Tuning the endothelial response: differential release of exocytic cargos from Weibel-Palade bodies

Essentials Endothelial activation initiates multiple processes, including hemostasis and inflammation. The molecules that contribute to these processes are co-stored in secretory granules. How can the cells control release of granule content to allow differentiated responses? Selected agonists recruit an exocytosis-linked actin ring to boost release of a subset of cargo.

SUMMARY

Background Endothelial cells harbor specialized storage organelles, Weibel-Palade bodies (WPBs). Exocytosis of WPB content into the vascular lumen initiates primary hemostasis, mediated by von Willebrand factor (VWF), and inflammation, mediated by several proteins including P-selectin. During full fusion, secretion of this large hemostatic protein and smaller pro-inflammatory proteins are thought to be inextricably linked. Objective To determine if secretagogue-dependent differential release of WPB cargo occurs, and whether this is mediated by the formation of an actomyosin ring during exocytosis. Methods We used VWF string analysis, leukocyte rolling assays, ELISA, spinning disk confocal microscopy, high-throughput confocal microscopy and inhibitor and siRNA treatments to demonstrate the existence of cellular machinery that allows differential release of WPB cargo proteins. Results Inhibition of the actomyosin ring differentially effects two processes regulated by WPB exocytosis; it perturbs VWF string formation but has no effect on leukocyte rolling. The efficiency of ring recruitment correlates with VWF release; the ratio of release of VWF to small cargoes decreases when ring recruitment is inhibited. The recruitment of the actin ring is time dependent (fusion events occurring directly after stimulation are less likely to initiate hemostasis than later events) and is activated by protein kinase C (PKC) isoforms. Conclusions Secretagogues differentially recruit the actomyosin ring, thus demonstrating one mechanism by which the prothrombotic effect of endothelial activation can be modulated. This potentially limits thrombosis whilst permitting a normal inflammatory response. These results have implications for the assessment of WPB fusion, cargo-content release and the treatment of patients with von Willebrand disease.

Advancing Edge Speeds of Epithelial Monolayers Depend on Their Initial Confining Geometry

Collective cell migrations are essential in several physiological processes and are driven by both chemical and mechanical cues. The roles of substrate stiffness and confinement on collective migrations have been investigated in recent years, however few studies have addressed how geometric shapes influence collective cell migrations. Here, we address the hypothesis that the relative position of a cell within the confinement influences its motility. Monolayers of two types of epithelial cells--MCF7, a breast epithelial cancer cell line, and MDCK, a control epithelial cell line--were confined within circular, square, and cross-shaped stencils and their migration velocities were quantified upon release of the constraint using particle image velocimetry. The choice of stencil geometry allowed us to investigate individual cell motility within convex, straight and concave boundaries. Cells located in sharp, convex boundaries migrated at slower rates than those in concave or straight edges in both cell types. The overall cluster migration occurred in three phases: an initial linear increase with time, followed by a plateau region and a subsequent decrease in cluster speeds. An acto-myosin contractile ring, present in the MDCK but absent in MCF7 monolayer, was a prominent feature in the emergence of leader cells from the MDCK clusters which occurred every ~125 μm from the vertex of the cross. Further, coordinated cell movements displayed vorticity patterns in MDCK which were absent in MCF7 clusters. We also used cytoskeletal inhibitors to show the importance of acto-myosin bounding cables in collective migrations through translation of local movements to create long range coordinated movements and the creation of leader cells within ensembles. To our knowledge, this is the first demonstration of how bounding shapes influence long-term migratory behaviours of epithelial cell monolayers. These results are important for tissue engineering and may also enhance our understanding of cell movements during developmental patterning and cancer metastasis.

Differential effect of actomyosin relaxation on the dynamic properties of focal adhesion proteins

Treatment of cultured cells with inhibitors of actomyosin contractility induces rapid deterioration of stress fibers, and disassembly of the associated focal adhesions (FAs). In this study, we show that treatment with the Rho kinase inhibitor Y-27632, which blocks actomyosin contractility, induces disarray in the FA-associated actin bundles, followed by the differential dissociation of eight FA components from the adhesion sites. Live-cell microscopy indicated that the drug triggers rapid dissociation of VASP and zyxin from FAs (τ values of 7-8 min), followed by talin, paxillin and ILK (τ ~16 min), and then by FAK, vinculin and kindlin-2 (τ = 25-28 min). Examination of the molecular kinetics of the various FA constituents, using Fluorescence Recovery After Photobleaching (FRAP), in the absence of or following short-term treatment with the drug, revealed major changes in the k_on and k_off values of the different proteins tested, which are in close agreement with their differential dissociation rates from the adhesion sites. These findings indicate that mechanical, actomyosin-generated forces differentially regulate the molecular kinetics of individual FA-associated molecules, and thereby modulate FA composition and stability.

[Thin filaments of molluscan catch muscles contain a calponin-like protein]

A novel 40 kDa protein was detected in native thin filaments from catch muscles of the mussel Crenomytilus grayanus. The MALDY-TOF analysis of the protein showed a 40% homology with the calponin-like protein from the muscle of Mytilus galloprovincialis (45 kDa), which has a 36% homology with smooth muscle calponin from chicken gizzard (34 kDa). The amount of the calponin-like protein in thin filaments depends on isolation conditions and varies from the complete absence to the presence in amounts comparable with that of tropomyosin. The most significant factor that determines the contact of the protein in thin filaments is the temperature of solution in which thin filaments are sedimented by ultracentrifugation during isolation. At 22 degrees C and optimal values of both pH and ionic strength of the extraction solution, total calponin-like protein coprecipitates with thin filaments. At 2 degrees C it remains in the supernatant. The 40 kDa calponin-like protein from the mussel Crenomytilus grayanus has similar properties with smooth muscle calponin (34 kDa). It is thermostable and inhibits the actin-activated Mg -ATPase activity of actomyosin. In addition, the 40 kDa calponin-like protein isolated without using thermal treatment contains endogenous kinases. It was found that the calponin-like protein can be phosphorylated by endogenous kinases in the Ca -independent manner. These results indicate that the calponin-like protein from the catch muscle of the mussel Crenomytilus grayanus is a new member of the calponin family. The role of proteins from this family both in muscle and ponmuscle cells is still obscure. We suggest that the calponin-like protein is involved in the Ca -independent regulation of smooth muscle contraction.

Attenuation of actinomyosinII contractile activity in growth cones accelerates filopodia-guided and microtubule-based neurite elongation

The myosinII-specific inhibitor blebbistatin was used to attenuate actinomyosinII contractility in E7-chicken retina explant, medulla and spinal cord neuronal cell cultures. Addition of 20-100 microM blebbistatin, a concentration range that reversibly disrupts actin stress fibers, led to a reduction of growth cone lamellipodial areas and to an elongation of filopodia within 5 to 10 min. These morphological changes were completely reversed after removing the inhibitor. In the continued presence of blebbistatin for several hours, a dose-dependent acceleration (up to 6-fold) of neurite outgrowth was observed. The rapidly elongating neuritic processes displayed narrowed growth cones with one to three long filopodia at the leading edge. At the same time, thin neuritic branches emerged in a "push"-like fashion guided by filopodial extensions. Immunocytochemical characterization of these thin sprouts revealed that they contained actin filaments, myosinIIA, phosphorylated neurofilament/tau epitopes, MAP2, NCAM-PSA, and microtubules, demonstrating that these processes presented neurites and not filopodia. The crucial involvement of microtubules in blebbistatin-induced accelerated neurite extension was confirmed by its inhibition in the presence of nocodazole or taxol. The promotion by blebbistatin of neurite outgrowth occurred on polylysine, laminin, as well as on fibronectin as substrate. The presence of the Rho/ROCK-inhibitor Y-27632 also caused a dose-dependent promotion of neurite growth which was, however, 3-fold less pronounced as compared to blebbistatin. In contrast to blebbistatin, Y-27632 led to the enlargement of growth cone lamellipodial extensions. Our data demonstrate that neurite outgrowth and branching are inversely correlated with the degree of actinomyosinII contractility which determines the speed of retrograde flow and turnover of actin filaments and, by this, microtubule extension.

Effect of an angiotensin II receptor blocker and two angiotensin converting enzyme inhibitors on transforming growth factor-beta (TGF-beta) and alpha-actomyosin (alpha SMA), important mediators of radiation-induced pneumopathy and lung fibrosis

Progressive, irreversible fibrosis is one of the most clinically significant consequences of ionizing radiation on normal tissue. When applied to lungs, it leads to a complication described as idiopathic pneumonia syndrome (IPS) and eventually to organ fibrosis. For its high mortality, the condition precludes treatment with high doses of radiation. There is widespread interest to understand the pathogenetic mechanisms of IPS and to find drugs effective in the prevention of its development. This report summarizes our experience with the protective effects of L 158,809, an angiotensin II (ANG II) receptor blocker, and two angiotensin converting enzyme (ACE) inhibitors in the development of IPS and the role of transforming growth factor beta (TGF-beta) and of alpha-actomyosin (alpha SMA) in pathogenesis of radiation induced pulmonary fibrosis in an experimental model of bone marrow transplant (BMT). Male WAG/Riji/MCV rats received total body irradiation and a regimen of cyclophosphamide (CTX) in preparation for bone marrow transplant. While one group of animals remained untreated, the remainders were subdivided into three groups, each of them receiving either the ANG II receptor blocker or one of the two ACE inhibitors (Captopril or Enalapril). Each of the three drugs was administered orally from 11 days before the transplant up to 56 days post transplant. At sacrifice time the irradiated rats receiving only CTX showed a chronic pneumonitis with septal fibrosis and vasculitis affecting, in particular, small caliber pulmonary arteries and arterioles. Their lung content of hydroxyproline was also markedly elevated in association with the lung concentrations of thromboxane (TXA2) and prostaglandin (PGI(2)), (two markers of pulmonary endothelial damage). A significant increase of alpha actomyosin staining was observed in vessels, septa and macrophages of the same animals which also overexpressed TGF-beta. When L 158,809, Captopril and Enalapril were added to the radiation and cytoxan treatment, a significant amelioration of the histological damage as well as the overexpression of alpha SMA was observed. Lung concentrations of hydroxyproline, PGI(2), TXA2 and TGF-beta were also observed in these animals so that the values of these compounds were closer to those measured in untreated control rats than to their irradiated and cytoxan treated counterparts. Angiotensin II plays an important role in the regulation of TGF-beta and alpha SMA, two proteins involved in the pathogenesis of pulmonary fibrosis. The finding that ACE inhibitors or ANG II receptor blockers protect the lungs from radiation induced pneumonitis and fibrosis reaffirms the role that ANG II plays in this inflammatory process and suggests an additional indication of treatment of this condition, thus opening a new potential pharmacologic use of these drugs.

Actomyosin-dependent microtubule rearrangement in lysophosphatidic acid-induced neurite remodeling of young cortical neurons

It has been shown that lysophosphatidic acid (LPA), a signaling phospholipid, induces neurite retraction and the formation of retraction fibers in young cortical neurons by actin rearrangement. This study examined the rearrangement of microtubules (MTs) during LPA-induced neurite remodeling by immunostaining with antibodies against several types of tubulin. The results showed that alpha-tubulin was present in growing neurites as well as in cell bodies with various localization profiles. Exposure of neurons to LPA resulted in neurite retraction, accompanied by the rearrangement of MTs in neurites and the accumulation of MTs in cell bodies, without significant changes in the total amount of MTs in the cytoskeletal fraction of cultured neurons. Similar findings were obtained when young neurons were stained for other types of tubulin, including beta-tubulin type III and posttranslationally acetylated and tyrosinated tubulin. LPA-induced MT rearrangement was accompanied by accumulation of myosin IIB and polymerized actin at the base of retraction fibers. These effects of LPA on MTs and myosin IIB were blocked by pretreatment with inhibitors of the actomyosin and Rho pathways (cytochalasin D, blebbistatin, and Y27632), but not by an MT stabilizer (taxol), whereas taxol inhibited neurite retraction and MT depolymerization induced by nocodazole. Furthermore, neurofilaments also showed rearrangement in response to LPA, which was blocked by cytochalasin D and Y27632, but not taxol. Taken together, these results suggested that LPA did not induce MT depolymerization and that LPA-induced actomyosin activation produced MT and neurofilament rearrangement, leading to neurite remodeling.

Localization of myosin II to chromosome arms and spindle fibers in PtK1 cells: a possible role for an actomyosin system in mitosis

The enzymes of importance in moving chromosomes are called motor proteins and include dynein, kinesin, and possibly myosin II. These three molecules are all included in the category of ATPases, in that they have the ability to convert chemical energy into mechanical energy. Both dynein and kinesin have been documented as molecules that "walk" along microtubules in the mitotic spindle, carrying cargo such as chromosomes. Myosin II, analogous to the muscle contraction system, transiently interacts along actin filaments and associates with kinetochore microtubules. In this paper we present evidence that a third ATPase, myosin II, may act as a "thruster" to propel chromosomes during the mitotic process. Double-label immunocytochemistry to actin and myosin II shows that myosin II is localized on chromosome arms at the beginning of mitosis and remains localized to the chromosomes throughout mitosis. Specific staining of myosin II is relegated to the outside of chromosomes with the highest density of staining occurring between the spindle poles and the chromosomes. This specific localization could account for the movement of chromosomes during mitosis, since they segregate towards the spindle poles, along kinetochore microtubules containing actin filaments, after aligning at the equatorial region of the cell at metaphase. We conclude from this study that there is an actomyosin system present in the mitotic spindle and that myosin is attached to chromosome arms and may act as a thruster in moving chromosomes during the mitotic process.

Caldesmon binding to actin is regulated by calmodulin and phosphorylation via different mechanisms

Smooth muscle caldesmon (CaD) binds F-actin and inhibits actomyosin ATPase activity. The inhibition is reversed by Ca2+/calmodulin (CaM). CaD is also phosphorylated upon stimulation at sites specific for mitogen-activated protein kinases (MAPKs). Because of these properties, CaD is thought to be involved in the regulation of smooth muscle contraction. The molecular mechanism of the reversal of inhibition is not well understood. We have expressed His6-tagged fragments containing the sequence of the C-terminal region of human (from M563 to V793) and chicken (from M563 to P771) CaD as well as a variant of the chicken isoform with a Q766C point mutation. By cleavages with proteases, followed by high-speed cosedimentation with F-actin and mass spectrometry, we found that within the C-terminal region of CaD there are multiple actin contact points forming two clusters. Intramolecular fluorescence resonance energy transfer between probes attached to cysteine residues (the endogenous C595 and the engineered C766) located in these two clusters revealed that the C-terminal region of CaD is elongated, but it becomes more compact when bound to actin. Binding of CaM restores the elongated conformation and facilitates dissociation of the C-terminal CaD fragment from F-actin. When the CaD fragment was phosphorylated with a MAPK, only one of the two actin-binding clusters dissociated from F-actin, whereas the other remained bound. Taken together, these results demonstrate that while both Ca2+/CaM and MAPK phosphorylation govern CaD's function via a conformational change, the regulatory mechanisms are different.

Actin cross-linking and inhibition of the actomyosin motor

Intrastrand cross-linking of actin filaments by ANP, N-(4-azido-2-nitrophenyl) putrescine, between Gln-41 in subdomain 2 and Cys-374 at the C-terminus, was shown to inhibit force generation with myosin in the in vitro motility assays [Kim et al. (1998) Biochemistry 37, 17801-17809]. To clarify the immobilization of which of these two sites inhibits the actomyosin motor, the properties of actins with partially overlapping cross-linked sites were examined. pPDM (N,N'-p-phenylenedimaleimide) and ABP [N-(4-azidobenzoyl) putrescine] were used to obtain actin filaments cross-linked ( approximately 50%) between Cys-374 and Lys-191 (interstrand) and Gln-41 and Lys-113 (intrastrand), respectively. ANP, ABP, and pPDM cross-linked filaments showed similar inhibition of their sliding speeds and force generation with myosin ( approximately 25%) in the in vitro motility assays. In analogy to ANP cross-linking of actin, pPDM and ABP cross-linkings did not change the strong S1 binding to actin and the V(max) and K(m) parameters of actomyosin ATPase. The similar effects of these three cross-linkings reveal the tight coupling between structural elements of the subdomain 2/subdomain 1 interface and show the importance of its dynamic flexibility to force generation with myosin. The possibility that actin cross-linkings inhibit rate-limiting steps in motion and force generation during myosin cross-bridge cycle was tested in stopped-flow experiments. Measurements of the rates of mantADP release from actoS1 and ATP-induced dissociation of actoS1 did not reveal any differences between un-cross-linked and ANP cross-linked actin in these complexes. These findings are discussed in terms of the uncoupling between force generation and other aspects of actomyosin interactions due to a constrained dynamic flexibility of the subdomain 2/subdomain 1 interface in cross-linked actin filaments.

Process formation results from the imbalance between motor-mediated forces

Several reports have suggested that neurite outgrowth is mediated by opposing forces generated on microtubules and microfilaments but the molecular basis underlying these forces have not been determined. Here, we show that in non-neuronal cell lines, the inhibition of actomyosin activity by acidic calponin promotes the formation of processes. This effect is blocked by inhibition of the motor activity of cytoplasmic dynein. Therefore, neurite formation is due to an imbalance between tensile and compressive forces mediated by myosins and dyneins, respectively. We propose a mechanism that involves the motor-mediated forces in a tight regulation of the process formation.

Evidence that actin and myosin are involved in the poleward flux of tubulin in metaphase kinetochore microtubules of crane-fly spermatocytes

We studied the effects of various drugs on the poleward flux of tubulin in kinetochore microtubules in metaphase-I crane-fly spermatocytes. We used as a measure of tubulin flux a ‘gap’ in acetylation of kinetochore microtubules immediately poleward from the kinetochore; the ‘gap’ is caused by a time lag between incorporation of new tubulin subunits at the kinetochore and subsequent acetylation of those subunits as they flux to the pole. We confirmed that the ‘gap’ is due to flux by showing that the ‘gap’ disappeared when cells were treated briefly with the anti-tubulin drug nocodazole, which decreases microtubule dynamics. The ‘gap’ disappeared when cells were treated for 10 minutes with anti-actin drugs (cytochalasin D, latrunculin B, swinholide A), or with the anti-myosin drug 2,3-butanedione 2-monoxime. The ‘gap’ did not disappear when cells were treated with the actin stabilizing drug jasplakinolide. We studied whether these drugs altered spindle actin. We used fluorescent phalloidin to visualize spermatocyte F-actin, which was associated with kinetochore spindle fibers as well as the cell cortex, the contractile ring and finger-like protrusions at the poles. Spindle F-actin was no longer seen after cells were treated with cytochalasin D, swinholide A or a high concentration of latrunculin B, whereas a low concentration of latrunculin B, which did not completely remove the ‘gap’, caused reduced staining of spindle actin. Neither 2,3-butanedione 2-monoxime nor jasplakinolide altered spindle actin. These data suggest that an actomyosin mechanism drives the metaphase poleward tubulin flux.

Phosphorylation of caldesmon by p21-activated kinase. Implications for the Ca(2+) sensitivity of smooth muscle contraction

We have previously shown that p21-activated kinase, PAK, induces Ca(2+)-independent contraction of Triton-skinned smooth muscle with concomitant increase in phosphorylation of caldesmon and desmin but not myosin-regulatory light chain (Van Eyk, J. E., Arrell, D. K., Foster, D. B., Strauss, J. D., Heinonen, T. Y., Furmaniak-Kazmierczak, E., Cote, G. P., and Mak, A. S. (1998) J. Biol. Chem. 273, 23433-23439). In this study, we provide biochemical evidence implicating a role for PAK in Ca(2+)-independent contraction of smooth muscle via phosphorylation of caldesmon. Mass spectroscopy data show that stoichiometric phosphorylation occurs at Ser(657) and Ser(687) abutting the calmodulin-binding sites A and B of chicken gizzard caldesmon, respectively. Phosphorylation of Ser(657) and Ser(687) has an important functional impact on caldesmon. PAK-phosphorylation reduces binding of caldesmon to calmodulin by about 10-fold whereas binding of calmodulin to caldesmon partially inhibits PAK phosphorylation. Phosphorylated caldesmon displays a modest reduction in affinity for actin-tropomyosin but is significantly less effective in inhibiting actin-activated S1 ATPase activity in the presence of tropomyosin. We conclude that PAK-phosphorylation of caldesmon at the calmodulin-binding sites modulates caldesmon inhibition of actin-myosin ATPase activity and may, in concert with the actions of Rho-kinase, contribute to the regulation of Ca(2+) sensitivity of smooth muscle contraction.

Spontaneous oscillatory contraction without regulatory proteins in actin filament-reconstituted fibers

Skinned skeletal and cardiac muscle fibers exhibits spontaneous oscillatory contraction (SPOC) in the presence of MgATP, MgADP, and inorganic phosphate (Pi)1 but the molecular mechanism underlying this phenomenon is not yet clear. We have investigated the role of regulatory proteins in SPOC using cardiac muscle fibers of which the actin filaments had been reconstituted without tropomyosin and troponin, according to a previously reported method (Fujita et al., 1996. Biophys. J. 71:2307-2318). That is, thin filaments in glycerinated cardiac muscle fibers were selectively removed by treatment with gelsolin. Then, by adding exogenous actin to these thin filament-free cardiac muscle fibers under polymerizing conditions, actin filaments were reconstituted. The actin filament-reconstituted cardiac muscle fibers generated active tension in a Ca(2+)-insensitive manner because of the lack of regulatory proteins. Herein we have developed a new solvent condition under which SPOC occurs, even in actin filament-reconstituted fibers: the coexistence of 2,3-butanedione 2-monoxime (BDM), a reversible inhibitor of actomyosin interactions, with MgATP, MgADP and Pi. The role of BDM in the mechanism of SPOC in the actin filament-reconstituted fibers was analogous to that of the inhibitory function of the tropomyosin-troponin complex (-Ca2+) in the control fibers. The present results suggest that SPOC is a phenomenon that is intrinsic to the actomyosin motor itself.

Uncoupling of actomyosin adenosinetriphosphatase by heparin and its fragments

Heparin and its enzymatic fragments, prepared by degradation of heparin with heparinase from Flavobacterium heparinum, were capable of inhibiting the actomyosin-ATPase activity obtained from striated and smooth vascular muscles. Heparin did not inhibit the myosin-ATPase activity in absence of actin. The results show that heparin changes the step of ATP hydrolysis of the complex actomyosin-ATPase by uncoupling the conformational transition on the myosin-head induced by actin upon the nucleotide-binding site. This mechanism is cooperative and dependent on conformational states of actomyosin complex which in turn is regulated by ATP and calcium levels. It was observed that in the presence of ATP, actin does not compete with heparin for binding to myosin showing that heparin and actin have different binding sites on myosin. The binding of heparin and ATP is cooperative suggesting that the nucleotide binding leads to an exposition of a second heparin-binding site. However, in the absence of ATP, actin competes with heparin for a binding site on the myosin. These results strongly suggest that in the weakly binding state of actin to myosin, the binding of heparin is powerful and in the rigor state its binding is decreased.

Modulatory role of drebrin on the cytoskeleton within dendritic spines in the rat cerebral cortex

Morphological changes in the dendritic spines have been postulated to participate in the expression of synaptic plasticity. The cytoskeleton is likely to play a key role in regulating spine structure. Here we examine the molecular mechanisms responsible for the changes in spine morphology, focusing on drebrin, an actin-binding protein that is known to change the properties of actin filaments. We found that adult-type drebrin is localized in the dendritic spines of rat forebrain neurons, where it binds to the cytoskeleton. To identify the cytoskeletal proteins that associated with drebrin, we isolated drebrin-containing cytoskeletons using immunoprecipitation with a drebrin antibody. Drebrin, actin, myosin, and gelsolin were co-precipitated. We next examined the effect of drebrin on actomyosin interaction. In vitro, drebrin reduced the sliding velocity of actin filaments on immobilized myosin and inhibited the actin-activated ATPase activity of myosin. These results suggest that drebrin may modulate the actomyosin interaction within spines and may play a role in the structure-based plasticity of synapses.

The effects of smooth muscle calponin on the strong and weak myosin binding sites of F-actin

We have investigated the mechanism of inhibition of the actomyosin MgATPase by the smooth muscle protein calponin. We have shown previously the specific interaction of calponin with Glu334 of actin (EL-Mezgueldi, M., Fattoum, A., Derancourt, J., and Kassab, R. (1992) J. Biol. Chem. 267, 15943-15951). This residue is within the sequence 332-334, which has been proposed to be an important part of the strong myosin binding site (Rayment, I., Holden, H. M., Whittaker, M., Yohn, C. B., Lorenz, M., Holmes, K. C., and Milligan, R. A. (1993) Science 261, 58-65). Therefore, we suggested that calponin will affect the strong binding actin-myosin interaction. To test this hypothesis we have investigated the effect of calponin on the strong binding of S-1.MgAMP-PNP (5'-adenylyl imidodiphosphate) and on the weak binding of S-1.MgADP.Pi to actin. We found that an inhibitory concentration of calponin decreased the binding of S-1. MgAMP-PNP to actin but had no effect on the binding of S-1.MgADP.Pi. Similar results were obtained with skeletal muscle and smooth muscle S-1. In competition experiments calponin was found to displace S-1. MgAMP-PNP and S-1.MgADP but not S-1.MgADP.Pi from the actin filament. S-1 displaced calponin from actin in the rigor state, in the presence of MgADP, and in the presence of MgAMP-PNP. We conclude that calponin inhibits the actin activated S-1 ATPase by blocking a strong S-1 binding site on actin and does not block the weak binding site.

Mapping myosin-binding sites on actin probed by peptides that inhibit actomyosin interaction

A 2-kDa peptide (2K peptide) which was derived from the neck region of porcine aorta smooth muscle myosin heavy chain binds to actin competitively with skeletal myosin subfragment 1 (S1) in the absence of ATP and inhibits acto-S1 ATPase activity [Katoh, T. and Morita, F. (1993) J. Biol. Chem. 268, 2380-2388]. Using this and other peptides, myosin-binding sites on actin were mapped and their functions were studied. The 2K peptide inhibited the acto-S1 ATPase activity without inhibiting the binding of S1 to actin in the presence of ATP. On the other hand, the dansylated 2K peptide (DNS-2K peptide) inhibited not only the acto-S1 ATPase activity but also the binding of S1 to actin in the presence of ATP. Then, DNS-2K peptide was crosslinked to actin with 1-ethyl-3[3-(dimethylamino)propyl] carbodiimide. Amino acid composition and sequencing analyses of the fluorescent lysylendopeptidase-peptides of the crosslinked product indicated that DNS-2K peptide was crosslinked to acidic residues within residues 1-18 (Asp1, Glu2, Asp3, Glu4, and/or Asp11), 19-50 (Asp25), and 85-113 (Glu99 or Glu100) of actin. A competition experiment for the crosslinking with unlabeled 2K peptide showed that the crosslinking to residues 85-113 of actin was specific for DNS-2K peptide. In addition, isolated actin peptide 85-113 was found to show the competitive inhibition of actin-activated ATPase activity of S1 with respect to actin. These results suggest that the site within residues 1-28 of actin participates in the actin-activation of myosin ATPase activity, and the site within residues 85-113 of actin participates in the weak binding of myosin to actin in the presence of ATP.

Stimulation of radial expansion in arabidopsis roots by inhibitors of actomyosin and vesicle secretion but not by various inhibitors of metabolism

Plant morphogenesis depends on accurate control over growth anisotropy. To learn to what extent the control of growth anisotropy depends on cellular metabolism, we surveyed the response of growing roots to a range of inhibitors. Seedlings of Arabidopsis thaliana L. (Heynh), 7-8 d old, were transplanted onto plates containing an inhibitor, and elongation and radial expansion of roots were measured over the subsequent 2-d period. Fourteen inhibitors of diverse metabolic processes inhibited root elongation but failed to stimulate radial expansion. These inhibitors were aluminum sulfate, aphidicolin (DNA synthesis), caffeine (cell-plate formation), cisplatin (DNA synthesis), cycloheximide (protein synthesis), 3,4-dehydro-L-proline (proline hydroxylation), 6-dimethylaminopurine (protein kinases), dinitrophenol (mitochondrial ATP synthesis), galactose (UDP-glucose formation), Lovastatin, formerly mevinolin (isoprenoid formation), methionine sulfoximine (glutamine synthetase), methotrexate (folate metabolism), XRD-489 (synthesis of branched-chain amino acids), and high or low calcium treatments. These results show that various types of metabolic disruption, although inhibitory to elongation, do not reduce the high degree of anisotropic growth of the root. However, five chemicals did stimulate radial expansion; namely, the detergent, digitonin; two inhibitors of vesicle secretion, monensin and brefeldin A; and two inhibitors of actomyosin, cytochalasin B and butanedione monoxime. The maximum radial expansion induced by these compounds (except butanedione monoxime) was greater than that caused by ethylene, and the morphology of treated roots did not resemble that of roots treated with ethylene. These results indicate that vesicle secretion and actomyosin play a role in controlling anisotropic expansion.

Is phosphorylation the main physiological action of myosin light chain kinase?

The 155-kDa component of bovine stomach, which exhibits a strong actomyosin (AM) activating activity and a relatively weak myosin light chain kinase (MLCK) activity, has a strong affinity for the actin filament and the actin-binding site is confined to an 80 amino acid residue on its N-terminal side. This affinity may play a crucial role in AM activation. Some reagents preferentially abolish either the AM-activating effect or MLCK activity. In conclusion, MLCK of the 155-kDa component does not play a fundamental role in activating the AM system as far as the in vitro system is concerned. The possible mechanism of AM activation by the component is discussed.

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.

Protein kinase C-mediated phosphorylation of troponin I and C-protein in isolated myocardial cells is associated with inhibition of myofibrillar actomyosin MgATPase

Phosphorylation of cardiac myofibrillar proteins by protein kinase C (PKC) in isolated adult rat cardiomyocytes has been compared with that mediated by the cAMP-dependent protein kinase (PKA). PKA activation by beta-adrenoreceptor (isoproterenol) stimulation results in stoichiometric phosphorylation of troponin I (TnI) and C-protein. PKC activation by either 12-O-tetradecanoylphorbol-13-acetate (TPA) or by alpha-adrenoreceptor (phenylephrine plus propranolol) stimulation results in phosphorylation of the same two proteins to similar extents. Two-dimensional phosphopeptide mapping shows that the same sites in TnI are modified by PKC in vitro and in TPA- or alpha-agonist-stimulated cells. These sites are distinct from those phosphorylated in isoproterenol-stimulated cells or by PKA in vitro. Phosphopeptide mapping analysis of C-protein shows that PKC and PKA phosphorylate identical residues in this protein in vitro and in situ. TPA-stimulated phosphorylation in myocytes is associated with a reduction in maximal activity of myofibrillar Ca(2+)-dependent actomyosin MgATPase. Isoproterenol-stimulated phosphorylation has no effect on maximal activity but reduces the Ca2+ sensitivity of the MgATPase. These data demonstrate that TnI and C-protein are phosphorylated in myocardial cells by both PKA and PKC, resulting in different functional consequences in each case.

Product inhibition of the actomyosin subfragment-1 ATPase in skeletal, cardiac, and smooth muscle

We studied product inhibition of the actin-activated ATPase of myosin subfragment-1 (S-1) from the three types of muscle tissue: skeletal, cardiac, and smooth. Increasing levels of [MgADP] in the 0-1-mM range caused significant inhibition of the actin-activated MgATPase activity of cardiac and gizzard but not skeletal muscle S-1. When total nucleotide concentration ([ATP] + [ADP]) was kept constant at 1 mM, ATPase activity was inhibited by 50% at an ADP/ATP ratio of 6:1 for cardiac S-1 and 3:1 for gizzard S-1. For skeletal S-1, however, even a 19:1 ratio did not cause 50% inhibition of ATPase activity. The observed effect was not due to changes in pH or inorganic phosphate concentration, nor could it be explained by substrate (ATP) depletion. In the absence of actin, ADP had little or no inhibitory effect on the ATPase activity of S-1, and these observations imply that ADP is competing directly for the ATP binding site of the actin-S1 complexes of cardiac and smooth muscle S-1. ADP has previously been shown to be a weak competitive inhibitor of the ATPase activity in skeletal muscle. The current data imply that ADP is a very effective competitive inhibitor for the actin-activated ATPase activity of cardiac and gizzard S-1 and, therefore, that ADP may be a physiologically important modulator of contractile activity in cardiac and smooth muscle.

Effects of 2,3-butanedione monoxime on contraction of frog skeletal muscles: an X-ray diffraction study

We studied the effects of BDM (2,3-butanedione monoxime) on the tetanic contraction of frog skeletal muscles using an X-ray diffraction technique. BDM significantly increased the resting equatorial intensity ratio (I1,0/I1,1). In sartorius muscle, 3 mM BDM suppressed tetanic tension by 40-70% whereas the equatorial intensity ratio, which is 2.6 at rest, decreased to 0.75 during tetanus, close to the value in normal contraction (about 0.50). BDM (3 mM) reduced the intensity increase of the 5.1-nm layer-line to 41%, that of the 5.9-nm layer-line to 24%, and the intensity decrease of the second myosin meridional reflection (at 1/21.5 nm-1) at 81%. In overstretched semitendinosus muscle, 3 mM BDM did not significantly reduce the intensity increase of the second actin layer-line during activation, suggesting that enough calcium is released to activate the regulatory system and the regulatory proteins are intact. These results indicate that BDM suppresses tetanic tension by mainly inhibiting actin-myosin interaction. It has a smaller effect on the equatorial reflections and myosin layer-lines than on the actin layer-lines, suggesting that BDM-influenced myosin heads may bind to actin without following the symmetry of the actin helix.

Molecular charge dominates the inhibition of actomyosin in skinned muscle fibers by SH1 peptides

It is not definitively known whether the highly conserved region of myosin heavy chain around SH1 (Cys 707) is part of the actin-binding site. We tested this possibility by assaying for competitive inhibition of maximum Ca-activated force production of skinned muscle fibers by synthetic peptides which had sequences derived from the SH1 region of myosin. Force was inhibited by a heptapeptide (IRICRKG) with an apparent K0.5 of about 4 mM. Unloaded shortening velocity of fibers, determined by the slack test, and maximum Ca-activated myofibrillar MgATPase activity were also inhibited by this peptide, but both required higher concentrations. We found that other cationic peptides also inhibited force in a manner that depended on the charge of the peptide; increasing the net positive charge of the peptide increased its efficacy. The inhibition was not significantly affected by altering solution ionic strength (100-200 mM). Disulfide bond formation was not involved in the inhibitory mechanism because a peptide with Thr substituted for Cys was inhibitory in the presence or absence of DTT. Our data demonstrate that the net charge was the predominant molecular characteristic correlated with the ability of peptides from this region of myosin heavy chain to inhibit force production. Thus, the hypothesis that the SH1 region of myosin is an essential part of the force-producing interaction with actin during the cross-bridge cycle (Eto, M., R. Suzuki, F. Morita, H. Kuwayama, N. Nishi, and S. Tokura., 1990, J. Biochem. 108:499-504; Keane et al., 1990, Nature (Lond.). 344:265-268) is not supported.

Localization of the calmodulin- and the actin-binding sites of caldesmon

Expression of the C-terminal third of chicken gizzard caldesmon in Escherichia coli, using the Nagai vector (Nagai, K., and Thøgersen, H.V. (1987) Methods Enzmol. 153, 461-481), produces a cII-caldesmon fusion protein (27 kDa) with caldesmon sequence beginning at Lys579. Degradation during purification yields five peptides with molecular masses of 24, 22, 19 (two peptides), and 15 kDa. The 24-kDa peptide begins at Phe581; the 22-kDa peptide begins at Leu597, the two 19-kDa peptides begin at Phe581 and Val629, respectively; the 15-kDa peptide also begins at Val629. We estimate that the 15-kDa and one of the 19-kDa peptides end near Leu710. Site-directed mutagenesis was used to produce truncated peptides with known C termini; one peptide (17 kDa) terminates at Asn675. Digestion of the fragments with chymotrypsin generates a second 15-kDa fragment that begins at Ser666 (15K'). All of the peptides, with the exception of 15K', bind Ca(2+)-calmodulin-Sepharose and share a common 37-amino acid peptide between Val629 and Ser666, suggesting this contains the calmodulin binding site. Comparison with published sequences (Takagi, T., Yazawa, M., Ueno, T., Suzuki, S., and Yagi, K. (1989) J. Biochem. (Tokyo) 106, 778-783 and Bartegi, A., Fattoum, A., Derancourt, J., and Kassab, R. (1990) J. Biol. Chem. 265, 15231-15238) for other calmodulin-binding fragments further restricts the binding site to 7 residues, Trp-Glu-Lys-Gly-Asn-Val-Phe, between Trp659 and Ser666. All of the fragments, except the two 15-kDa peptides, co-sediment with F-actin, indicating that there are two segments in the C-terminal third of caldesmon that can interact with F-actin: one between Leu597 and Val629, the other between Arg711 and Pro756. Although separated in the primary sequence, these domains may interact with the calmodulin-binding region in the folded structure.

Smooth muscle calponin. Inhibition of actomyosin MgATPase and regulation by phosphorylation

Calponin isolated from chicken gizzard smooth muscle inhibits the actin-activated MgATPase activity of smooth muscle myosin in a reconstituted system composed of contractile and regulatory proteins. ATPase inhibition is not due to inhibition of myosin phosphorylation since, at calponin concentrations sufficient to cause maximal ATPase inhibition, myosin phosphorylation was unaffected. Furthermore, calponin inhibited the actin-activated MgATPase of fully phosphorylated or thiophosphorylated myosin. Although calponin is a Ca2(+)-binding protein, inhibition did not require Ca2+. Furthermore, although calponin also binds to tropomyosin, ATPase inhibition was not dependent on the presence of tropomyosin. Calponin was phosphorylated in vitro by protein kinase C and Ca2+/calmodulin-dependent protein kinase II, but not by cAMP- or cGMP-dependent protein kinases, or myosin light chain kinase. Phosphorylation of calponin by either kinase resulted in loss of its ability to inhibit the actomyosin ATPase. The phosphorylated protein retained calmodulin and tropomyosin binding capabilities, but actin binding was greatly reduced. The calponin-actin interaction, therefore, appears to be responsible for inhibition of the actomyosin ATPase. These observations suggest that calponin may be involved in regulating actin-myosin interaction and, therefore, the contractile state of smooth muscle. Calponin function in turn is regulated by Ca2(+)-dependent phosphorylation.

Tropomyosin-caldesmon/actomyosin systems in platelets and arterial smooth muscle: results from exchange experiments

Tropomyosin and caldesomon reciprocally control the actomyosin system in smooth muscle and some non-muscle cells. To compare this mechanism between arterial smooth muscle and platelets, we carried out extensive exchange experiments. Actin, myosin, tropomyosin from arterial smooth muscle cells and platelets were recombined and the effects of two species of caldesmon ('caldesmon77' and 'caldesmon140') on the ATPase activities of both systems were examined and analyzed by the method of analysis of variance. (a) The actomyosin system itself is different between artery and platelets, the difference being determined by myosin (P less than 0.05) and not by actin. (b) Platelet tropomyosin differentiates platelet actin from arterial actin (P less than 0.01), while arterial tropomyosin does not. Neither does tropomyosin differentiate myosin. (c) The effect of caldesmon77 differentiates the origins of myosin (P less than 0.01), actin (P less than 0.05) and tropomyosin (P less than 0.05). The effect of caldesmon140 differentiates the origin of myosin (P less than 0.05) and the actin-myosin 'interaction' (combination) (P less than 0.01), but not the origin of tropomyosin (P greater than 0.1). (1) It is concluded that actomyosin/tropomyosin-caldesmon system is distinguishable between platelets and artery. (2) It is suggested that caldesmon is an actomyosin inhibitor which may interact with myosin, in addition to actin and tropomyosin.

Physiological and ultrastructural analysis of elongating mitotic spindles reactivated in vitro

We have developed a simple procedure for isolating mitotic spindles from the diatom Stephanopyxis turris and have shown that they undergo anaphase spindle elongation in vitro upon addition of ATP. The isolated central spindle is a barrel-shaped structure with a prominent zone of microtubule overlap. After ATP addition greater than 75% of the spindle population undergoes distinct structural rearrangements: the spindles on average are longer and the two half-spindles are separated by a distinct gap traversed by only a small number of microtubules, the phase-dense material in the overlap zone is gone, and the peripheral microtubule arrays have depolymerized. At the ultrastructural level, we examined serial cross-sections of spindles after 1-, 5-, and 10-min incubations in reactivation medium. Microtubule depolymerization distal to the poles is confirmed by the increased number of incomplete, i.e., c-microtubule profiles specifically located in the region of overlap. After 10 min we see areas of reduced microtubule number which correspond to the gaps seen in the light microscope and an overall reduction in the number of half-spindle microtubules to about one-third the original number. The changes in spindle structure are highly specific for ATP, are dose-dependent, and do not occur with nonhydrolyzable nucleotide analogues. Spindle elongation and gap formation are blocked by 10 microM vanadate, equimolar mixtures of ATP and AMPPNP, and by sulfhydryl reagents. This process is not affected by nocodazole, erythro-9-[3-(2-hydroxynonyl)]adenine, cytochalasin D, and phalloidin. In the presence of taxol, the extent of spindle elongation is increased; however, distinct gaps still form between the two half-spindles. These results show that the response of isolated spindles to ATP is a complex process consisting of several discrete steps including initiation events, spindle elongation mechanochemistry, controlled central spindle microtubule plus-end depolymerization, and loss of peripheral microtubules. They also show that the microtubule overlap zone is an important site of ATP action and suggest that spindle elongation in vitro is best explained by a mechanism of microtubule-microtubule sliding. Spindle elongation in vitro cannot be accounted for by cytoplasmic forces pulling on the poles or by microtubule polymerization.

Pharmacologic levels of nitrendipine do not affect actin-myosin interaction in the human uterus and placenta

Because of the potential of dihydropyridine calcium channel blockers in the management of premature labor, we have studied the direct effects of nitrendipine on actomyosin in the pregnant and nonpregnant uterus and in the term human placenta. Actomyosin adenosinetriphosphatase in the three tissues and another model of actin-myosin interaction, superprecipitation of placental actomyosin, were inhibited by nitrendipine. The inhibition was not diminished by high concentrations of calcium. To identify the mechanism, placental myosin was phosphorylated in the absence and presence of 0.8 X 10(-4) mol/L of nitrendipine. The myosin phosphorylated in the presence of nitrendipine had lower actin-activated adenosinetriphosphatase, which is consistent with the inhibition of myosin light chain phosphorylation. However, nitrendipine did not affect the adenosinetriphosphatase activity of myosin nor did further reduce the adenosinetriphosphatase of the already phosphorylated placental actomyosin. Thus nitrendipine inhibition is directed to the phosphorylation reaction but not to the adenosinetriphosphatase site of myosin. Myometrial relaxation in vivo or in vitro occurs at the pharmacologic nitrendipine levels of 10(-9) to 10(-8) mol/L, which is at least 10,000 times lower than that of the concentration of 50% inhibition of myosin light chain phosphorylation (0.0026 +/- 0.00015 mol/L of nitrendipine, mean +/- SEM) demonstrated in the present work. Because of this difference, the direct intracellular actions of dihydropyridine calcium channel blockers are not expected to cause adverse effects in the uteroplacental system when these drugs are used in the prevention or treatment of premature labor.

6-Tridecylresorcylic acid, a novel ATPase inhibitor that blocks the contractile apparatus of skeletal muscle proteins

6-Tridecylresorcylic acid (TRA) isolated from a primula Lysimachia japonica Thunb. inhibited contraction of myofibrils, superprecipitation of myosin B, and ATPase activities of myosin and actomyosin prepared from rabbit skeletal muscle in a dose-dependent manner. The IC50 values in molarity of TRA were as follows: myosin (K+,EDTA)-ATPase, 3.5 X 10(-6); myosin Ca2+-ATPase, 3.5 X 10(-5); and actomyosin Mg2+-ATPase, 1.6 X 10(-5). The inhibition of ATPase activity of myofibrils by TRA was virtually reversed by washing with the fresh saline solution. Kinetic analysis of inhibitory effects of TRA suggests that the inhibition of (K+,EDTA)-ATPase activity of myosin or subfragment-1 is parabolic noncompetitive. TRA had no effect on alkaline phosphatase and choline acetyltransferase activities. TRA may provide a useful chemical tool for the study of the molecular mechanisms of actin-myosin contractile systems.

Burn toxin and its competitin

A "toxic factor' has been isolated and purified from scalded normal human skin; it is lethal to mice and toxic to HeLa and HEP2 cell lines in tissue culture. The toxic factor in both crude and purified form is antigenic in rabbits, producing an antisera that neutralizes the in vivo and in vitro effects of "burn toxin', similar to that of the convalescent sera of burned human subjects. The sonicate of the toxic glycoprotein named competitin is relatively non-lethal and protects against the lethal effect of the toxic factor. The action of the purified burn toxic factor and its competitin is at the ATP site of actomyosin preparations. The data presented suggests that the purified burn toxic factor and its competitin compete for the same receptor sites in the myocardium. A thesis is presented that states that it is vital to neutralize the toxic effects of burn breakdown tissue products in severely burned subjects before the vicious cycle of depressed immunological function and malnutrition ensues. Competitin produced in vitro from toxic factor(s) generated scalded normal human skin is offered as a means of neutralizing the toxic factors(s).

Mechanism of action of troponin . tropomyosin. Inhibition of actomyosin ATPase activity without inhibition of myosin binding to actin

The regulation of vertebrate skeletal muscle contraction by the troponin . tropomyosin complex is generally thought to be the result of tropomyosin physically blocking the myosin binding site of actin in the absence of Ca2+. This mechanism was tested during steady state ATP hydrolysis by comparing the degree of association of myosin subfragment 1 (S-1) with the actin . troponin . tropomyosin complex in the absence and presence of Ca2+. Binding in the presence of ATP was determined by stopped flow absorbance measurements at 25 degrees C. Although the steady state ATPase rate was reduced 96% in the absence of Ca2+, the association constant of S-1 with regulated actin was virtually the same in the absence of Ca2+ (1.3 X 10(4) M-1) as in the presence of Ca2+ (2.3 X 10(4) M-1). The association constant of S-1 to regulated actin in the presence of Ca2+ was similar to the association constant of S-1 to unregulated actin. These results suggest that the troponin . tropomyosin complex does not inhibit the actin-activated ATPase activity by preventing the binding of S-1 . ATP or S-1 . ADP . Pi to actin; rather, it may act by blocking the release of Pi from the acto-S-1 . ADP . Pi complex.

Synthesis and biological activity of an icosapeptide analog of the actomyosin ATPase inhibitory region of troponin I

A 20-residue peptide analog of the actomyosin ATPase inhibitory region of rabbit skeletal troponin I (Tn-I) has been synthesized by the solid phase method. The analog exhibited biological activity similar to both Tn-I and a 21-residue cyanogen bromide fragment of Tn-I. At ionic strengths where the inhibition of the actomyosin ATPase due to tropomyosin alone is low, the synthetic peptide in the presence of tropomyosin inhibits 90% of the original ATPase activity. In the absence of tropomyosin, the inhibition due to the peptide is much reduced. In contrast, salmine, a basic protein also known to inhibit the actomyosin ATPase, shows less inhibition in the presence of tropomyosin than it does in its absence. Gel electrophoresis data showed that the enhancement of the analog's inhibition by tropomyosin may be related to the analog's promotion of tropomyosin binding to F-actin similar to that reported for Tn-I and that the reduction of salmine inhibition by tropomyosin may be due to the binding of salmine by tropomyosin. At ionic strengths where binding and inhibition of tropomyosin is significant, the analog enhanced inhibition in a manner similar to that reported for whole Tn-I.

Inhibition of actomyosin ATPase by high concentrations of 5-hydroxytryptamine. Possible basis of lesion in 5HT-induced experimental myopathy

The effect of 5-hydroxytryptamine (5HT) on the ATPase activity and sulphydryl group reactivity of mammalian skeletal muscle actomyosin has been studied. 5HT inhibited the Mg2+-activated but not the Ca2+-activated ATPase activity of actomyosin. It slightly activated myosin ATPase. The sulphydryl groups of actomyosin reacting with 5,5'-dithiobis-(2-nitrobenzoic acid) were blocked by concentrations of 5HT which inhibited the Mg2+-activated ATPase. The significance of the results are discussed in relation to the muscle lesions in the experimental myopathy induced by 5HT and imipramine.

Changes in the activity of sarcoplasmic reticulum fragments and actomyosin isolated from skeletal muscle of thyroxine-treated cats

1. Sarcoplasmic reticulum fragments (SRF) and actomyosin were isolated from skeletal muscle of cats treated either with thyroxine or with placebo tablets, for 5-16 months.2. Ca(2+) uptake and binding by SRF, measured in the presence and absence of oxalate respectively, were reduced by thyroxine treatment.3. Actomyosin from the thyroxine-treated animals underwent ATP-induced syneresis at a faster rate than did that from the controls.4. Syneresis of the actomyosin preparations from controls and treated animals was inhibited to the same extent by EGTA when rates of syneresis were made the same by adjustment of the KCl concentration in the assay media. In contrast, at any given KCl concentration, syneresis of actomyosin from thyroxine-treated animals was inhibited to a lesser extent by EGTA.5. At the end of the isolation procedure, the amount of Ca(2+) remaining in the actomyosin suspension was similar for both treated and control animals.6. It was concluded that the effect of thyroxine on skeletal muscle may be the result of an action on both the sarcoplasmic reticulum and the contractile protein complex.