuclear-edited and -filtered NMR experiments has revolutionized the field of protein structure determination by NMR, making it possible to extend the methodology from the 10-kDa limit of conventional 2-dimensional NMR to systems up to potentially 35-40 kDa. The basic strategy for solving 3-dimensional structures of larger proteins and protein-ligand complexes in solution using 3- and 4-dimensional NMR spectroscopy is summarized, and the power of these methods is illustrated using 3 examples: interleukin-1 beta, the complex of calmodulin with a target peptide, and the specific complex of the transcription factor GATA-1 with its cognate DNA target site.

Expression, purification, characterization, and deletion mutations of phosphorylase kinase gamma subunit: identification of an inhibitory domain in the gamma subunit

A catalytic fragment, gamma 1-298, derived from limited chymotryptic digestion of phosphorylase b kinase (Harris, W.R. et al., J. Biol. Chem., 265: 11740-11745, 1990), is reported to have about six-fold greater specific activity than does the gamma subunit-calmodulin complex. To test whether there is an inhibitory domain located outside the catalytic core of the gamma subunit, full-length wild-type and seven truncated forms of gamma were expressed in E. coli. Recombinant proteins accumulate in the inclusion bodies and can be isolated, solubilized, renatured, and purified further by ammonium sulfate precipitation and Q-Sepharose column. Four out of seven truncated mutants show similar (gamma 1-353 and gamma 1-341) or less (gamma 1-331 and gamma 1-276) specific activity than does the full-length wild-type gamma, gamma 1-386. Three truncated forms, gamma 1-316, gamma 1-300, and gamma 1-290 have molar specific activities approximately twice as great as those of the full-length wild-type gamma and the nonactivated holoenzyme. All recombinant gamma s exhibit similar Km values for both substrates, i.e., about 18 microM for phosphorylase b and about 75 microM for MgATP. Three truncated gamma s, gamma 1-316, gamma 1-300, and gamma 1-290, have a 1.9- to 2.5-fold greater catalytic efficiency (Vmax/Km) than that of the full-length wild-type gamma and a 3.5- to 4.5-fold greater efficiency than that of the truncated gamma 1-331. This evidence suggests that there is at least one inhibitory domain in the C-terminal region of gamma, which is located at gamma 301-331. gamma 1-290, but not gamma 1-276, which contains the highly conserved kinase domain, is the minimum sequence required for the gamma subunit to exhibit phosphotransferase activity. Both gamma 1-290 and gamma 1-300 have several properties similar to full-length wild-type gamma, including metal ion responses (activation by free Mg2+ and inhibition by free Mn2+), pH dependency, and substrate specificities.

Interaction sites on phosphorylase kinase for calmodulin

Holophosphorylase kinase was digested with Glu-C specific protease; from the peptide mixture calmodulin binding peptides were isolated by affinity chromatography and identified by N-terminal sequence analysis. Two peptides originating from the alpha subunit, having a high tendency to form a positively charged amphiphilic helix and containing tryptophane, were synthesized. Additionally, a homologous region of the beta subunit and a peptide from the alpha subunit present in a region deleted in the alpha' isoform were also selected for synthesis. Binding stoichiometry and affinity were determined by following the enhancement in tryptophane fluorescence occurring upon 1:1 complex formation between these peptides and calmodulin. Finally, Ca2+ binding to calmodulin in presence of peptides was measured. By this way, the peptides alpha 542-566, alpha 547-571, alpha 660-677 and beta 597-614 have been found to bind specifically to calmodulin. Together with previously predicted and synthesized calmodulin binding peptides four calmodulin binding regions have been characterized on each the alpha and beta subunits. It can be concluded that endogenous calmodulin can bind to two calmodulin binding regions in gamma as well as to two regions in alpha and beta. Exogenous calmodulin can bind to two regions in alpha and in beta. A binding stoichiometry of 0.8 mol of calmodulin/alpha beta gamma delta promoter of phosphorylase kinase has been determined by inhibiting the ubiquitination of calmodulin with phosphorylase kinase. Phosphorylase kinase is half maximally activated by 23 nM calmodulin which is in the affinity range of calmodulin binding peptides from beta to calmodulin. Therefore, binding of exogenous calmodulin to beta activates the enzyme. A model for switching endogenous calmodulin between alpha, beta and gamma and modulation of ATP binding to alpha as well as Mg2+/ADP binding to beta by calmodulin is presented.