Target enzyme recognition by calmodulin: 2.4 A structure of a calmodulin-peptide complex

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.

Comparative modeling of the three-dimensional structure of the calmodulin-related TCH2 protein from Arabidopsis

Plants adapt to various stresses by developmental alterations that render them less easily damaged. Expression of the TCH2 gene of Arabidopsis is strongly induced by stimuli such as touch and wind. The gene product, TCH2, belongs to the calmodulin (CaM) family of proteins and contains four highly conserved Ca(2+)-binding EF-hands. We describe here the structure of TCH2 in the fully Ca(2+)-saturated form, constructed using comparative molecular modeling, based on the x-ray structure of paramecium CaM. Like known CaMs, the overall structure consists of two globular domains separated by a linker helix. However, the linker region has added flexibility due to the presence of 5 glycines within a span of 6 residues. In addition, TCH2 is enriched in Lys and Arg residues relative to other CaMs, suggesting a preference for targets which are more negatively charged. Finally, a pair of Cys residues in the C-terminal domain, Cys126 and Cys131, are sufficiently close in space to form a disulfide bridge. These predictions serve to direct future biochemical and structural studies with the overall aim of understanding the role of TCH2 in the cellular response of Arabidopsis to environmental stimuli.

A model of Ca(2+)-free calmodulin binding to unconventional myosins reveals how calmodulin acts as a regulatory switch

BACKGROUND

In contrast to conventional muscle myosins, where two different light chains (LCs) stabilize the elongated regulatory domain (RD) region of the head portion of the molecule, unconventional myosins are a diverse group of motors in which from one to six calmodulin (CaM) subunits are bound tandemly to the RD. In both cases, the heavy chains of the RDs have special sequences called "IQ motifs' to which the LCs or CaM bind. A previously puzzling aspect of certain unconventional myosins is their unusual mode of regulation, where activation of motility occurs at low levels of Ca2+. Although the atomic structure of the conventional muscle myosin RD has been determined, no crystallographic structure of the RD of an unconventional myosin is yet available.

RESULTS

We have constructed a model of vertebrate CaM bound to the first IQ motif present in the neck region of an unconventional myosin (chicken brush border myosin I), using strict binding rules derived from the crystal structure of the scallop RD. The model accounts for aspects of the regulation of many unconventional myosins where CaM is bound at low levels of Ca2+ and released or changed in conformation at high levels of Ca2+. The conformational changes as a function of Ca2+ depend not only on the precise sequence of the IQ motifs but also on the interactions between CaM molecules bound to adjacent sites on the myosin heavy chain.

CONCLUSIONS

According to our model, the full versatility of CaM binding to target peptides is displayed in the regulation of unconventional myosins. At low concentrations of Ca2+, CaM binds in a manner similar to the LCs of conventional myosins. At higher Ca2+ concentrations, CaM changes conformation and acts as a switch to regulate the activity of the unconventional myosin molecules.

Localization of unique functional determinants in the calmodulin lobes to individual EF hands

We have investigated the functional interchangeability of EF hands I and III or II and IV, which occupy structurally analogous positions in the native I-II and III-IV EF hand pairs of calmodulin. Our approach was to functionally characterize four engineered proteins, made by replacing in turn each EF hand in one pair by a duplicate of its structural analog in the other. In this way functional determinants we define as unique were localized to the component EF hands in each pair. Replacement of EF hand I by III reduces calmodulin-dependent activation of cerebellar nitric oxide synthase activity by 50%. Replacement of EF hand IV by II reduces by 60% activation of skeletal muscle myosin light chain kinase activity. There appear to be no major unique determinants for activation of these enzyme activities in the other EF hands. Replacement of EF hand III by I or IV by II reduces by 50-80% activation of smooth muscle myosin light chain kinase activity, and replacement of EF hand I by III or II by IV reduces by 90% activation of this enzyme activity. Thus, calmodulin-dependent activation of each of the enzyme activities examined, even the closely related kinases, is dependent upon a distinct pattern of unique determinants in the four EF hands of calmodulin. All the engineered proteins examined bind four Ca2+ ions with high affinity. Comparison of the Ca2+-binding properties of native and engineered CaMs indicates that the Ca2+-binding affinity of an engineered I-IV EF hand pair and a native I-II pair are similar, but an engineered III-II EF hand pair is intermediate in affinity to the native III-IV and I-II pairs, minimally suggesting that EF hands I and III contain unique determinants for the formation and function of EF hand pairs. The residues directly coordinating Ca2+ ion appear to play little or no role in establishing the different Ca2+-binding properties of the EF hand pairs in calmodulin.

Calcineurin-immunosuppressor complexes

Crystal structures of the Ser/Thr phosphatase calcineurin (protein phosphatase 2B) have recently been solved by X-ray crystallography, both in the free-protein state, and complexed with the immunophilin/immunosuppressant FKBP12/FK506. Core elements of the calcineurin phosphatase have been found to be similar to the corresponding elements of Ser/Thr phosphatase 1 and purple acid phosphatase. The structures provide a basis for understanding calcineurin inhibition by a ternary complex of immunophilin and immunosuppressant proteins.

Methionine to glutamine substitutions in the C-terminal domain of calmodulin impair the activation of three protein kinases

The 9 methionine residues of vertebrate calmodulin (CaM) were individually changed to glutamine residues in order to investigate their roles in enzyme binding and activation. The mutant proteins showed three classes of effect on the activation of smooth muscle myosin light chain kinase, CaM-dependent protein kinase IIalpha, and CaM-dependent protein kinase IV. First, some mutations had no appreciable effect on the ability of CaM to activate the three protein kinases. Included in this category were glutamine substitutions at residues 36 and 51 in the N-terminal domain, at residue 76 in the domain linker sequence, and at residues 144 and 145 in the C-terminal domain. Second, glutamine substitutions in the N-terminal domain of CaM, particularly those at positions 71 and 72, lowered the maximal activity of smooth muscle myosin light chain kinase while having no effect on the other two enzymes. Finally the affinity of CaM for all three enzymes was lowered by glutamine mutations at the neighboring methionines 109 and 124, located on a solvent-accessible surface of the C-terminal domain of Ca2+/CaM. This last result provides the first demonstration of the involvement of the same hydrophobic groups in the high affinity binding of CaM to three different enzymes.

Structure of the MDM2 oncoprotein bound to the p53 tumor suppressor transactivation domain

The MDM2 oncoprotein is a cellular inhibitor of the p53 tumor suppressor in that it can bind the transactivation domain of p53 and downregulate its ability to activate transcription. In certain cancers, MDM2 amplification is a common event and contributes to the inactivation of p53. The crystal structure of the 109-residue amino-terminal domain of MDM2 bound to a 15-residue transactivation domain peptide of p53 revealed that MDM2 has a deep hydrophobic cleft on which the p53 peptide binds as an amphipathic alpha helix. The interface relies on the steric complementarity between the MDM2 cleft and the hydrophobic face of the p53 alpha helix and, in particular, on a triad of p53 amino acids-Phe19, Trp23, and Leu26-which insert deep into the MDM2 cleft. These same p53 residues are also involved in transactivation, supporting the hypothesis that MDM2 inactivates p53 by concealing its transactivation domain. The structure also suggests that the amphipathic alpha helix may be a common structural motif in the binding of a diverse family of transactivation factors to the TATA-binding protein-associated factors.

Drosophila melanogaster contains both X-linked and autosomal homologues of the gene encoding calcineurin B

A transcription unit was identified in the 43E polytene band region of the second chromosome of Drosophila melanogaster (Dm) whose putative translation product has 85% amino acid (aa) identity with the B subunit of the calcineurin protein (CnB) from humans. Unlike the previously described intronless Dm CnB gene homologue, which is located within the 4F band region of the X chromosome, the coding region of this second CnB is found to be interrupted by three introns. Conceptual translation of both Dm CnB genes predict proteins of identical size that are 98% identical in aa sequence. Northern blot analyses indicate that Dm pupae and adults express two different CnB-encoding transcripts that are differentially regulated.

Helix unwinding in the effector region of elongation factor EF-Tu-GDP

BACKGROUND

Elongation factor Tu (EF-Tu) in its GTP conformation is a carrier of aminoacylated tRNAs (aa-tRNAs) to the ribosomal A site during protein biosynthesis. The ribosome triggers GTP hydrolysis, resulting in the dissociation of EF-Tu-GDP from the ribosome. The affinity of EF-Tu for other molecules involved in this process, some of which are unknown, is regulated by two regions (Switch I and Switch II) that have different conformations in the GTP and GDP forms. The structure of the GDP form of EF-Tu is known only as a trypsin-modified fragment, which lacks the Switch I, or effector, domain. The aim of this work was to establish the overall structure of intact EF-Tu-GDP, in particular the structure of the effector domain.

RESULTS

The crystal structures of intact EF-Tu-GDP from Thermus aquaticus and Escherichia coli have been determined at resolutions of 2.7 A and 3.8 A, respectively. The structures confirm the domain orientation previously found in the structure of partially trypsin-digested EF-Tu-GDP. The structures of the effector region in T. aquaticus and E. coli EF-Tu-GDP are very similar. The C-terminal part of the effector region of EF-Tu-GDP is a beta hairpin; in EF-Tu-GTP, this region forms an alpha helix. This conformational change is not a consequence of crystal packing.

CONCLUSIONS

EF-Tu undergoes major conformational changes upon GTP hydrolysis. Unlike other GTP-binding proteins, EF-Tu exhibits a dramatic conformational change in the effector region, involving an unwinding of a small helix and the formation of a beta hairpin structure. This change is presumably involved in triggering the release of tRNA, and EF-Tu, from the ribosome.

Heme-CO binding to tryptophan-containing calmodulin mutants

The binding of heme-CO to genetically engineered calmodulin containing a single tryptophan residue has been studied. A tryptophan residue was integrated at one of five positions: 26 or 62 of the N-terminal, 81 in the central helix, or 99 or 135 of the C-terminal. As for the wild type, the mutant calmodulins bind four molecules of heme-CO with an average affinity of 1 microM. (i) Homotropic effect. The quenching of the tryptophan fluorescence by energy transfer to the hemes indicates that there is no preference between the N- or C-terminal pockets for heme binding. The quenching is less than expected for a binomial distribution of four sites. This could indicate a lower energy transfer rate due to a specific orientation factor. The weak quenching as a function of the number of hemes bound may also reveal a cooperativity in the heme binding; the data can be simulated assuming two pairs of sites, where each pocket shows a cooperative binding for two hemes. (ii) Heterotropic effect. As observed for the wild type, addition of melittin does not displace the hemes from the mutant calmodulins; the affinity of heme-CO for the calmodulin.melittin complex is higher than that for calmodulin alone. The affinity of heme-CO for native calmodulin is also higher in the presence of trifluoperazine.

Interactions of amphiphilic peptides derived from alpha s2-casein with calmodulin

Calmodulin-binding peptides, which had previously been isolated from a pepsin digest of alpha-CN, were synthesized and then examined for their inhibitory effects on the activation of cyclic nucleotide phosphodiesterase that was induced by calmodulin. The concentrations of the synthetic peptides corresponding to 164-179, LKKISQRYQKFALPQY; 183-206, VYQHQKAMKPWIQPKTKVIPYVRY; and 183-207, VYQHQKAMKPWIQPKTKVIPYVRYL, of alpha s2-CN that gave half-maximal inhibition were 65, 7.0, and 2.6 microM, respectively. These inhibitory effects were reversed by increasing the amount of calmodulin. Fragments and analogs were prepared to study the interactions of the peptides with calmodulin in more detail. The results indicated that modification of the carboxyl terminus enhanced the affinities of the three peptides for calmodulin, and a region involved in the inhibition by alpha s2-CN (f183-207) was located at the carboxyl terminus 191-207. Two predicted calmodulin-binding sequences, 164-179 and 191-207 of alpha s2-CN, despite rather divergent primary structures, shared the structural motif common to the calmodulin-binding domains of the target proteins in the previously proposed complex model.

Bending of the calmodulin central helix: a theoretical study

The crystal structure of calcium-calmodulin (CaM) reveals a protein with a typical dumbbell structure. Various spectroscopic studies have suggested that the central linker region of CaM, which is alpha-helical in the crystal structure, is flexible in solution. In particular, NMR studies have indicated the presence of a flexible backbone between residues Lys 77 and Asp 80. This flexibility is related directly to the function of the protein because it enables the N- and C-terminal domains of the protein to move toward each other and bind to the CaM-binding domain of a target protein. We have investigated the flexibility of the CaM central helix by a variety of computational techniques: molecular dynamics (MD) simulations, normal mode analysis (NMA), and essential dynamics (ED) analysis. Our MD results reproduce the experimentally determined location of the bend in a simulation of only the CaM central helix, indicating that the bending point is an intrinsic property of the alpha-helix, for which the remainder of the protein is not important. Interestingly, the modes found by the ED analysis of the MD trajectory are very similar to the lowest frequency modes from the NM analysis and to modes found by an ED analysis of different structures in a set of NMR structures. Electrostatic interactions involving residues Arg 74 and Asp 80 seem to be important for these bending motions and unfolding, which is in line with pH-dependent NMR and CD studies.

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.

A role in enzyme activation for the N-terminal leader sequence in calmodulin

We have found that deletion of residues 2-8 from the N-terminal leader sequence: Ala1-Asp2-Gln-Leu4-Thr-Glu6-Glu-Gln8, in calmodulin abolishes calmodulin-dependent activation of skeletal muscle myosin light chain kinase activity and reduces calmodulin-dependent activation of smooth muscle myosin light chain kinase activity to approximately 50% of the maximum level measured at a saturating calmodulin concentration. Calmodulin-dependent activation of cerebellar nitric oxide synthase activity is not affected by this deletion. Overlapping tripeptide deletions from the leader sequence indicate that the acidic cluster, Glu6-Glu-Gln8, contains the determinants necessary for activation of myosin light chain kinase activity. Deletion of Asp2-Gln-Leu4 has no effect on activation of enzyme activity. Based on enzyme kinetic analyses, deletions in the leader sequence have little or no effect on the apparent affinities of calmodulin for the synthase or the two kinases. Since the N-terminal leader does not appear to play a significant structural role in the complexes between calmodulin and peptides representing the calmodulin-binding domains in the two kinases, our results indicate that it participates in secondary interactions with these enzymes that are important to activation, but not to recognition or binding of calmodulin.

Identification of cyclized calmodulin antagonists from a phage display random peptide library

To isolate peptide ligands that bound calmodulin (CaM) specifically, we screened an M13 phage library displaying cyclized octamer random peptides with immobilized bovine CaM. Isolates were recovered, sequenced, and deduced to express nine independent peptides, five of which contained the sequence Trp-Gly-Lys (WGK). Four of the nine peptide sequences were synthesized in cyclized, biotinylated form. All of the peptides required Ca2+ to bind CaM. The cyclized, disulfide-bonded form of one such peptide, SCLRWGKWSNCGS, bound CaM better than its reduced form or an analogue in which the cysteine residues were replaced by serine. The cyclized peptide also exhibited the ability to inhibit CaM-dependent kinase activity. Systematic alanine substitution of residues in this peptide sequence implicate the tryptophan residue as being critical for binding, with other residues contributing to binding to varying degrees. Cloning of ligand targets (COLT) confirmed the specificity of one of the cyclized peptides, yielding full-length and C-terminal CaM clones, in addition to a full-length clone of troponin C, a CaM-related protein. This study has demonstrated that conformationally constrained peptides isolated from a phage library acted as specific, Ca(2+)-dependent CaM ligands.

Decreased plasma membrane calcium transport activity in aging brain

We have assessed the functional properties of both calmodulin (CaM) and the plasma membrane Ca(2+)-ATPase in brains of young, middle aged, and old Fisher 344 rats. Under optimal conditions of saturating Ca2+ and ATP, the CaM-activated Ca(2+)-ATPase activity was decreased with increasing age, particularly when CaM isolated from the brains of aged rats was used to stimulate the enzyme. In the case of CaM, structural modifications within the primary sequence of the protein from aged brains were identified. We found that during normal biological aging approximately 6 methionine residues were modified to their corresonding sulfoxide per CaM, and no other amino acids were modified. Some aspects of the age-related decline in the effectiveness of CaM as an activator of Ca(2+)-ATPase could be simulated using a range of reactive oxygen species (including hydrogen peroxide and oxoperoxynitrite) and, in the latter case, the extent of oxidative modification of specific methionine residues was directly related to their surface accessibility. The pattern of oxidative modification of the methionines in the aged CaM was less straightforward, though both in vitro oxidation of CaM and aging within the brain markedly decreased the functional properties of this important Ca(2+)-regulating protein.

Analysis of phosphorylation and mutation of tyrosine residues of calmodulin on its activation of the erythrocyte Ca(2+)-transporting ATPase

The role played by the phosphorylation sites of calmodulin on its ability to activate the human erythrocyte Ca(2+)-transporting ATPase (Ca(2+)-ATPase) was evaluated. Phosphorylation of mammalian calmodulin on serine/threonine residues by casein kinase II decreased its affinity for Ca(2+)-ATPase by twofold. In contrast, tyrosine phosphorylation of mammalian calmodulin by the insulin-receptor kinase did not significantly alter calmodulin-stimulated Ca(2+)-ATPase activity. Two variant calmodulins, each containing only one tyrosine residue (the second Tyr is replaced by Phe) were also examined: [F138]calmodulin, a mutant containing tyrosine at position 99, and wheat germ calmodulin which has tyrosine at position 139. The concentrations of [F138]calmodulin and wheat germ calmodulin required for half-maximal activation of Ca(2+)-ATPase were tenfold and fourfold higher, respectively, than mammalian calmodulin. Phosphorylation at Tyr99 of [F138]calmodulin shifted its affinity for Ca(2+)-ATPase towards that of mammalian calmodulin. However, phosphorylation at Tyr139 of wheat germ calmodulin had essentially no effect on its interaction with Ca(2+)-ATPase. Thus, all of the observed effects of both phosphorylation and substitution of residues of calmodulin are on its affinity for Ca(2+)-ATPase, not on Vmax. The effects are dependent on the site of phosphate incorporation. Replacement of tyrosine with phenylalanine has a larger effect than phosphorylation of tyrosine, suggesting that the observed functional alterations reflect a secondary conformational change in the C-terminal half of calmodulin, the region that is important in its activation of Ca(2+)-ATPase.

Distance measurements near the myosin head-rod junction using fluorescence spectroscopy

We reacted a fluorescent probe, N-methyl-2-anilino-6-naphthalenesulfonyl chloride (MNS-Ci), with a specific lysine residue of porcine cardiac myosin located in the S-2 region of myosin. We performed fluorescence resonance energy transfer (FRET) spectroscopy measurements between this site and three loci (Cys109, Cys125, and Cys154) located within different myosin light-chain 2s (LC2) bound to the myosin "head". We used LC2s from rabbit skeletal muscle myosin (Cys125), chicken gizzard smooth muscle myosin (Cys109), or a genetically engineered mutant of chicken skeletal muscle myosin (Cys154). The atomic coordinates of these LC2 loci can be closely approximated, and the FRET measurements were used to determine the position of the MNS-labeled lysine with respect to the myosin head. The C-terminus of myosin subfragment-1 determined by Rayment et al. ends abruptly after a sharp turn of its predominantly alpha-helical structure. We have constructed a model based on our FRET distance data combined with the known structure of chicken skeletal muscle myosin subfragment-1. This model suggests that the loci that bracket the head-rod junction will be useful for evaluating dynamic changes in this region.

Target recognition by calmodulin: dissecting the kinetics and affinity of interaction using short peptide sequences

The interaction between calmodulin (CaM) and peptide M13, its target binding sequence from skeletal muscle myosin light chain kinase, involves predominantly two sets of interactions, between the N-terminal target residues and the C-domain of calmodulin, and between the C-terminal target residues and the N-domain of calmodulin (Ikura M et al., 1992, Science 256:632-638). Using short synthetic peptides based on the two halves of the target sequence, the interactions with calmodulin and its separate C-domain have been studied by fluorescence and CD spectroscopy, calcium binding, and kinetic techniques. Peptide WF10 (residues 1-10 of M13) binds to CaM with Kd approximately 1 microM; peptide FW10 (residues 9-18 of M13, with Phe-17-->Trp substitution) binds to CaM with Kd approximately 100 microM. The effect of peptide WF10 on calcium binding to calmodulin produces a biphasic saturation curve, with marked enhancement of affinity for the binding of two calcium ions to the C-domain, forming a stable half-saturated complex, Ca2-CaM-peptide, and confirming the functional importance of the interaction of this sequence with the C-domain. Stopped-flow studies show that the EGTA-induced dissociation of WF10 from Ca4-CaM proceeds by a reversible relaxation mechanism from a kinetic intermediate state, also involving half-saturation of CaM, and the same mechanism is evident for the full target peptide. Interaction of the N-terminal target residues with the C-domain is energetically the most important component, but interaction of calmodulin with the whole target sequence is necessary to induce the full cooperative interaction of the two contiguous elements of the target sequence with both N- and C-domains of calmodulin. Thus, the interaction of calmodulin with the M13 sequence can be dissected on both a structural and kinetic basis into partial reactions involving intermediates comprising distinct regions of the target sequence. We propose a general mechanism for the calcium regulation of calmodulin-dependent enzyme activation, involving an intermediate complex formed by interaction of the calmodulin C-domain and the corresponding part of the target sequence. This intermediate species can function to regulate the overall calcium sensitivity of activation and to determine the affinity of the calmodulin target interaction.

Multiple-sited interaction of caldesmon with Ca(2+)-calmodulin

The binding of Ca(2+)- and Ba(2+)-calmodulin to caldesmon and its functional consequence was investigated with three different calmodulin mutants. Two calmodulin mutants have pairs of cysteine residues substituted and oxidized to a disulphide bond in either the N- or C-terminal lobe (C41/75 and C85/112). The third mutant has phenylalanine-92 replaced by alanine (F92A). Binding measurements in the presence of Ca2+ by separation on native gels and by carbodiimide-induced cross-linking showed a lower affinity for caldesmon in all the mutants. When Ca2+ was replaced by Ba2+ the affinity of calmodulin for caldesmon was further reduced. The ability of Ca(2+)-calmodulin to release caldesmon's inhibition of the actin-tropomyosin-activated myosin ATPase was virtually abolished by mutation of phenylalanine-92 to alanine or by replacing Ba2+ for Ca2+ in native calmodulin. Both cysteine mutants retained their functional ability, but the increased concentration needed for 50% release of caldesmon inhibition reflected their decreased affinity. Ca2+ -calmodulin produced a broadening in the signals of the NMR spectrum of the 10 kDa Ca(2+)-calmodulin-binding C-terminal fragment of caldesmon arising from tryptophans -749 and -779 and caused an enhancement of maximum tryptophan fluorescence of 49% and a 16 nm blue shift of the maximum. Ca(2+)-calmodulin F92A produced a change in wavelength of 4 nm but no change in maximum, whereas Ca(2+)-calmodulin C41/75 binding produced a decrease in fluorescence with no shift of the maximum. We conclude that functional binding of Ca(2+)-calmodulin to caldesmon requires multiple interaction sites on both molecules. However, some structural modification in calmodulin does not abolish the caldesmon-related functionality. This suggests that various EF hand proteins can substitute for the calmodulin molecule.

The role of Phe-92 in the Ca(2+)-induced conformational transition in the C-terminal domain of calmodulin

Recent studies have shown that substitution of Ala for one or more Phe residues in calmodulin (CaM) imparts a temperature-sensitive phenotype to yeast (Ohya, Y., and Botstein, D. (1994) Science 263, 963-966). The Phe residue immediately preceding the first Ca(2+) ligand in site III of CaM (Phe-92) was found to be of particular importance because the mutation at this position alone was sufficient to induce this phenotype. In the present work we have studied the functional and structural consequences of the Phe-92 --> Ala mutation in human liver calmodulin. We found that the mutant (CaMF92A) is incapable of activating phosphodiesterase, and the maximal activation of calcineurin is reduced by 40% as compared with the wild type CaM. Impaired regulatory properties of CaMF92A are accompanied by an increase in affinity for Ca(2+) at the C-terminal domain. To investigate the structural consequences of the F92A mutation, we constructed four recombinant C-terminal domain fragments (C-CaM) of calmodulin (residues 78-148): 1) wild type (C-CaMW); 2) Ala substituted for Phe-92 (C-CaMF92A); 3) cysteine residues introduced at position 85 and 112 to lock the domain with a disulfide bond in the Ca(2+)-free (closed) conformation (C-CaM85/112); and 4) mutations 2 and 3 combined (C-CaM85/112F92A). The Cys-containing mutants readily form intramolecular disulfide bonds regardless whether Phe or Ala is present at position 92. The F92A mutation causes a decrease in stability of the domain in the absence of Ca(2+) as indicated by an 11.8 degree C shift in the far UV circular dichroism thermal unfolding curve. This effect is reversed by the disulfide bond in the C-CaM85/112F92A mutant. The C-CaMW peptide shows a characteristic Ca(2+)-dependent increase in solvent-exposed hydrophobic surface which was monitored by an increase in the fluorescence of the hydrophobic probe 1,1'-bis(4-anilino)-naphthalene-5,5'-disulfonic acid. The fluorescence increase induced by C-CaMF92A is approximately 45% lower than that induced by C-CaMW suggesting that the F92A mutation causes a decrease in the accessibility of several hydrophobic side chains in the C-terminal domain of CaM in the presence of Ca(2+). The Cys-85-Cys-112 disulfide bond causes a 10- or 5.9-fold decrease in Ca(2+) affinity depending on whether Phe or Ala is present at position 92, respectively, suggesting that coupling between Ca(2+) binding and the conformational transition is weaker in the absence of the phenyl ring at position 92. Our results indicate that Phe-92 makes an important contribution to the Ca(2+)-induced transition in the C-terminal domain of CaM. This is most likely the reason for the severely impaired regulatory properties of the CaM mutants having Ala substituted for Phe-92.

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.

Interaction of proteolytic fragments of calmodulin with caldesmon and calponin

Interaction of five tryptic fragments of calmodulin with caldesmon and calponin was analysed by native gel electrophoresis. In the presence of Ca2+ intact calmodulin interacts with caldesmon and calponin with apparent Kd values equal to 0.23 and 1.3 microM respectively. The interaction was abolished in the absence of Ca2+. Two large tryptic fragments of calmodulin obtained in the presence of Ca2+ (TR1C, residues 1-77, and TR2C, residues 78-148) interact with caldesmon with apparent Kd values of 11.9 and 4.6 microM. Affinity of TR2C to calponin (Kd 3.8 microM) was comparable with that of native calmodulin and was much higher than the corresponding value for TR1C (Kd 41 microM). The short C-terminal tryptic peptide of calmodulin obtained in the presence of EGTA (TR3E, residues 107-148) interacts with caldesmon and calponin with Kd values of 23.9 and 12.1 microM, whereas the large N-terminal peptide TR1E (residues 1-106) interacts with both caldesmon and calponin with a very low affinity (Kd 60 microM). Thus although both N- and C-terminal domains of calmodulin are involved in the interaction with caldesmon and calponin, the C-terminal part of calmodulin (residues 78-148) is of special importance and has the highest contribution for caldesmon and calponin binding.

Ca2+/S100 regulation of giant protein kinases

Protein phosphorylation by protein kinases plays a central regulatory role in cellular processes and these kinases are themselves tightly regulated. One common mechanism of regulation involves Ca2+-binding proteins (CaBP) such as calmodulin (CaM). Here we report a Ca2+-effector mechanism for protein kinase activation by demonstrating the specific and >1,000-fold activation of the myosin-associated giant protein kinase twitchin by Ca2+/S100A1(2). S100A1(2) is a member of a large CaBP family that is implicated in various cellular processes, including cell growth, differentiation and motility, but whose molecular actions are largely unknown. The S100A1(2)-binding site is a part of the autoregulatory sequence positioned in the active site that is responsible for intrasteric autoinhibition of twitchin kinase; the mechanism of autoinhibition based on the crystal structures of two twitchin kinase fragments is described elsewhere. Ca2+/S100 represents a likely physiological activator for the entire family of giant protein kinases involved in muscle contractions and cytoskeletal structure.

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.

Role of calmodulin and Spc110p interaction in the proper assembly of spindle pole body compenents

Previously we demonstrated that calmodulin binds to the carboxy terminus of Spc110p, an essential component of the Saccharomyces cerevisiae spindle pole body (SPB), and that this interaction is required for chromosome segregation. Immunoelectron microscopy presented here shows that calmodulin and thus the carboxy terminus of Spc110p localize to the central plaque. We created temperature-sensitive SPC110 mutations by combining PCR mutagenesis with a plasmid shuffle strategy. The temperature-sensitive allele spc110-220 differs from wild type at two sites. The cysteine 911 to arginine mutation resides in the calmodulin-binding site and alone confers a temperature-sensitive phenotype. Calmodulin overproduction suppresses the temperature sensitivity of spc110-220. Furthermore, calmodulin levels at the SPB decrease in the mutant cells at the restrictive temperature. Thus, calmodulin binding to Spc110-220p is defective at the nonpermissive temperature. Synchronized mutant cells incubated at the nonpermissive temperature arrest as large budded cells with a G2 content of DNA and suffer considerable lethality. Immunofluorescent staining demonstrates failure of nuclear DNA segregation and breakage of many spindles. Electron microscopy reveals an aberrant nuclear structure, the intranuclear microtubule organizer (IMO), that differs from a SPB but serves as a center of microtubule organization. The IMO appears during nascent SPB formation and disappears after SPB separation. The IMO contains both the 90-kD and the mutant 110-kD SPB components. Our results suggest that disruption of the calmodulin Spc110p interaction leads to the aberrant assembly of SPB components into the IMO, which in turn perturbs spindle formation.

Blocking the Ca2+-induced conformational transitions in calmodulin with disulfide bonds

Calcium-dependent regulation of intracellular processes is mediated by proteins that on binding Ca2+ assume a new conformation, which enables them to bind to their specific target proteins and to modulate their function. Calmodulin (CaM) and troponin C, the two best characterized Ca2+-regulatory proteins, are members of the family of Ca2+-binding proteins utilizing the helix-loop-helix structural motif (EF-hand). Herzberg, Moult, and James (Herzberg, O., Moult, J., and James, M.N.G. (1986) J. Biol. Chem. 261, 2638-2644) proposed that the Ca2+-induced conformational transition in troponin C involves opening of the interface between the alpha-helical segments in the N-terminal domain of this protein. Here we have tested the hypothesis that a similar transition is the key Ca2+-induced regulatory event in calmodulin. Using site-directed mutagenesis we have substituted cysteine residues for Gln41 and Lys75 (CaM41/75) or Ile85 and Leu112 (CaM85/112) in the N-terminal and C-terminal domains, respectively, of human liver calmodulin. Based on molecular modeling, cysteines at these positions were expected to form intramolecular disulfide bonds in the Ca2+-free conformation of the protein, thus blocking the putative Ca2+-induced transition. We found that intramolecular disulfide bonds are readily formed in both mutants causing a decrease in affinity for Ca2+ and the loss of ability to activate target enzymes, phosphodiesterase and calcineurin. The regulatory activity is fully recovered in CaM41/75 and partially recovered in CaM85/112 upon reduction of the disulfide bonds with dithiothreitol and blocking the Cys residues by carboxyamidomethylation or cyanylation. These results indicate that the Ca2+-induced opening of the interfaces between helical segments in both domains of CaM is critical for its regulatory properties consistent with the Herzberg-Moult-James model.

Structural basis for the autoinhibition of calcium/calmodulin-dependent protein kinase I

The crystal structure of calcium/calmodulin-dependent protein kinase I has been determined in the autoinhibited form. The C-terminal regulatory region of the enzyme forms a helix-loop-helix segment that extends across the two domains of the catalytic core, making multiple inhibitory interactions. Elements of the first regulatory alpha helix and the loop interfere with the binding site for peptide substrates, while the loop and the second helix interact with the ATP-binding domain to induce conformational changes that obstruct the nucleotide binding pocket. One part of the calmodulin recognition element protrudes away from the catalytic domain and is potentially available for an initial interaction with calmodulin. The structure provides a view of an intact calmodulin target and suggests that substantial structural changes will accompany kinase activation by calmodulin binding to the regulatory region.

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.

Homologs of idoxifene: variation of estrogen receptor binding and calmodulin antagonism with chain length

A series of homologs of idoxifene [1a, (E)-1-[4-(N-pyrrolidinoethoxy)phenyl]-1-(4-iodophenyl)-2-phenyl-1-butene ] and selected homologs of 4-iodotamoxifen [2a,(E)-1-[4-(N-dimethylamino)-ethoxy]phenyl]-1-(4-iodophenyl)-2-phenyl -1-butene] with the side chain (CH(2))(n) varying in length from n=3 (1b,2b) to n=10(1i,2i) have been synthesized and tested for antagonism of of the calmodulin-dependent activity of cAMP phosphodiesterase and for binding affinity to rat uterine estrogen receptor. Compared with 1a (IC(50) =1.5 microM), the homologs showed a progressive increase in calmodulin antagonism with a maximum inhibition at n=7-9 (1f-h)(IC(50)=0.2 microM), declining at n=10 (1i) to IC(50) =1.6 microM. In the pyrrolidino series, estrogen receptor binding affinity peaked at n=3 (1b, RBA= 23; estradiol = 100), declining by n=10 (1i) to RBA = 0.4, but the homolog n=8 (1g, RBA = 3.5) was still comparable to tamoxifen (RBA = 3.9). A similar pattern of activity was seen for the dimethylamino counterparts. These compounds represent a new class of antiestrogens with potent calmodulin antagonism.

Effects of myosin light chain kinase and peptides on Ca2+ exchange with the N- and C-terminal Ca2+ binding sites of calmodulin

Myosin light chain kinase and peptides from the calmodulin (CaM) binding domains of myosin light chain kinase (RS-20, M-13), CaM kinase II, and the myristoylated alanine-rich protein kinase C substrate protein slowed Ca2+ dissociation from CaM's N-terminal sites from 405 +/- 75/s to 1.8-2.9/s and from CaM's C-terminal sites from 2.4 +/- 0.2/s to 0.1-0.4/s at 10 degrees C. Since Ca2+ dissociates 5-29 times faster from the N-terminal in these CaM.peptide complexes and both lobes are required for activation, Ca2+ dissociation from the N-terminal would control target protein inactivation. Ca2+ binds 70 times faster to the N-terminal (1.6 x 10(8) M-1 s-1) than the C-terminal sites (2.3 x 10(6) M-1 s-1). In a 0.6-ms half-width Ca2+ transient, Ca2+ occupied > 70% of the N-terminal but only 20% of the C-terminal sites. RS-20 produced a 9-fold and CaM kinase II a 6.3-fold increase in C-terminal Ca2+ affinity, suggesting that some target proteins may be bound to the C-terminal at resting [Ca2+]. When this is the case, Ca2+ exchange with the faster N-terminal sites may regulate CaM's activation and inactivation of these target proteins during a Ca2+ transient.

Protein-protein interactions in the rigor actomyosin complex

Since it has not been possible to crystallize the actomyosin complex, the x-ray structures of the individual proteins together with data obtained by fiber diffraction and electron microscopy have been used to build detailed models of filamentous actin (f-actin) and the actomyosin rigor complex. In the f-actin model, a single monomer uses 10 surface loops and two alpha-helices to make sometimes complicated interactions with its four neighbors. In the myosin molecule, both the essential and regulatory light chains show considerable structural homology to calmodulin. General principles are evident in their mode of attachment to the target alpha-helix of the myosin heavy chain. The essential light chain also makes contacts with other parts of the heavy chain and with the regulatory light chain. The actomyosin rigor interface is extensive, involving interaction of a single myosin head with regions on two adjacent actin monomers. A number of hydrophobic residues on the apposing faces of actin and myosin contribute to the main binding site. This site is flanked on three sides by charged myosin surface loops that form predominantly ionic interactions with adjacent regions of actin. Hydrogen bonding is likely to play a significant role in actin-actin and actin-myosin interactions since many of the contacts involve loops. The model building approach used with actomyosin is applicable to other multicomponent assemblies of biological interest and is a powerful method for revealing molecular interactions and providing insights into the mode of action of the assemblies.

Covalent binding of peptides to the N-terminal hydrophobic region of cardiac troponin C has limited effects on function

Exposure of an N-terminal hydrophobic region in troponin C is thought to be important for the regulation of contraction in striated muscle. To test this hypothesis, single Cys residues were engineered at positions 45, 81, 84, or 85 in the N-terminal hydrophobic region of cardiac troponin C (cTnC) to provide specific sites for attachment of blocking groups. A synthetic peptide, Ac-Val-Arg-Ala-Ile-Gly-Lys-Leu-Ser-Ser, or biotin was coupled to these Cys residues, and the covalent adducts were tested for activity in TnC-extracted myofibrils. Covalent modification of cTnC(C45) had no effect on maximal myofibril ATPase activity. Greatly decreased myofibril ATPase activity (70-80% inhibited) resulted when the peptide was conjugated to Cys-81 in cTnC(C81), while a lesser degree of inhibition (10-25% inhibited) resulted from covalent modification of cTnC(C84) and cTnC(C85). Inhibition was not due to an altered affinity of the cTnC(C81)/peptide conjugate for the myofibrils, and the Ca2+ dependence of ATPase activity was essentially identical to the unmodified protein. Thus, a subregion of the N-terminal hydrophobic region in cTnC is sensitive to disruption, while other regions are less important or can adapt to rather bulky blocking groups. The data suggest that Ca(2+)-sensitizing drugs may bind to the N-terminal hydrophobic region on cTnC but not interfere with transmission of the Ca2+ signal.

Structure of the pseudosubstrate recognition site of chicken smooth muscle myosin light chain kinase

The structure of the chicken smooth muscle myosin light chain kinase pseudosubstrate sequence MLCK(774-807)amide was studied using two-dimensional proton NMR spectroscopy. Resonance assignments were made with the aid of totally correlated and nuclear Overhauser effect spectroscopy. A distance geometry algorithm was used to process the body of NMR distance and angle data and the resulting family of structures was further refined using dynamic simulated annealing. The major structural features determined include two helical segments extending from Asp-777 to Lys-785 and from Arg-790/Met-791 to Trp-800 connected by a turn region from Leu-786 to Asp-789 enabling the helices to interact in solution. The C-terminal helix incorporates the bulk of the pseudosubstrate recognition site which is partially overlapped by the calmodulin binding site while the N-terminal helix forms the bulk of the connecting peptide. The demonstrated turn between the helices may assist in enabling the autoregulatory or pseudosubstrate recognition sequence to be rotated out of the active site of the catalytic core following calmodulin binding.

Calmodulin is the divalent cation receptor for rapid endocytosis, but not exocytosis, in adrenal chromaffin cells

Exocytosis and the ensuing rapid endocytosis in adrenal chromaffin cells are both Ca(2+)-dependent phenomena but differ in their divalent cation specificity, implying distinct Ca2+ receptors for the two processes. To ascertain whether calmodulin is the Ca2+ receptor for either process, we blocked its function by introducing calmodulin-binding peptides or anti-calmodulin antibodies into these cells. Exo/endocytosis was followed by measurement of cell membrane capacitance. Rapid endocytosis, but not exocytosis, was abolished by these treatments, indicating that calmodulin is the Ca2+ receptor for rapid endocytosis but is not involved in exocytosis. The principal calmodulin target is not protein phosphatase-2B, as antagonism of this enzyme did not inhibit but accelerated rapid endocytosis. Calmodulin may thus regulate both the rate and extent of rapid endocytosis by distinct pathways.

Structure of a novel extracellular Ca(2+)-binding module in BM-40

The EF-hand is a highly conserved Ca(2+)-binding motif found in many cytosolic Ca(2+)-modulated proteins. Here we report the crystal structure at 2.0 A resolution of the carboxy-terminal domain of human BM-40 (SPARC, osteonectin), an extracellular matrix protein containing an EF-hand pair. The two EF-hands interact canonically but their detailed structures are unusual. In the first EF-hand a one-residue insertion is accommodated by a cis-peptide bond and by substituting a carboxylate by a peptide carbonyl as a Ca2+ ligand. The second EF-hand is stabilized by a disulphide bond. The EF-hand pair interacts tightly with an amphiphilic amino-terminal helix, reminiscent of target peptide binding by calmodulin. The present structure defines a novel protein module occurring in several other extracellular proteins.

Proximity relationships between engineered cysteine residues in chicken skeletal myosin regulatory light chain. A resonance energy transfer study

Resonance energy transfer was used to measure the distances between pairs of cysteines, Cys2 and Cys155 and Cys73 and Cys155, in recombinant chicken skeletal myosin regulatory light chains in the free and bound states. The fluorescent and nonfluorescent probes N-iodoacetyl-N'-(5-sulfo-1-naphthyl) ethylenediamine and N-(4-dimethylamino-3,5-dinitrophenyl)maleimide were used as the donor and the acceptor, respectively. The distance between Cys2 and Cys155 was measured to be 35 and 30 A in the absence and presence of myosin heavy chain, respectively, suggesting a slightly more compact structure for the light chain in the bound state. The distance between Cys73 and Cys155 measured in a similar manner was 31 and 30 A in the free and bound states, respectively; this latter value is in good agreement with that derived from crystallographic structures. For heavy chain-bound light chains, no measurable distance changes were detected with the binding of ATP or actin. These results show that no gross structural changes occur within the regulatory light chain during the contraction cycle, but that resonance energy transfer between other sites could be used to monitor potential changes in the myosin head upon the binding of nucleotides and actin.

The calmodulin-nitric oxide synthase interaction. Critical role of the calmodulin latch domain in enzyme activation

The neuronal isoform of nitric oxide synthase (nNOS) requires calmodulin for nitric oxide producing activity. Calmodulin functions as a molecular switch, allowing electron transport from the carboxyl-terminal reductase domain of nitric oxide synthase to its heme-containing amino-terminal domain. Available evidence suggests that calmodulin binds to a site between the two domains of nNOS, but it is not known how calmodulin then executes its switch function. To study the calmodulin-nNOS interaction, we created a series of chimeras between calmodulin and cardiac troponin C (cTnC, a homologue of calmodulin that does not activate nNOS). Although a few chimeras showed good ability to activate nNOS, most failed to activate. A subset of the inactive chimeras retained the ability to bind to nNOS and therefore functioned as potent competitive inhibitors of nNOS activation by calmodulin (CaM). The observed inhibition was additive with the arginine antagonists NG-monomethyl-L-arginine and 7-nitroindazole, indicating a distinct and independent mechanism of nNOS inhibition. To localize the calmodulin residues that account for impaired activation in the inhibitory CaM-cTnC chimeras, we conducted a detailed mutagenesis study, replacing CaM subdomains and individual amino acid residues with the corresponding residues from cTnC. This revealed that mutations in CaM helices 2 and 6 (its latch domain) have a disproportionate negative effect on nNOS activation. Thus, our evidence suggests that the CaM latch domain plays a critical role in its molecular switch function.

Identification of a Ca(2+)-binding light chain within Chlamydomonas outer arm dynein

We describe here the molecular cloning of the M(r) 18,000 dynein light chain from the outer arm of Chlamydomonas flagella. In vivo, this molecule is directly associated with the gamma dynein heavy chain. Sequence analysis indicates that this light chain is a novel member of the calmodulin superfamily of Ca2+ binding regulatory proteins; this molecule is 42, 37 and 36% identical to calmodulin, centrin/caltractin and troponin C, respectively, and also shows significant similarity to myosin light chains. Although four helix-loop-helix elements are evident, only two conform precisely to the EF hand consensus and are therefore predicted to bind Ca2+ in vivo. In vitro Ca2+ binding studies indicate that this dynein light chain (expressed as a C-terminal fusion with maltose binding protein) has at least one functional Ca2+ binding site with an apparent affinity for Ca2+ of approximately 3 × 10(−5) M. Within the Chlamydomonas flagellum, the transition from an assymmetric to a symmetric waveform (which implies an alteration in dynein activity) is mediated by an increase in intraflagellar Ca2+ from 10(−6) to 10(−1) M; this transition is altered in mutants that lack the outer arm. The data presented here suggest that a Ca(2+)-dependent alteration in the interaction of this dynein light chain with the motor containing heavy chain may affect outer arm function during flagellar reversal.

Both the amino and carboxyl termini of Dictyostelium myosin essential light chain are required for binding to myosin heavy chain

Dictyostelium myosin deficient in the essential light chain (ELC) does not function normally either in vivo or in vitro (Pollenz, R. S., Chen, T. L., Trivinos-Lagos, L., and Chisholm, R. L. (1992) Cell 69, 951-962). Since normal myosin function requires association of ELC, we investigated the domains of ELC that are necessary for binding to the myosin heavy chain (MHC). Deleting the NH2-terminal 11 or 28 amino acid residues (delta N11 or delta N28) or the COOH-terminal 15 amino acid residues (delta C15) abolished binding of the ELC to the MHC when the mutants were expressed in wild-type (WT) cells. In contrast, the ELC carrying deletion or insertion of four amino acid residues (D4 or I4) in the central linker segment bound the MHC in WT cells, although less efficient competition with WT ELC suggested that the affinity for the MHC is reduced. When these mutants were expressed in ELC-minus (mlcE-) cells, where the binding to the heavy chain is not dependent on efficient competition with the endogenous ELC, delta N28 and delta N11 bound to the MHC at 15% of WT levels and delta C15 did not bind to a significant degree. I4 and D4, however, bound with normal stoichiometry. These data indicate that residues at both termini of the ELC are required for association with the MHC, while the central linker domain appears to be less critical for binding. When the mutants were analyzed for their ability to complement the cytokinesis defect displayed by mlcE- cells, a correlation to the level of ELC carried by the MHC was observed, indicating that a stoichiometric ELC-MHC association is necessary for normal myosin function in vivo.

Role of the N-terminal region of the skeletal muscle myosin light chain kinase target sequence in its interaction with calmodulin

The binding of calmodulin (CaM) to four synthetic peptide analogues of the skeletal muscle myosin light chain kinase (sk-MLCK) target sequence has been studied using 1H-NMR. The 18-residue peptide WFF is anchored to CaM via the interaction of the Trp 4 side chain with the C-domain and the Phe 17 side chain with the N-domain of the protein. A peptide corresponding to the first 10 residues (WF10) does not provide the second anchoring residue and is not long enough to span both domains of CaM. 1H-NMR spectroscopy indicates that the WF10 peptide interacts specifically with the C-domain of CaM, and the chemical shifts of the bound Trp side chain are very similar in the CaM:WF10 and CaM:WFF complexes. Binding of the C-domain of CaM to the strongly basic region around Trp 4 of this MLCK sequence may be an important step in target recognition. Comparison of 1H-NMR spectra of CaM bound to WFF, a Trp 4-->Phe analogue (FFF), or a Trp 4-->Phe/Phe 17-->Trp analogue (FFW) suggests that all three peptides bind to CaM in the same orientation, i.e., with the peptide side chain in position 4 interacting with the C-domain and the side chain in position 17 interacting with the N-domain. This indicates that a Trp residue in position 4 is not an absolute requirement for binding this target sequence and that interchanging the Trp 4 and Phe 17 residues does not reverse the orientation of the bound peptide, in confirmation of the deduction from previous indirect studies using circular dichroism (Findlay WA, Martin SR, Beckingham K, Bayley PM, 1995, Biochemistry 34:2087-2094). Molecular modeling/energy minimization studies indicate that only minor local changes in the protein structure are required to accommodate binding of the bulkier Trp 17 side chain of the FFW peptide to the N-domain of CaM.