Photocrosslinking of calmodulin and/or actin to chicken gizzard caldesmon

Regulation of PKC autophosphorylation by calponin in contractile vascular smooth muscle tissue

Protein kinase C (PKC) is a key enzyme involved in agonist-induced smooth muscle contraction. In some cases, regulatory phosphorylation of PKC is required for full activation of the enzyme. However, this issue has largely been ignored with respect to PKC-dependent regulation of contractile vascular smooth muscle (VSM) contractility. The first event in PKC regulation is a transphosphorylation by PDK at a conserved threonine in the activation loop of PKC, followed by the subsequent autophosphorylation at the turn motif and hydrophobic motif sites. In the present study, we determined whether phosphorylation of PKC is a regulated process in VSM and also investigated a potential role of calponin in the regulation of PKC. We found that calponin increases the level of in vitro PKCα phosphorylation at the PDK and hydrophobic sites, but not the turn motif site. In vascular tissues, phosphorylation of the PKC hydrophobic site, but not turn motif site, as well as phosphorylation of PDK at S241 increased in response to phenylephrine. Calponin knockdown inhibits autophosphorylation of cellular PKC in response to phenylephrine, confirming results with recombinant PKC. Thus these results show that autophosphorylation of PKC is regulated in dVSM and calponin is necessary for autophosphorylation of PKC in VSM.

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.

The major myosin-binding site of caldesmon resides near its N-terminal extreme

The primary myosin-binding site of caldesmon was thought to be in the N-terminal region of the molecule, but the exact nature of the caldesmon-myosin interaction has not been well characterized. A caldesmon fragment that encompasses residues 1-240 (N240) was found to bind full-length smooth muscle myosin on the basis of co-sedimentation experiments. The interaction between myosin and N240 was not affected by phosphorylation of myosin, but it was weakened by the presence of Ca(2+)/calmodulin. To locate the myosin-binding site, we have designed several synthetic peptides based on the N-terminal caldesmon sequence. We found that a peptide stretch corresponding to the first 27 residues (Met-1 to Tyr-27), but not that of the first 22 residues (Met-1 to Ala-22), exhibited a moderate affinity toward myosin. We also found that a peptide containing the segment from Ile/Leu-25 to Lys-53 bound both myosin and heavy meromyosin more strongly and was capable of displacing caldesmon from myosin. Our results demonstrate that the sequence near the N-terminal extreme of caldesmon harbors a major myosin-binding site of caldesmon, in which both the nonpolar residues and clusters of positively and negatively charged residues confer the specificity and affinity of the caldesmon-myosin interaction.

Regulation of vascular smooth muscle tone by N-terminal region of caldesmon. Possible role of tethering actin to myosin

To assess the functional significance of tethering actin to myosin by caldesmon in the regulation of smooth muscle contraction, we investigated the effects of synthetic peptides, containing the myosin-binding sequences in the N-terminal region of caldesmon, on force directly recorded from single permeabilized smooth muscle cells of ferret portal vein. Two peptides were used, IK29C and MY27C, containing residues from Ile(25) to Lys(53) and from Met(1) to Tyr(27) of the human and chicken caldesmon sequence, respectively, plus an added cysteine at the C terminus. In cells clamped at pCa 6. 7, both peptides increased basal tone. Pretreatment of cells at pCa 6.7 with IK29C or MY27C decreased the amplitude of subsequent phenylephrine-induced contractions but not microcystin-racemic mixture-induced contractions. In all cases the effects of the peptides were concentration-dependent, and IK29C was more potent than MY27C, in agreement with their relative affinity toward myosin. The peptides were ineffective after the phenylephrine contraction was established. MY27C did not further increase the magnitude of contraction caused by a maximally effective concentration of IK29C, consistent with the two peptides having the same mechanism of action. Neither polylysine nor two control peptides containing scrambled sequences of IK29C, which do not bind myosin, had any effect on basal or phenylephrine-induced force. Our results suggest that IK29C and MY27C induce contraction by competing with the myosin-binding domain of endogenous caldesmon. Digital imaging of fluoroisothiocyanate-tagged IK29C confirmed the association of the peptide with intracellular filamentous structures. The results are consistent with a model whereby tethering of actin to myosin by caldesmon may play a role in regulating vascular tone by positioning the C-terminal domain of caldesmon so that it is capable of blocking the actomyosin interaction.

Interaction of isoforms of S100 protein with smooth muscle caldesmon

Interaction of S100a and S100b with duck gizzard caldesmon was investigated by means of native gel electrophoresis, fluorescent spectroscopy and disulfide crosslinking. Both isoforms of S100 interact with intact caldesmon and its C-terminal deletion mutant 606C (residues 606-756) with apparent Kd of 0.2-0.6 microM thus indicating that the S100-binding site is located in the C-terminal domain of caldesmon. The single SH group of duck gizzard caldesmon can be crosslinked to Cys-84 of the beta-chain or to Cys-85 of the alpha-chain of S100. Crosslinking of S100 reduces the inhibitory action of caldesmon on actomyosin ATPase activity. S100 reverses the inhibitory action of intact caldesmon and its deletion mutants 606C (residues 606-756) and H9 (residues 669-737) as effectively as calmodulin. S100a has higher affinity to caldesmon and is more effective than S100b in reversing caldesmon-induced inhibition of actomyosin ATPase activity. Although monomeric (calmodulin, troponin C) and dimeric (S100) Ca-binding proteins have different sizes and structures they interact with caldesmon in a very similar fashion.

Heat treatment could affect the biochemical properties of caldesmon

Smooth muscle caldesmon (CaD) exhibits apparent heat stability. A widely used purification procedure of CaD involves extensive heat treatment (Bretscher, A. (1984) J. Biol. Chem. 259, 12873-12880). CaD thus purified co-sediments with actin, inhibits actomyosin ATPase activity, and interacts with Ca2+/calmodulin, similarly to the unheated protein. On the other hand, heat-treated CaD binds to actin filaments in a tether-like fashion, whereas lengthwise binding dominates in vivo (Mabuchi, K., Lin, J. J.-C., and Wang, C.-L. A. (1993) J. Muscle Res. Cell Motil. 14, 54-64), suggesting that differences do exist between heat-purified CaD and the native protein. We have isolated, without heat treatment, full-length recombinant chicken gizzard CaD overexpressed in insect cells (High-FiveTM) using a baculovirus expression system. We found that such unheated CaD interacts with calmodulin 10 times stronger than does the heated CaD; its inhibitory action on actomyosin ATPase is reversed by a much lesser amount of calmodulin. Moreover, electron microscopic examination indicated that actin binding at the N-terminal region is more frequent in the unheated CaD, resulting in more lengthwise binding. These findings point to the fact that CaD is not entirely heat-stable; the C-terminal CaM-binding regions and the N-terminal actin-binding region are possibly affected by heat treatment.

Location of smooth-muscle myosin and tropomyosin binding sites in the C-terminal 288 residues of human caldesmon

We have produced nine recombinant fragments, H1 to H9, from a human cDNA that codes for the C-terminal 288 residues of caldesmon. The fragment H1, encompassing the 288 residues, is equivalent to domains 3 and 4 of caldesmon (amino acids 506-793 in human, 476-737 in the chicken gizzard sequence). It has been shown [Huber, Redwood, Avent, Tanner and Marston (1993) J. Muscle Res. Cell Motil. 14, 385-391] to bind to actin, Ca(2+)-calmodulin, tropomyosin and myosin. The fragments, H2 to H9, differ in length between 60 and 176 residues and cover the whole of domains 3 and 4 with many of the fragments overlapping. We have characterized the myosin and tropomyosin binding of these fragments. The binding of both tropomyosin and myosin is highly dependent on salt concentration, indicating the ionic nature of these interactions. The location of the myosin binding is an extended region encompassing the junction of domains 3/4 and domain 4a (residues 622-714, human; 566-657, chicken gizzard). Tropomyosin binds in a smaller region within domain 4a of caldesmon (residues 663-714, human; 606-657 chicken gizzard). We confirmed predictions based on sequence similarities of a tropomyosin binding site in domain 3 of caldesmon; however, this site bound to skeletal-muscle tropomyosin and had little affinity for the smooth-muscle tropomyosin isoform. None of the protein fragments H2-H9 retained the affinity of the parent fragment H1 for either myosin or tropomyosin. This indicates the need for several interaction sites scattered over an extended region to attain higher affinity. The regions interacting with caldesmon in both tropomyosin and myosin are coiled-coil structures. This is probably the reason for their shared interaction sites on caldesmon and their similar natures of binding.

Identification of the functionally relevant calmodulin binding site in smooth muscle caldesmon

The C-terminal region of smooth muscle caldesmon (CaD) interacts with calmodulin (CaM) and reverses CaD's inhibitory effect on the actomyosin ATPase activity. We have previously shown that the major CaM-binding site (site A) in this region is within the segment from Met-658 to Ser-666 (Zhan, Q., Wong, S. S., and Wang, C.-L. A. (1991) J. Biol. Chem. 266, 21810-21814). Recently, another segment (site B), Asn-675 to Lys-695, was reported to bind CaM (Mezgueldi, M., Derancourt, J., Calas, B., Kassab, R., and Fattoum, A. (1994) J. Biol. Chem. 269, 12824-12832). To assess the functional relevance of these two putative CaM-binding sites, we have examined three synthetic peptides regarding their effects on CaM's ability to reverse CaD-induced inhibition of actomyosin ATPase activity: GS17C (Gly-651 to Ser-667), VG29C (Val-685 to Gly-713), each containing one CaM-binding site, and MG56C (Met-658 to Gly-713), which contains both sites. We found that although VG29C did bind CaM, its affinity was weakened by GS17C, and it failed to compete with CaD for CaM under the conditions where GS17C effectively displaced CaD from CaM. MG56C had an effect similar to that of GS17C. These experiments demonstrated that site A for CaM binding is involved in regulating the inhibitory property of CaD.

Benzophenone photophores in biochemistry

The photoactivatable aryl ketone derivatives have been rediscovered as biochemical probes in the last 5 years. The expanding use of benzophenone (BP) photoprobes can be attributed to three distinct chemical and biochemical advantages. First, BPs are chemically more stable than diazo esters, aryl azides, and diazirines. Second, BPs can be manipulated in ambient light and can be activated at 350-360 nm, avoiding protein-damaging wavelengths. Third, BPs react preferentially with unreactive C-H bonds, even in the presence of solvent water and bulk nucleophiles. These three properties combine to produce highly efficient covalent modifications of macromolecules, frequently with remarkable site specificity. This Perspectives includes a brief review of BP photochemistry and a selection of specific applications of these photoprobes, which address questions in protein, nucleic acid, and lipid biochemistry.

Electron microscopic images suggest both ends of caldesmon interact with actin filaments

An improved rotary shadowing technique enabled us to visualize chicken gizzard caldesmon (CaD) and its complexes with one or two covalently linked calmodulin (CaM) molecules by electron microscopy. Using a monoclonal antibody against an epitope in the N-terminal region of CaD (anti-N), we can now identify the end of the molecule that is involved in binding to another protein molecule. Thus in the 1:1 complex of CaD and CaM, the CaM molecule was almost always associated with the C-terminus of CaD, indicating preferential CaM-binding to the C-terminal region. We have also studied binding of CaD to filamentous actin (F-actin), using an EM technique that avoids spraying or freeze drying and thereby preserves the structure of F-actin. Only one end of CaD appeared to bind to F-actin, leaving the rest of the molecule projecting away from the filament. While the majority of anti-N bound at the free end of CaD, some antibody molecules were found on F-actin. These findings suggest that either end of CaD can bind to F-actin. Experiments using a monoclonal antibody against the C-terminus of CaD (anti-C) supported this idea. When the native thin filaments that contain endogenous CaD were incubated with anti-N, almost all the bound antibodies were found on the filaments, indicating that the N-terminal regions of CaD interact with actin, and that the binding affinity of the N-terminal region of CaD for actin is higher in vivo than that in vitro, either because the properties of CaD have been altered during purification, or because of the presence of some other component(s) associated with the native filaments.

Interaction of calmodulin with phospholamban and caldesmon: comparative studies by 1H-NMR spectroscopy

In order to identify comparative aspects of the interaction of calmodulin with its target proteins, proton magnetic-resonance studies of complex formation between calmodulin and defined segments of phospholamban and caldesmon have been undertaken. Residues 3-15 in the cytoplasmic region of phospholamban, an integral membrane protein of cardiac sarcoplasmic reticulum believed to regulate the calcium pumping ATPase, are shown to contribute to interaction with calmodulin. Using wheat germ calmodulin specifically modified with a spin-label to provide the spectral means for spatial localisation, these residues of phospholamban were correlated with binding in the vicinity of the probe attached to Cys-27 in the N-terminal domain of calmodulin. This interaction, relevant to the mechanism of calmodulin-dependent phosphorylation of phospholamban that relieves its inhibitory influence on the calcium pump, provides a useful model system for comparative study of the properties of calmodulin-binding domains. We contrast here a calmodulin-binding segment in the C-terminal region of caldesmon localised by 1H-NMR study of the interface(s) between the two proteins. These observations are discussed in the context of other calmodulin-binding sequences.

Cloning of cDNAs encoding human caldesmons

Caldesmon (CDM) is a potential actomyosin regulatory protein found in smooth muscle and nonmuscle cells. Domain mapping and physical studies suggest that CDM is an elongated molecule with an N-terminal myosin/calmodulin-binding domain and a C-terminal tropomyosin/actin/calmodulin-binding domain separated by a 40-nm-long central helix. An 1100-nucleotide (nt) cDNA probe encoding the C terminus of avian caldesmon (aCDM) was used to screen a human aorta library and clone smooth-muscle and non-muscle CDM-encoding cDNAs (CDM). The human (h) smooth-muscle hCDM is 3050-3630 nt long, having variation in length in the 3'-untranslated region. The predicted hCDM protein has a high degree of identity, greater than 90%, to aCDM in the N- and C-terminal-binding domains. The central helical domain is more variable, but retains characteristic repeated peptides and an 'i, i + 4' acidic/basic amino acid (aa) motif found in aCDM which can form intra-helical salt bridges to stabilize the central helix. The predicted smooth-muscle protein is 793 aa long (93,262 Da) with a calculated pI of 5.75. As is the case for the chicken, nonmuscle hCDM is missing the central helical domain, 256 aa overall. Our nonmuscle clone is not full length, but the C-terminal end is identical to the smooth-muscle form. If the N-terminal domain is identical, as it is in the chicken, the predicted protein is 537 aa (62,558 Da). Examination of the 'junctions' at either end of the deleted central domain gives a clear indication of the splice sites and suggests that the nonmuscle form is generated by exon skipping.(ABSTRACT TRUNCATED AT 250 WORDS)

The molecular anatomy of caldesmon

Images

Electron microscopic studies of chicken gizzard caldesmon and its complex with calmodulin

Caldesmon samples mounted on a stage rotating about a horizontal axis were shadowed keeping the shadow angle at about 3 degrees. This technique minimizes background metal deposits compared with the conventional method. The identity of caldesmon was confirmed by comparing the images of caldesmon alone with those of the caldesmon-calmodulin complex. In these samples the caldesmon molecules appeared to be elongated; most were between 30 and 80 nm in length. The maximum length was in good agreement with the earlier estimate of 74 nm based on hydrodynamic studies. Our observations also suggested the presence of a rather rigid 30-40 nm stretch in the middle of the caldesmon molecule, which was always visible under rotary shadowing, and a flexible structure of about 20 nm in length at each end of the molecule, which may or may not be visible depending on their orientation on the mica surface. In the samples of caldesmon crosslinked with calmodulin, we noticed the existence of complexes containing two calmodulin molecules per caldesmon molecule, separated by a distance of 60 nm, consistent with the suggestion that each end of caldesmon can interact with calmodulin.

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.

Epitope mapping of monoclonal antibodies against caldesmon and their effects on the binding of caldesmon to Ca++/calmodulin and to actin or actin-tropomyosin filaments

The effects of monoclonal anti-caldesmon antibodies, C2, C9, C18, C21, and C23, on the binding of caldesmon to F-actin/F-actin-tropomyosin filaments and to Ca++/calmodulin were examined in an in vitro reconstitution system. In addition, the antibody epitopes were mapped by Western blot analysis of NTCB (2-nitro-5-thiocyanobenzoic acid) and CNBr (cyanogen bromide) fragments of caldesmon. Both C9 and C18 recognize an amino terminal fragment composed of amino acid residues 19 to 153. The C23 epitope lies within a fragment ranging from residues 230 to 386. Included in this region is a 13-residue repeat sequence. Interestingly this repetitive sequence shares sequence similarity with a sequence found in nuclear lamin A, a protein which is also recognized by C23 antibody. Therefore, it is likely that the C23 epitope corresponds to this 13-residue repeat sequence. A carboxyl-terminal 10K fragment contains the epitopes for antibodies C2 and C21. Among these antibodies, only C21 drastically inhibits the binding of caldesmon to F-actin/F-actin-tropomyosin filaments and to Ca++/calmodulin. When the molar ratio of monoclonal antibody C21 to caldesmon reached 1.0, a maximal inhibition (90%) on the binding of caldesmon to F-actin filaments was observed. However, it required double amounts of C21 antibody to exhibit a maximal inhibition of 70% on the binding of caldesmon to F-actin-tropomyosin filaments. These results suggest that the presence of tropomyosin in F-actin enhances caldesmon's binding. Furthermore, C21 antibody also effectively inhibits the caldesmon binding to Ca++/calmodulin. The kinetics of C21 inhibition on caldesmon's binding to Ca++/calmodulin is very similar to the inhibition obtained by preincubation of caldesmon with free Ca++/calmodulin. This result suggests that there is only one Ca++/calmodulin binding domain on caldesmon and this domain appears to be very close to the C21 epitope. Apparently, the Ca++/calmodulin-binding domain and the actin-binding domain are very close to each other and may interfere with each other. In an accompanying paper, we have further demonstrated that microinjection of C21 antibody into living chicken embryo fibroblasts inhibit intracellular granule movement, suggesting an in vivo interference with the functional domains [Hegmann et al., 1991: Cell Motil. Cytoskeleton 20:109-120].

Caldesmon has two calmodulin-binding domains

Chicken gizzard caldesmon was cleaved with chymotrypsin or CNBr, and the calmodulin-binding fragments were isolated using an affinity column. Limited chymotryptic digestion gives rise to a 38 kDa calmodulin-binding fragment (CT40) as described previously (Szpacenko, A. & Dabrowska, R., FEBS Lett. 202, 182-186, 1986; Fujii, T., Imai, M., Rosenfeld, G. C. & Bryan, J., J. Biol. Chem. 261, 16155-16160, 1987; Yazawa, M., Yagi, K. & Sobue, K., J. Biochem. 102, 1065-1073, 1987). In the case of CNBr cleavage a 37 kDa calmodulin-binding fragment (CB40) was obtained. Both CT40 and CB40 contain a reactive thiol group, but these thiols are apparently in different environments as judged by the responses of attached fluorescent labels to calmodulin-binding. A comparison of the N-terminal sequences of CB40 and CT40 with the complete sequence of caldesmon shows that the two calmodulin-binding fragments in fact originate from different parts of the parent molecule. Thus there exist two calmodulin-binding sites in caldesmon, one in the N-terminal half and the other in the C-terminal half of the molecule. This is consistent with the recent finding that up to two calmodulin molecules can be crosslinked to each caldesmon molecule (Wang, C.-L.A., Biochem. Biophys. Res. Commun., 156, 1033-1038, 1988).

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.

Caldesmon, acidic amino acids and molecular weight determinations

Recent estimates of molecular weight and cDNA sequencing indicate that smooth muscle caldesmons are considerably smaller than previously thought. The anomalous behaviour of these proteins during SDS-polyacrylamide gel electrophoresis can be correlated with their high acidic amino acid content. The results suggest a need to re-evaluate the stoichiometric relations of caldesmon to tropomyosin and actin in thin filaments and its presumed 1:1 interaction with calmodulin.