A 35-kilodalton fragment from gizzard smooth muscle caldesmon that induces F-actin bundles

There is more than one way to model an elephant. Experiment-driven modeling of the actin cytoskeleton

Mathematical modeling has established its value for investigating the interplay of biochemical and mechanical mechanisms underlying actin-based motility. Because of the complex nature of actin dynamics and its regulation, many of these models are phenomenological or conceptual, providing a general understanding of the physics at play. But the wealth of carefully measured kinetic data on the interactions of many of the players in actin biochemistry cries out for the creation of more detailed and accurate models that could permit investigators to dissect interdependent roles of individual molecular components. Moreover, no human mind can assimilate all of the mechanisms underlying complex protein networks; so an additional benefit of a detailed kinetic model is that the numerous binding proteins, signaling mechanisms, and biochemical reactions can be computationally organized in a fully explicit, accessible, visualizable, and reusable structure. In this review, we will focus on how comprehensive and adaptable modeling allows investigators to explain experimental observations and develop testable hypotheses on the intracellular dynamics of the actin cytoskeleton.

Differential effects of caldesmon on the intermediate conformational states of polymerizing actin

The actin-binding protein caldesmon (CaD) reversibly inhibits smooth muscle contraction. In non-muscle cells, a shorter CaD isoform co-exists with microfilaments in the stress fibers at the quiescent state, but the phosphorylated CaD is found at the leading edge of migrating cells where dynamic actin filament remodeling occurs. We have studied the effect of a C-terminal fragment of CaD (H32K) on the kinetics of the in vitro actin polymerization by monitoring the fluorescence of pyrene-labeled actin. Addition of H32K or its phosphorylated form either attenuated or accelerated the pyrene emission enhancement, depending on whether it was added at the early or the late phase of actin polymerization. However, the CaD fragment had no effect on the yield of sedimentable actin, nor did it affect the actin ATPase activity. Our findings can be explained by a model in which nascent actin filaments undergo a maturation process that involves at least two intermediate conformational states. If present at early stages of actin polymerization, CaD stabilizes one of the intermediate states and blocks the subsequent filament maturation. Addition of CaD at a later phase accelerates F-actin formation. The fact that CaD is capable of inhibiting actin filament maturation provides a novel function for CaD and suggests an active role in the dynamic reorganization of the actin cytoskeleton.

Caldesmon and the regulation of cytoskeletal functions

Caldesmon (CaD) is an extraordinary actin-binding protein, because in addition to actin, it also bindsmyosin, calmodulin and tropomyosin. As a component of the smoothmuscle and nonmuscle contractile apparatus CaD inhibits the actomyosin ATPase activity and its inhibitory action is modulated by both Ca2+ and phosphorylation. The multiplicity of binding partners and diverse biochemical properties suggest CaD is a potent and versatile regulatory protein both in contractility and cell motility. However, after decades ofinvestigation in numerous laboratories, hard evidence is still lacking to unequivocally identify its in vivo functions, although indirect evidence is mounting to support an important role in connection with the actin cytoskeleton. This chapter reviews the highlights of the past findings and summarizes the current views on this protein, with emphasis of its interaction with tropomyosin.

Caldesmon is a cytoskeletal target for PKC in endothelium

We have previously shown that treatment of bovine endothelial cell (EC) monolayers with phorbol myristate acetate (PMA) leads to the thinning of cortical actin ring and rearrangement of the cytoskeleton into a grid-like structure, concomitant with the loss of endothelial barrier function. In the current work, we focused on caldesmon, a cytoskeletal protein, regulating actomyosin interaction. We hypothesized that protein kinase C (PKC) activation by PMA leads to the changes in caldesmon properties such as phosphorylation and cellular localization. We demonstrate here that PMA induces both myosin and caldesmon redistribution from cortical ring into the grid-like network. However, the initial step of PMA-induced actin and myosin redistribution is not followed by caldesmon redistribution. Co-immunoprecipitation experiments revealed that short-term PMA (5 min) treatment leads to the weakening of caldesmon ability to bind actin and, to the lesser extent, myosin. Prolonged incubation (15-60 min) with PMA, however, strengthens caldesmon complexes with actin and myosin, which correlates with the grid-like actin network formation. PMA stimulation leads to an immediate increase in caldesmon Ser/Thr phosphorylation. This process occurs at sites distinct from the sites specific for ERK1/2 phosphorylation and correlates with caldesmon dissociation from the actomyosin complex. Inhibition of ERK-kinase MEK fails to abolish grid-like structure formation, although reducing PMA-induced weakening of the cortical actin ring, whereas inhibition of PKC reverses PMA-induced cytoskeletal rearrangement. Our results suggest that PKC-dependent phosphorylation of caldesmon is involved in PMA-mediated complex cytoskeletal changes leading to the EC barrier compromise.

A caldesmon peptide activates smooth muscle via a mechanism similar to ERK-mediated phosphorylation

Caldesmon (CaD) is thought to regulate smooth muscle contraction, because it binds actin and inhibits actomyosin interactions. A synthetic actin-binding peptide (GS17C) corresponding to Gly666-Ser682 of chicken gizzard CaD has been shown to induce force development in permeabilized smooth muscle cells. The mechanism of GS17C's action remains unclear, although a structural effect was postulated. By photo-crosslinking and fluorescence quenching experiments with a gizzard CaD fragment (H32K; Met563-Pro771) and its mutants, we showed that GS17C indeed dissociated the C-terminal region of H32K from actin, in a manner similar to extracellular signal-regulated kinase-mediated phosphorylation, thereby reversing the CaD-imposed inhibition and enabling the actomyosin interaction.

Calcium-dependent regulation of interactions of caldesmon with calcium-binding proteins found in growth cones of chick forebrain neurons

1. This study was undertaken to determine if caldesmon, calmodulin, S100beta, and neurocalcin delta were present in chick forebrain neurons, and if so, to investigate the interactions of these proteins in the presence of different concentrations of calcium. 2. Immunocytochemistry was used to determine the presence and localization of these proteins in cultured forebrain neurons. Western blotting, gel electrophoresis in the presence of different concentrations of calcium, chemical cross-linking, and affinity chromatography were used to investigate the interactions of these proteins with each other. 3. Our data show that caldesmon and three calcium-binding proteins (S100beta, calmodulin, and neurocalcin 3) are localized in growth cones and neurites of chick forebrain neurons in culture. In the presence of different concentration of calcium, these calcium-binding proteins have different affinities to caldesmon and to each other. S100beta binds with greater affinity than calmodulin to caldesmon, and its ability to bind to caldesmon is regulated by neurocalcin delta. 4. These findings suggest a specific calcium-dependent regulatory pathway for modulating actomyosin during growth cone motility.

Polymerization of actin induced by actin-binding fragments of caldesmon

Our earlier studies revealed that caldesmon causes assembly of G-actin into polymers morphologically indistinguishable from those formed in the presence of salt (Gałazkiewicz, B., Belagyi, J. and Dabrowska, R. (1989) Eur. J. Biochem. 181, 607-614). In this work we have investigated the effect of actin-binding fragments of caldesmon on actin polymerization process followed by measurements of the changes in fluorescence of pyrenyl conjugated with G-actin and ATP hydrolysis. The results indicate that C-terminal 34 kDa fragment of caldesmon containing two actin-binding sites and 19 kDa containing high-affinity binding site have similar capability to polymerize actin to that of intact molecule. Binding of each of these fragments to G-actin causes bypassing of nucleation phase. The 11.5 kDa fragment comprising low affinity actin-binding site has much lower potency to polymerize actin. Conformation of actin monomers in filaments formed upon 19 kDa fragment and that formed upon 11.5 kDa fragment differs. The former fragment seems to resemble more conformation of monomers in filaments formed upon intact caldesmon than the latter one.

Identification of functioning regulatory sites and a new myosin binding site in the C-terminal 288 amino acids of caldesmon expressed from a human clone

A partial clone of caldesmon, coding for the C-terminal 288 amino acids, was isolated from a human fetal liver cDNA library and sequenced. Expression of the clone in Escherichia coli produced a peptide called H1 (M(r) 32,549), which inhibited tropomyosin-enhanced actomyosin Mg(2+)-ATPase activity by 90% with half maximal inhibition at 0.03-0.04 mol H1 per mol actin. The inhibition could be reversed by Ca(2+)-calmodulin. H1 bound actin, Ca(2+)-calmodulin and tropomyosin and smooth muscle myosin with high affinities. This latter finding shows the presence of a second myosin-binding site in caldesmon. This was confirmed in thrombic digests of native sheep aorta and chicken gizzard caldesmon.

Involvement of caldesmon at the actin-myosin interface

Addition of myosin subfragment 1 (S-1) to the actin-caldesmon binary complex, which forms bundles of actin filaments resulted in the formation of actin/caldesmon-decorated filaments [Harricane, Bonet-Kerrache, Cavadore & Mornet (1991) Eur. J. Biochem. 196, 219-224]. The present data provide further evidence that caldesmon and S-1 compete for a common actin-binding region and demonstrate that a change occurs in the actin-myosin interface induced by caldesmon. S-1 digested by trypsin, which has an actin affinity 100-fold weaker than that of native S-1, was efficiently removed from actin by caldesmon, but not completely dissociated. This particular ternary complex was stabilized by chemical cross-linking with carbodi-imide, which does not have any spacer arm, and revealed contact interfaces between the different protein components. Cross-linking experiments showed that the presence of caldesmon had no effect on stabilization of actin-(20 kDa domain), whereas the actin-(50 kDa domain) covalent association was significantly decreased, to the point of being virtually abolished.

Regulation by Ca(2+)-calmodulin of the actin-bundling activity of Physarum 210-kDa protein

From the plasmodia of a lower eukaryote, Physarum polycephalum, we have previously purified a 210-kDa protein that showed similar properties to those of smooth muscle caldesmon. Further characterization of the 210-kDa protein revealed that it bundled actin filaments. This bundling activity was inhibited by calmodulin in the presence of Ca2+. Unlike smooth muscle caldesmon, the 210-kDa protein bundled actin filaments whether or not a reducing agent, such as dithiothreitol, was present. The protein was shown to have two (or more) different actin-binding sites which were classified into salt-sensitive and salt-insensitive sites. Electron microscopy revealed that the 210-kDa protein was an elongated molecule (mean length, 97 +/- 25 nm) which was bent in the middle. The Stokes radius and sedimentation coefficient of the 210-kDa protein were 130 A and 2.9 S, respectively. An immunofluorescence study revealed that the 210-kDa protein colocalized with the bundles of actin filaments in thin-spread preparations of Physarum plasmodia, suggesting that the 210-kDa protein was regulating the appearance and disappearance of the actin bundles that are associated with the contraction-relaxation cycle of the plasmodia.

Actin-caldesmon-myosin-subfragment-1 ternary complex viewed by electron microscopy. Competitive actin binding region for caldesmon and myosin subfragment-1

An earlier electron microscopic study using different caldesmon forms complexed with actin revealed that the aggregates produced display regular periodic striation after antibody labeling of the 35-kDa caldesmon fragment. This approach provides further evidence that a caldesmon fragment, even as small as 15 kDa, can induce actin filaments to assemble into bundles. The observed difference in the compactness of these structures, depending on the use of the 15-kDa fragment instead of the 35-kDa fragment, suggests the existence of more than one actin-binding site in the caldesmon molecule. In this study, the caldesmon-induced process of F-actin association was investigated in the presence of skeletal myosin subfragment-1, using light-scattering methods, cosedimentation experiment and electron microscopic techniques. We show that the actin-caldesmon association is partially destabilized in the presence of subfragment-1 and this leads to a ternary complex formation. Immunogold labelling of the actin filaments still reveals the presence of caldesmon within this structure. This latter result strengthens the hypothesis that actin has a site(s) able to bind both caldesmon and myosin subfragment-1, as detected by recent NMR observations. This evidence is discussed with respect to the regulatory function of caldesmon during smooth muscle contraction.

Conformational change of turkey-gizzard caldesmon induced by specific chemical modification with carbodiimide

Water soluble 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide was used to internally cross-link carboxyl and lysyl groups of caldesmon. The modification did not involve the two cysteines of the molecule which were previously labelled with N-iodoacetyl-N'-(5-sulfo-1-naphthyl)ethylenediamine. The modified caldesmon exhibited a smaller Stokes radius (4.0 nm instead of 6.3 nm) and its electrophoretic mobility corresponded to an apparent molecular mass of approximately 82 kDa, appreciably lower than that of the native molecule (120 kDa), but more similar to the reported true molecular mass of 86,974 Da of chicken-gizzard caldesmon (Bryan, J., Imai, M., Lee, R., Moore, P., Cook. R. G. & Lin, W. (1989) J. Biol. Chem. 264, 13,873-13,879). Comparative circular dichroism analysis indicated a decrease of the alpha-helix content from 43% to 36% resulting from the chemical modification. The 1H-NMR spectra of the native and modified caldesmon showed that the covalent cross-linking affected mainly the central and N-terminal parts of the molecule. The C-terminal part, rich in aromatic amino acids, was unmodified by the carbodiimide treatment. This was also corroborated by the continued ability of the modified caldesmon to bind to actin and calmodulin, and by the property of the 90-kDa proteolytic N-terminal fragment to give an internally cross-linked species of 60 kDa. Using electron microscopy, the modified protein was shown to have a more compact shape and a reduced capacity to induce tight and long F-actin bundles. These conformational changes were obtained when the carbodiimide reaction was conducted at pH 6.0 and were not observed at pH 8.0. This suggests that local variation of the pH might affect the conformation of caldesmon which changes from an elongated to more compact shape, stabilized by electrostatic interactions. It is proposed that the flexibility of caldesmon might be involved in the regulatory function of this protein in the smooth muscle and might favour tightly packed F-actin bundles or weaker interactions between actin filaments.

Structural study of gizzard caldesmon and its interaction with actin. Binding involves residues of actin also recognised by myosin subfragment 1

The interaction between actin and caldesmon that is associated with the inhibition of actomyosin ATPase activity in smooth muscle has been studied using 1H-NMR spectroscopy. Binding studies using the intact molecules were complemented by the use of thrombic cleavage fragments of both turkey and chicken gizzard caldesmon as well as defined peptides of actin, in order to investigate the conformational properties of caldesmon and to localise regions of the primary structures that participate in protein-protein contacts. The binding of caldesmon is shown to involve distinct segments on the N-terminal region (residues 1-44) of actin, as previously observed for the inhibitory component of the thin filament of striated muscle, troponin I [Levine et al. (1988) Eur. J. Biochem. 153, 389-397]. The comparable structural properties of these tissue-specific inhibitors of actomyosin ATPase and the similarities in their mode of interaction at the N-terminal region of actin suggest common aspects to the structural mechanism for thin-filament regulation in smooth and striated muscle. Unlike the inhibitory interaction of troponin I, however, the binding of caldesmon to the N-terminal region of actin directly involves groups within residues 20-41 of actin that are also recognised by myosin subfragment 1. The complementary segment of caldesmon has been localised to a 15-kDa thrombic fragment (residues 483-578) derived from the N-terminal portion of a 35-kDa proteolytic cleavage product from the C-terminal of caldesmon whose interaction with actin is modulated by calmodulin. The results are discussed in relation to the calcium-mediated mechanism for thin-filament regulation in smooth and striated muscle.

Complexes between 15 kDa caldesmon fragment and actin investigated by immuno-electron microscopy

The regulatory system of smooth muscle thin filaments is thought to involve a major calcium-calmodulin and actin binding protein: caldesmon. A dissective approach was used to isolate a 35 kDa C-terminal fragment of the molecule and to produce antibodies reacting against both the intact and the 15 kDa N-terminal end of this parental fragment. While this purified 15 kDa caldesmon fragment demonstrates a weak actin association, we observed that it cross-links actin filaments into loose bundles. These structures were labelled with a selective antibody and showed regular periodic striation with repeats of approximately 40 nm. This work brings additional information to previous reports using an actin and calmodulin binding 25 kDa C-terminal fragment of the caldesmon molecule [(1989) J. Biol. Chem. 264, 2869-2875]. We demonstrate that a purified fragment corresponding to a sequence smaller than 96 amino acids, which contains no cystein residue, is able to interact with actin at a single site which is not the calmodulin modulated.

New subfragment 1 of skeletal muscle myosin obtained by thrombin cleavage

The head of the myosin molecule (i.e., subfragment 1 with a heavy chain of 95 kDa) is usually obtained by chymotryptic cleavage in the presence of a divalent cation chelator. In the present work, we used another specific proteolytic enzyme, thrombin, to produce a limited cut within the myosin molecule, resulting in a new species of N-terminal fragment. Treatment of skeletal muscle myosin yielded a 97-kDa split heavy chain associated with intact light chains, corresponding to a single cut. The ATPase activities of this new S-1 derivative were slightly affected by the breakdown. It recognized actin in an ATP-dependent manner, as expected, with an affinity 2-5 times higher than that of the usual chymotryptic S-1 preparation but with a very different electron microscopic pattern. Functional differences are noted, and we involve them more precisely in relation to possible structural aspects of the additional C-terminal segment extending the usual S-1 heavy chain from 95 to 97 kDa.

The effect of caldesmon on dynamic properties of F-actin alone and bound to heavy meromyosin and/or tropomyosin

The effect of caldesmon on the rotational dynamics of actin filaments alone or conjugated with heavy meromyosin and/or tropomyosin has been measured by the electron paramagnetic resonance (EPR) technique using a maleimide spin label rigidly bound to Cys374 of actin. The rotation of actin protomers in filaments and the angular distribution of spin probes on actin were determined by conventional EPR spectroscopy, while torsional motions within actin filaments were detected by saturation transfer EPR measurements. Binding of caldesmon to F-actin resulted in the reduction of torsional mobility of actin filaments. The maximum effect was produced at a ratio of about one molecule of caldesmon/seven actin protomers. Smooth muscle tropomyosin enhanced the effect of caldesmon, i.e. caused further slowing down of internal motions within actin filaments. Caldesmon increased the degree of order of spin labels on F-actin in macroscopically oriented pellets in the presence of tropomyosin but not in its absence. Computer analysis of the spectra revealed that caldesmon alone slightly changed the orientation of spin probes relative to the long axis of the filament. In the presence of tropomyosin this effect of caldesmon was potentiated and then approximately every twentieth protomer along the actin filament was affected. Caldesmon weakened the effect of heavy meromyosin both on the polarity of environment of the spin label attached to F-actin and on the degree of order of labels on actin in macroscopically oriented pellets. Whereas the former effect of caldesmon was independent of tropomyosin, the latter one was observed only in the absence of tropomyosin.

Caldesmon structure and function: sequence analysis of a 35 kilodalton actin- and calmodulin-binding fragment from the C-terminus of the turkey gizzard protein

We have determined the amino acid sequence of a 35 kDa proteolytic fragment ("CaD35") derived from the C-terminus of turkey gizzard caldesmon. This 239-residue peptide contains binding sites for actin and calmodulin. Residues 1-96 of CaD35 comprise "CaD15", an actin-binding subfragment which we previously showed to resemble the tropomyosin-binding segment of troponin T. The remainder of the CaD35 sequence shows no significant similarity to other proteins. Residues 111-128 may form a basic, amphipathic helix which interacts with calmodulin.

Location of the calmodulin- and actin-binding domains at the C-terminus of caldesmon

Digestion of caldesmon with carboxypeptidase Y is accompanied by loss of its ability to inhibit actomyosin ATPase activity and to bind actin and calmodulin. Similarly, carboxypeptidase Y digestion of a terminal 40 kDa chymotryptic fragment of caldesmon abolishes its inhibition of the actomyosin ATPase and binding to actin and calmodulin. This represents the first direct demonstration that these functional domains of caldesmon are located close to the carboxy-terminus of the molecule.

Amino acid sequence of a 15 kilodalton actin-binding fragment of turkey gizzard caldesmon: similarity with dystrophin, tropomyosin and the tropomyosin-binding region of troponin T

We have determined the amino acid sequence of a 15 kDa actin-binding fragment of turkey gizzard caldesmon. The 96-residue fragment contains 29 acidic and 29 basic residues, and is predicted to have an extended helical conformation stabilized by numerous internal salt bridges. CaD15 bears some resemblance to dystrophin, tropomyosin and several other proteins, but is most strikingly similar to the tropomyosin-binding segment of troponin T.