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

The role of caldesmon and its phosphorylation by ERK on the binding force of unphosphorylated myosin to actin

BACKGROUND

Studies conducted at the whole muscle level have shown that smooth muscle can maintain tension with low Adenosine triphosphate (ATP) consumption. Whereas it is generally accepted that this property (latch-state) is a consequence of the dephosphorylation of myosin during its attachment to actin, free dephosphorylated myosin can also bind to actin and contribute to force maintenance. We investigated the role of caldesmon (CaD) in regulating the binding force of unphosphorylated tonic smooth muscle myosin to actin.

METHODS

To measure the effect of CaD on the binding of unphosphorylated myosin to actin (in the presence of ATP), we used a single beam laser trap assay to quantify the average unbinding force (Funb) in the absence or presence of caldesmon, extracellular signal-regulated kinase (ERK)-phosphorylated CaD, or CaD plus tropomyosin.

RESULTS

Funb from unregulated actin (0.10±0.01pN) was significantly increased in the presence of CaD (0.17±0.02pN), tropomyosin (0.17±0.02pN) or both regulatory proteins (0.18±0.02pN). ERK phosphorylation of CaD significantly reduced the Funb (0.06±0.01pN). Inspection of the traces of the Funb as a function of time suggests that ERK phosphorylation of CaD decreases the binding force of myosin to actin or accelerates its detachment.

CONCLUSIONS

CaD enhances the binding force of unphosphorylated myosin to actin potentially contributing to the latch-state. ERK phosphorylation of CaD decreases this binding force to very low levels.

GENERAL SIGNIFICANCE

This study suggests a mechanism that likely contributes to the latch-state and that explains the muscle relaxation from the latch-state.

Inverse interaction between tropomyosin and phosphorylated myosin in the presence or absence of caldesmon

In the present study, co-sedimentation assay, intrinsic fluorescence intensity measurement, and Mg²⁺-ATPase activity analysis were carried out to investigate the direct effect of tropomyosin (TM) on unphosphorylated myosin (UM) or phosphorylated myosin (PM) in the presence or absence of caldesmon (CaD). Results showed that TM significantly decreased the sedimentation, intrinsic fluorescence intensity, and the Mg²⁺-ATPase activity of PM, but not UM. In the presence of CaD, TM also significantly decreased these parameters irrespective of myosin phosphorylation, suggesting that the interaction between TM and CaD abolished the effects of TM on PM or UM and that there was an inverse interaction between TM and PM, characterized by the decreased PM sedimentation and intrinsic fluorescence intensity.

cGMP-dependent protein kinase Iβ regulates breast cancer cell migration and invasion via interaction with the actin/myosin-associated protein caldesmon

The two isoforms of type I cGMP-dependent protein kinase (PKGIα and PKGIβ) differ in their first ∼100 amino acids, giving each isoform unique dimerization and autoinhibitory domains. The dimerization domains form coiled-coil structures and serve as platforms for isoform-specific protein-protein interactions. Using the PKGIβ dimerization domain as an affinity probe in a proteomic screen, we identified the actin/myosin-associated protein caldesmon (CaD) as a PKGIβ-specific binding protein. PKGIβ phosphorylated human CaD on serine 12 in vitro and in intact cells. Phosphorylation on serine 12 or mutation of serine 12 to glutamic acid (S12E) reduced the interaction between CaD and myosin IIA. Because CaD inhibits myosin ATPase activity and regulates cell motility, we examined the effects of PKGIβ and CaD on cell migration and invasion. Inhibition of the NO/cGMP/PKG pathway reduced migration and invasion of human breast cancer cells, whereas PKG activation enhanced their motility and invasion. siRNA-mediated knockdown of endogenous CaD had pro-migratory and pro-invasive effects in human breast cancer cells. Reconstituting cells with wild-type CaD slowed migration and invasion; however, CaD containing a phospho-mimetic S12E mutation failed to reverse the pro-migratory and pro-invasive activity of CaD depletion. Our data suggest that PKGIβ enhances breast cancer cell motility and invasive capacity, at least in part, by phosphorylating CaD. These findings identify a pro-migratory and pro-invasive function for PKGIβ in human breast cancer cells, suggesting that PKGIβ is a potential target for breast cancer treatment.

Ablation of smooth muscle caldesmon affects the relaxation kinetics of arterial muscle

Smooth muscle caldesmon (h-CaD) is an actin- and myosin-binding protein that reversibly inhibits the actomyosin ATPase activity in vitro. To test the function of h-CaD in vivo, we eliminated its expression in mice. The h-CaD-null animals appeared normal and fertile, although the litter size was smaller. Tissues from the homozygotes lacked h-CaD and exhibited upregulation of the non-muscle isoform, l-CaD, in visceral, but not vascular tonic smooth muscles. While the Ca(2+) sensitivity of force generation of h-CaD-deficient smooth muscle remained largely unchanged, the kinetic behavior during relaxation in arteries was different. Both intact and permeabilized arterial smooth muscle tissues from the knockout animals relaxed more slowly than those of the wild type. Since this difference occurred after myosin dephosphorylation was complete, the kinetic effect most likely resulted from slower detachment of unphosphorylated crossbridges. Detailed analyses revealed that the apparently slower relaxation of h-CaD-null smooth muscle was due to an increase in the amplitude of a slower component of the biphasic tension decay. While the identity of this slower process has not been unequivocally determined, we propose it reflects a thin filament state that elicits fewer re-attached crossbridges. Our finding that h-CaD modulates the rate of smooth muscle relaxation clearly supports a role in the control of vascular tone.

Caldesmon regulates axon extension through interaction with myosin II

To begin the process of forming neural circuits, new neurons first establish their polarity and extend their axon. Axon extension is guided and regulated by highly coordinated cytoskeletal dynamics. Here we demonstrate that in hippocampal neurons, the actin-binding protein caldesmon accumulates in distal axons, and its N-terminal interaction with myosin II enhances axon extension. In cortical neural progenitor cells, caldesmon knockdown suppresses axon extension and neuronal polarity. These results indicate that caldesmon is an important regulator of axon development.

Diversification of caldesmon-linked actin cytoskeleton in cell motility

The actin cytoskeleton plays a key role in regulating cell motility. Caldesmon (CaD) is an actin-linked regulatory protein found in smooth muscle and non-muscle cells that is conserved among a variety of vertebrates. It binds and stabilizes actin filaments, as well as regulating actin-myosin interaction in a calcium (Ca2+)/calmodulin (CaM)- and/or phosphorylation-dependent manner. CaD function is regulated qualitatively by Ca2+/CaM and by its phosphorylation state and quantitatively at the mRNA level, by three different transcriptional regulation of the CALD1 gene. CaD has numerous functions in cell motility, such as migration, invasion, and proliferation, exerted via the reorganization of the actin cytoskeleton. Here we will outline recent findings regarding CaD's structural features and functions.

Effects of genistein on beta-catenin signaling and subcellular distribution of actin-binding proteins in human umbilical CD105-positive stromal cells

This study was performed to define the roles of actin-binding proteins in the regulation of actin filament assembly associated with cellular signal transduction pathways in stromal cell proliferation. Genistein, a tyrosine protein kinase inhibitor, decreased the intracellular Ca(2+) and attenuated cell proliferation and DNA synthesis through the beta-catenin and cyclin D1 pathway in human umbilical CD105-positive cells. Immunoprecipitation studies using anti-beta-actin antibody revealed that several actin-binding proteins implicated in cells include formin-2 (FMN-2), caldesmon (CaD), tropomyosin (Tm), and profilin. Protein levels of these proteins in whole cell lysates were not significantly changed by genistein. Three Tm isoforms, Tm-1, Tm-2, and Tm-4, were found to be present in cells. Genistein caused a reduction in levels of mRNAs coding for Tm-1 and Tm-4, but had no significant effect on Tm-2 mRNA levels. Immunofluorescence confocal scanning microscopy indicated that changes in the subcellular distribution of Tm and CaD, in which the diffuse cytosolic staining was shifted to show colocalization with actin stress fibers. In contrast, genistein-induced accumulation of FMN-2 and profilin in the peri-nuclear area. Silencing of FMN-2 by small interfering RNA resulted in increases of intracellular Ca(2+) and rendered genistein resistance in decreasing intracellular Ca(2+) in cells. These results provide the novel findings that genistein acts by modulating the cellular distribution of actin-binding proteins in association with alterations of cellular signal transduction pathways in human stromal cell proliferation.

Phosphorylated HSP27 modulates the association of phosphorylated caldesmon with tropomyosin in colonic smooth muscle

Thin-filament regulation of smooth muscle contraction involves phosphorylation, association, and dissociation of contractile proteins in response to agonist stimulation. Phosphorylation of caldesmon weakens its association with actin leading to actomyosin interaction and contraction. Present data from colonic smooth muscle cells indicate that acetylcholine induced a significant association of caldesmon with PKCalpha and sustained phosphorylation of caldesmon at ser789. Furthermore, acetylcholine induced significant and sustained increase in the association of phospho-caldesmon with heat-shock protein (HSP)27 with concomitant increase in the dissociation of phospho-caldesmon from tropomyosin. At the thin filament level, HSP27 plays a crucial role in acetylcholine-induced association of contractile proteins. Present data from colonic smooth muscle cells transfected with non-phospho-HSP27 mutant cDNA indicate that the absence of phospho-HSP27 inhibits acetylcholine-induced caldesmon phosphorylation. Our results further indicate that the presence of phospho-HSP27 significantly enhances acetylcholine-induced sustained association of phospho-caldesmon with HSP27 with a concomitant increase in acetylcholine-induced dissociation of phospho-caldesmon from tropomyosin. We thus propose a model whereby upon acetylcholine-induced phosphorylation of caldesmon at ser789, the association of phospho-caldesmon (ser789) with phospho-HSP27 results in an essential conformational change leading to dissociation of phospho-caldesmon from tropomyosin. This leads to the sliding of tropomyosin on actin thus exposing the myosin binding sites on actin for actomyosin interaction.

Direct interaction between caldesmon and cortactin

Actin polymerization and depolymerization plays a central role in controlling a wide spectrum of cellular processes. There are many actin-binding proteins in eukaryotic cells. Their roles in the remodeling of the actin architecture and whether they work cooperatively await further study. Caldesmon (CaD) is an actin-binding protein present in nearly all mammalian cells. Cortactin is another actin-binding protein found mainly in the cell cortex. There have been no reports suggesting that CaD and cortactin interact with each other or work as partners. Here, we present evidence that CaD binds cortactin directly by overlay, pull-down assays, ELISA, and by column chromatography. The interaction involves the N-terminal region of cortactin and the C-terminal region of CaD, and appears to be enhanced by divalent metal ions. Cortactin competes with both full-length CaD and its C-terminal fragment for actin binding. Binding of cortactin partially alleviates the inhibitory effect of CaD on the actomyosin ATPase activity. Not only can binding be demonstrated in vitro, the two proteins also co-localize in activated cells at the cortex. Whether such interactions bear any functional significance awaits further investigation.

Phosphorylation of caldesmon during smooth muscle contraction and cell migration or proliferation

The actin-binding protein caldesmon (CaD) exists both in smooth muscle (the heavy isoform, h-CaD) and non-muscle cells (the light isoform, l-CaD). In smooth muscles h-CaD binds to myosin and actin simultaneously and modulates the actomyosin interaction. In non-muscle cells l-CaD binds to actin and stabilizes the actin stress fibers; it may also mediate the interaction between actin and non-muscle myosins. Both h- and l-CaD are phosphorylated in vivo upon stimulation. The major phosphorylation sites of h-CaD when activated by phorbol ester are the Erk-specific sites, modification of which is attenuated by the MEK inhibitor PD98059. The same sites in l-CaD are also phosphorylated when cells are stimulated to migrate, whereas in dividing cells l-CaD is phosphorylated more extensively, presumably by cdc2 kinase. Both Erk and cdc2 are members of the MAPK family. Thus it appears that CaD is a downstream effector of the Ras signaling pathways. Significantly, the phosphorylatable serine residues shared by both CaD isoforms are in the C-terminal region that also contains the actin-binding sites. Biochemical and structural studies indicated that phosphorylation of CaD at the Erk sites is accompanied by a conformational change that partially dissociates CaD from actin. Such a structural change in h-CaD exposes the myosin-binding sites on the actin surface and allows actomyosin interactions in smooth muscles. In the case of non-muscle cells, the change in l-CaD weakens the stability of the actin filament and facilitates its disassembly. Indeed, the level of l-CaD modification correlates very well in a reciprocal manner with the level of actin stress fibers. Since both cell migration and cell division require dynamic remodeling of actin cytoskeleton that leads to cell shape changes, phosphorylation of CaD may therefore serve as a plausible means to regulate these processes. Thus CaD not only links the smooth muscle contractility and non-muscle motility, but also provides a common mechanism for the regulation of cell migration and cell proliferation.

Caldesmon is an integral component of podosomes in smooth muscle cells

Podosomes are highly dynamic actin-based structures commonly found in motile and invasive cells such as macrophages, osteoclasts and vascular smooth muscle cells. Here, we have investigated the role of caldesmon, an actin-binding protein, in the formation of podosomes in aortic smooth muscle A7r5 cells induced by the phorbol ester PDBu. We found that endogenous low molecular weight caldesmon (l-caldesmon), which was normally localised to actin-stress fibres and membrane ruffles, was recruited to the actin cores of PDBu-induced podosomes. Overexpression of l-caldesmon in A7r5 cells caused dissociation of actin-stress fibres and disruption of focal adhesion complexes, and significantly reduced the ability of PDBu to induce podosome formation. By contrast, siRNA interference of caldesmon expression enhanced PDBu-induced formation of podosomes. The N-terminal fragment of l-caldesmon, CaD40, which contains the myosin-binding site, did not label stress fibres and was not translocated to PDBu-induced podosomes. Cad39, the C-terminal fragment housing the binding sites for actin, tropomyosin and calmodulin, was localised to stress fibres and was translocated to podosomes induced by PDBu. The caldesmon mutant, CadCamAB, which does not interact with Ca2+/calmodulin, was not recruited to PDBu-induced podosomes. These results show that (1) l-caldesmon is an integral part of the actin-rich core of the podosome; (2) overexpression of l-caldesmon suppresses podosome formation, whereas siRNA knock-down of l-caldesmon facilitates its formation; and (3) the actin-binding and calmodulin-binding sites on l-caldesmon are essential for the translocation of l-caldesmon to the podosomes. In summary, this data suggests that caldesmon may play a role in the regulation of the dynamics of podosome assembly and that Ca2+/calmodulin may be part of a regulatory mechanism in podosome formation.

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.

Regulation of microfilament organization by Kaposi sarcoma-associated herpes virus-cyclin.CDK6 phosphorylation of caldesmon

Kaposi sarcoma-associated herpes virus (KSHV) encodes a D-like cyclin (K-cyclin) that is thought to contribute to the viral oncogenicity. K-cyclin activates cellular cyclin-dependent kinases (CDK) 4 and 6, generating enzymes with a substrate selectivity deviant from CDK4 and CDK6 activated by D-type cyclins, suggesting different biochemical and biological functions. Here we report the identification of the actin- and calmodulin-binding protein caldesmon (CALD1) as a novel K-cyclin.CDK substrate, which is not phosphorylated by D.CDK. CALD1 plays a central role in the regulation of microfilament organization, consequently controlling cell shape, adhesion, cytokinesis and motility. K-cyclin.CDK6 specifically phosphorylates four Ser/Thr sites in the human CALD1 carboxyl terminus, abolishing CALD1 binding to its effector protein, actin, and its regulator protein, calmodulin. CALD1 is hyperphosphorylated in cells following K-cyclin expression and in KSHV-transformed lymphoma cells. Moreover, expression of exogenous K-cyclin results in microfilament loss and changes in cell morphology; both effects are reliant on CDK catalysis and can be reversed by the expression of a phosphorylation defective CALD1. Together, these data strongly suggest that K-cyclin expression modulates the activity of caldesmon and through this the microfilament functions in cells. These results establish a novel link between KSHV infection and the regulation of the actin cytoskeleton.

The role of caldesmon in the regulation of endothelial cytoskeleton and migration

The actin- and myosin-binding protein, caldesmon (CaD) is an essential component of the cytoskeleton in smooth muscle and non-muscle cells and is involved in the regulation of cell contractility, division, and assembly of actin filaments. CaD is abundantly present in endothelial cells (EC); however, the contribution of CaD in endothelial cytoskeletal arrangement is unclear. To examine this contribution, we generated expression constructs of l-CaD cloned from bovine endothelium. Wild-type CaD (WT-CaD) and truncated mutants lacking either the N-terminal myosin-binding site or the C-terminal domain 4b (containing actin- and calmodulin-binding sites) were transfected into human pulmonary artery EC. Cell fractionation experiments and an actin overlay assay demonstrated that deleting domain 4b, but not the N-terminal myosin-binding site, resulted in decreased affinity to both the detergent-insoluble cytoskeleton and soluble actin. Recombinant WT-CaD co-localized with acto-myosin filaments in vivo, but neither of CaD mutants did. Thus both domain 4b and the myosin-binding site are essential for proper localization of CaD in EC. Overexpression of WT-CaD led to cell rounding and formation of a thick peripheral subcortical actin rim in quiescent EC, which correlated with decreased cellular migration. Pharmacological inhibition of p38 MAPK, but not ERK MAPK, caused disassembly of this peripheral actin rim in CaD-transfected cells and decreased CaD phosphorylation at Ser531 (Ser789 in human h-CaD). These results suggest that CaD is critically involved in the regulation of the actin cytoskeleton and migration in EC, and that p38 MAPK-mediated CaD phosphorylation may be involved in endothelial cytoskeletal remodeling.

Modes of Caldesmon Binding to Actin

Smooth muscle caldesmon binds actin and inhibits actomyosin ATPase activity. Phosphorylation of caldesmon by extracellular signal-regulated kinase (ERK) reverses this inhibitory effect and weakens actin binding. To better understand this function, we have examined the phosphorylation-dependent contact sites of caldesmon on actin by low dose electron microscopy and three-dimensional reconstruction of actin filaments decorated with a C-terminal fragment, hH32K, of human caldesmon containing the principal actin-binding domains. Helical reconstruction of negatively stained filaments demonstrated that hH32K is located on the inner portion of actin subdomain 1, traversing its upper surface toward the C-terminal segment of actin, and forms a bridge to the neighboring actin monomer of the adjacent long pitch helical strand by connecting to its subdomain 3. Such lateral binding was supported by cross-linking experiments using a mutant isoform, which was capable of cross-linking actin subunits. Upon ERK phosphorylation, however, the mutant no longer cross-linked actin to polymers. Three-dimensional reconstruction of ERK-phosphorylated hH32K indeed indicated loss of the interstrand connectivity. These results, together with fluorescence quenching data, are consistent with a phosphorylation-dependent conformational change that moves the C-terminal end segment of caldesmon near the phosphorylation site but not the upstream region around Cys(595), away from F-actin, thus neutralizing its inhibitory effect on actomyosin interactions. The binding pattern of hH32K suggests a mechanism by which unphosphorylated, but not ERK-phosphorylated, caldesmon could stabilize actin filaments and resist F-actin severing or depolymerization in both smooth muscle and nonmuscle cells.

Alpha1-adrenergic signaling mechanisms in contraction of resistance arteries

Our goal in this review is to provide a comprehensive, integrated view of the numerous signaling pathways that are activated by alpha(1)-adrenoceptors and control actin-myosin interactions (i.e., crossbridge cycling and force generation) in mammalian arterial smooth muscle. These signaling pathways may be categorized broadly as leading either to thick (myosin) filament regulation or to thin (actin) filament regulation. Thick filament regulation encompasses both "Ca(2+) activation" and "Ca(2+)-sensitization" as it involves both activation of myosin light chain kinase (MLCK) by Ca(2+)-calmodulin and regulation of myosin light chain phosphatase (MLCP) activity. With respect to Ca(2+) activation, adrenergically induced Ca(2+) transients in individual smooth muscle cells of intact arteries are now being shown by high resolution imaging to be sarcoplasmic reticulum-dependent asynchronous propagating Ca(2+) waves. These waves differ from the spatially uniform increases in [Ca(2+)] previously assumed. Similarly, imaging during adrenergic activation has revealed the dynamic translocation, to membranes and other subcellular sites, of protein kinases (e.g., Ca(2+)-activated protein kinases, PKCs) that are involved in regulation of MLCP and thus in "Ca(2+) sensitization" of contraction. Thin filament regulation includes the possible disinhibition of actin-myosin interactions by phosphorylation of CaD, possibly by mitogen-activated protein (MAP) kinases that are also translocated during adrenergic activation. An hypothesis for the mechanisms of adrenergic activation of small arteries is advanced. This involves asynchronous Ca(2+) waves in individual SMC, synchronous Ca(2+) oscillations (at high levels of adrenergic activation), Ca(2+) sparks, "Ca(2+)-sensitization" by PKC and Rho-associated kinase (ROK), and thin filament mechanisms.

Twitchin from molluscan catch muscle: primary structure and relationship between site-specific phosphorylation and mechanical function

The phosphorylation state of the myosin thick filament-associated mini-titin, twitchin, regulates catch force maintenance in molluscan smooth muscle. The full-length cDNA for twitchin from the anterior byssus retractor muscle of the mussel Mytilus was obtained using PCR and 5'rapid amplification of cDNA ends, and its derived amino acid sequence showed a large molecule ( approximately 530 kDa) with a motif arrangement as follows: (Ig)11(IgFn2)2Ig(Fn)3Ig(Fn)2Ig(Fn)3(Ig)2(Fn)2(Ig)2 FnKinase(Ig)4. Other regions of note include a 79-residue sequence between Ig domains 6 and 7 (from the N terminus) in which more than 60% of the residues are Pro, Glu, Val, or Lys and between the 7th and 8th Ig domains, a DFRXXL motif similar to that thought to be necessary for high affinity binding of myosin light chain kinase to F-actin. Two major phosphorylation sites, i.e. D1 and D2, were located in linker regions between Ig domains 7 and 8 and Ig domains 21 and 22, respectively. Correlation of the phosphorylation state of twitchin, using antibodies specific to D1 and D2, with mechanical properties suggested that phosphorylation of both D1 and D2 is required for relaxation from the catch state.

Role of CaM kinase II and ERK activation in thrombin-induced endothelial cell barrier dysfunction

We have previously shown that thrombin-induced endothelial cell barrier dysfunction involves cytoskeletal rearrangement and contraction, and we have elucidated the important role of endothelial cell myosin light chain kinase and the actin- and myosin-binding protein caldesmon. We evaluated the contribution of calmodulin (CaM) kinase II and extracellular signal-regulated kinase (ERK) activation in thrombin-mediated bovine pulmonary artery endothelial cell contraction and barrier dysfunction. Similar to thrombin, infection with a constitutively active adenoviral alpha-CaM kinase II construct induced significant ERK activation, indicating that CaM kinase II activation lies upstream of ERK. Thrombin-induced ERK-dependent caldesmon phosphorylation (Ser789) was inhibited by either KN-93, a specific CaM kinase II inhibitor, or U0126, an inhibitor of MEK activation. Immunofluorescence microscopy studies revealed phosphocaldesmon colocalization within thrombin-induced actin stress fibers. Pretreatment with either U0126 or KN-93 attenuated thrombin-mediated cytoskeletal rearrangement and evoked declines in transendothelial electrical resistance while reversing thrombin-induced dissociation of myosin from nondenaturing caldesmon immunoprecipitates. These results strongly suggest the involvement of CaM kinase II and ERK activities in thrombin-mediated caldesmon phosphorylation and both contractile and barrier regulation.

Caldesmon exhibits a clustered distribution along individual chicken gizzard native thin filaments

Our earlier immuno-gold electron microscopic study indicated that the distribution of caldesmon (CaD) on actin filaments is not uniform and is restricted to the vicinity of the myosin filaments (Mabuchi K, Li Y, Tao T, Wang CLA (1996) J Muscle Res Cell Motil 17: 243). This suggested that CaD could effectively inhibit muscle contraction, if those actin filaments in the vicinity of myosin filaments were saturated with CaD. In the present study we further examined the distribution of CaD along isolated, crude and purified native thin filaments (NTF). Individual CaD molecules on purified NTF were visualized with the aid of a chemical crosslinker, 5,5'-dithiobis(2-nitrobenzoic acid), which efficiently crosslinks CaD to actin (Graceffa P, Adam LP, Lehman W (1993) Biochem J294: 63), and of a monoclonal anti-CaD antibody. The results indicated that individual NTF had alternating CaD-rich and CaD-deficient regions. Moreover, we found that the N-termini of all CaD molecules in a given cluster appeared on the same side of an actin filament. Electron microscopic images of crude NTF immunoprecipitated by a polyclonal antibody clearly indicated that the spacing between the CaD clusters is wide enough for myosin heads to interact with actin subunits. Similar clustering of CaD was also observed in plastic embedded tissue sections. These observations raise the possibility that CaD is not acting as a simple on/off switch, but more likely as a modulator, of smooth muscle contraction.

Invited review: cross-bridge regulation by thin filament-associated proteins

This minireview will cover current concepts on the identity and mechanistic function of smooth muscle actin binding proteins that may regulate actin-myosin interactions. The potential roles of tropomyosin, caldesmon, calponin, and SM22 will be discussed. The review, for purposes of brevity, will be nonexhaustive but will give an overview of available information on the in vitro biochemistry and potential in vivo function of these proteins. Preterm labor is discussed as a possible example of where thin filament regulation may be relevant. Considerable controversy surrounds the putative physiological significance of these proteins, and emphasis will be placed on the need for more experimental work to determine the degree to which tissue- and species-specific effects have clouded the interpretation of functional data.

Actomyosin cross-linking by caldesmon in non-muscle cells

The role of myosin-binding in cytoskeletal arrangement of non-muscle low molecular weight caldesmon (l-caldesmon) was studied. The N-terminal myosin-binding domain of caldesmon N152 colocalized with myosin in transiently transfected chicken fibroblasts. When added exogenously to the Triton-insoluble cytoskeleton, N152 enhanced l-caldesmon displacement by exogenous C-terminal actin-binding fragment (H1). Thus, a significant fraction of l-caldesmon cross-links actin and myosin. In contrast, in epithelioid HeLa cells most of l-caldesmon was only actin-bound as H1 alone was enough for its displacement. Phosphorylation by mitogen-activated protein kinase reduced the capability of H1 to displace endogenous l-caldesmon, suggesting it may represent a regulatory mechanism for actin-caldesmon interaction in vivo.