Probing the phalloidin binding site of actin

Conserved actin machinery drives microtubule-independent motility and phagocytosis in Naegleria

Much of our understanding of actin-driven phenotypes in eukaryotes has come from the "yeast-to-human" opisthokont lineage and the related amoebozoa. Outside of these groups lies the genus Naegleria, which shared a common ancestor with humans >1 billion years ago and includes the "brain-eating amoeba." Unlike nearly all other known eukaryotic cells, Naegleria amoebae lack interphase microtubules; this suggests that actin alone drives phenotypes like cell crawling and phagocytosis. Naegleria therefore represents a powerful system to probe actin-driven functions in the absence of microtubules, yet surprisingly little is known about its actin cytoskeleton. Using genomic analysis, microscopy, and molecular perturbations, we show that Naegleria encodes conserved actin nucleators and builds Arp2/3-dependent lamellar protrusions. These protrusions correlate with the capacity to migrate and eat bacteria. Because human cells also use Arp2/3-dependent lamellar protrusions for motility and phagocytosis, this work supports an evolutionarily ancient origin for these processes and establishes Naegleria as a natural model system for studying microtubule-independent cytoskeletal phenotypes.

Solid-phase synthesis of a novel phalloidin analog with on-bead and off-bead actin-binding activity

Specific effectors of actin polymerization have found use as dynamic probes of cellular morphology that may be used to gauge cellular response to stimuli and drugs. Of various natural products that target actin, phalloidin is one of the most potent and selective inhibitors of actin depolymerization. Phalloidin and related members of the phallotoxin family are macrocyclic heptapeptides bearing a characteristic and rigidifying transannular tryptathionine bridge. Here we describe a solid-phase synthesis of a new phalloidin analog as a prototype for library development with the potential for on- and off-bead screening. To validate our method, we labelled the phalloidin derivative with a fluorescent dye which stained actin in CHO cells. Furthermore, a bioassay was developed allowing actin polymerization on beads carrying a phalloidin derivative.

Covalent and non-covalent chemical engineering of actin for biotechnological applications

The cytoskeletal filaments are self-assembled protein polymers with 8-25nm diameters and up to several tens of micrometres length. They have a range of pivotal roles in eukaryotic cells, including transportation of intracellular cargoes (primarily microtubules with dynein and kinesin motors) and cell motility (primarily actin and myosin) where muscle contraction is one example. For two decades, the cytoskeletal filaments and their associated motor systems have been explored for nanotechnological applications including miniaturized sensor systems and lab-on-a-chip devices. Several developments have also revolved around possible exploitation of the filaments alone without their motor partners. Efforts to use the cytoskeletal filaments for applications often require chemical or genetic engineering of the filaments such as specific conjugation with fluorophores, antibodies, oligonucleotides or various macromolecular complexes e.g. nanoparticles. Similar conjugation methods are also instrumental for a range of fundamental biophysical studies. Here we review methods for non-covalent and covalent chemical modifications of actin filaments with focus on critical advantages and challenges of different methods as well as critical steps in the conjugation procedures. We also review potential uses of the engineered actin filaments in nanotechnological applications and in some key fundamental studies of actin and myosin function. Finally, we consider possible future lines of investigation that may be addressed by applying chemical conjugation of actin in new ways.

The expression of microvesicles in the blood of patients with Graves' disease and its relationship to treatment

OBJECTIVE

Graves' disease (GD) is an autoimmune disease characterized by the presence of circulating autoantibodies against thyroid-stimulating hormone (TSH) receptor. Despite extensive research, the pathogenic mechanisms remain unclear. Immune responses associated with the disease may lead to cell activation/apoptosis and the release of microvesicles (MVs) into the circulation. MVs can display biological activities which may aggravate GD further. We studied immune mechanisms in GD by investigating the numbers and phenotype of circulating MVs in patients before and after antithyroid therapy with thiamazole.

PATIENTS AND MEASUREMENTS

Samples were obtained from 15 patients with GD in the acute phase of hyperthyroidism and following 17-26 months treatment and 14 healthy controls. MVs from platelets, endothelial cells and monocytes exposing inflammation/activation markers (P-selectin, CD40 ligand, E-selectin and HMGB1) and MVs containing nuclear molecules were measured with flow cytometry.

RESULTS

Patients had elevated baseline values of MVs (P < 0·001 for all types of MVs), while the levels decreased during thiamazole treatment (P < 0·05 for all types of MVs). The majority of MV populations remained, however, significantly higher in patients after treatment compared to levels in controls.

CONCLUSIONS

GD patients have elevated levels of MVs that carry molecules with potential biological activities. MVs are significantly reduced after antithyroid treatment with thiamazole but still higher compared to levels in healthy controls. Assessment of MV levels and pattern may therefore provide additional information on underlying immune disturbances not obtained by measurements of hormone levels alone.

Nrf2 protection against liver injury produced by various hepatotoxicants

To investigate the role of Nrf2 as a master defense against the hepatotoxicity produced by various chemicals, Nrf2-null, wild-type, Keap1-knock down (Keap1-Kd) and Keap1-hepatocyte knockout (Keap1-HKO) mice were used as a “graded Nrf2 activation” model. Mice were treated with 14 hepatotoxicants at appropriate doses, and blood and liver samples were collected thereafter (6 h to 7 days depending on the hepatotoxicant). Graded activation of Nrf2 offered a Nrf2-dependent protection against the hepatotoxicity produced by carbon tetrachloride, acetaminophen, microcystin, phalloidin, furosemide, cadmium, and lithocholic acid, as evidenced by serum alanine aminotransferase (ALT) activities and by histopathology. Nrf2 activation also offered moderate protection against liver injury produced by ethanol, arsenic, bromobenzene, and allyl alcohol but had no effects on the hepatotoxicity produced by D-galactosamine/endotoxin and the Fas ligand antibody Jo-2. Graded Nrf2 activation reduced the expression of inflammatory genes (MIP-2, mKC, IL-1β, IL-6, and TNFα), oxidative stress genes (Ho-1, Egr1), ER stress genes (Gadd45 and Gadd153), and genes encoding cell death (Noxa, Bax, Bad, and caspase3). Thus, this study demonstrates that Nrf2 prevents the liver from many, but not all, hepatotoxicants. The Nrf2-mediated protection is accompanied by induction of antioxidant genes, suppression of inflammatory responses, and attenuation of oxidative stress.

Evolutionarily divergent, unstable filamentous actin is essential for gliding motility in apicomplexan parasites

Apicomplexan parasites rely on a novel form of actin-based motility called gliding, which depends on parasite actin polymerization, to migrate through their hosts and invade cells. However, parasite actins are divergent both in sequence and function and only form short, unstable filaments in contrast to the stability of conventional actin filaments. The molecular basis for parasite actin filament instability and its relationship to gliding motility remain unresolved. We demonstrate that recombinant Toxoplasma (TgACTI) and Plasmodium (PfACTI and PfACTII) actins polymerized into very short filaments in vitro but were induced to form long, stable filaments by addition of equimolar levels of phalloidin. Parasite actins contain a conserved phalloidin-binding site as determined by molecular modeling and computational docking, yet vary in several residues that are predicted to impact filament stability. In particular, two residues were identified that form intermolecular contacts between different protomers in conventional actin filaments and these residues showed non-conservative differences in apicomplexan parasites. Substitution of divergent residues found in TgACTI with those from mammalian actin resulted in formation of longer, more stable filaments in vitro. Expression of these stabilized actins in T. gondii increased sensitivity to the actin-stabilizing compound jasplakinolide and disrupted normal gliding motility in the absence of treatment. These results identify the molecular basis for short, dynamic filaments in apicomplexan parasites and demonstrate that inherent instability of parasite actin filaments is a critical adaptation for gliding motility.

Cellular evaluation of synthesized insulin/transferrin bioconjugates for oral insulin delivery using intelligent complexation hydrogels

Insulin transport across the epithelial cell layer in the small intestine was studied using insulin/transferrin conjugates with and without the presence of P(MAA-g-EG) microparticles in contact with a co-culture of Caco-2/HT29-MTX cells. The insulin/transferrin conjugate was shown to increase transport relative to pure insulin by a factor of 7, achieving an apparent permeability of 37 x 10(9) cm . s(-1). The presence of P(MAA-g-EG) microparticles increased conjugate transport by a factor of 14 times relative to insulin, achieving an apparent permeability of 72.8 x 10(9) cm . s(-1). The presence of the microparticles in solution was found to improve conjugate transport by nearly 100% with little to no change in cell monolayer integrity.

Pulsed-laser creation and characterization of giant plasma membrane vesicles from cells

Femtosecond-pulsed laser irradiation was found to initiate giant plasma membrane vesicle (GPMV) formation on individual cells. Laser-induced GPMV formation resulted from intracellular cavitation and did not require the addition of chemical stressors to the cellular environment. The viscosity, structure, and contents of laser-induced GPMVs were measured with fluorescence microscopy and single-particle tracking. These GPMVs exhibit the following properties: (1) GPMVs grow fastest immediately after laser irradiation; (2) GPMVs contain barriers to free diffusion of incorporated fluorescent beads; (3) materials from both the cytoplasm and surrounding media flow into the growing GPMVs; (4) the GPMVs are surrounded by phospholipids, including phosphatidylserine; (5) F-actin is incorporated into the vesicles; and (6) caspase activity is not essential for GPMV formation. The effective viscosity of 65 nm polystyrene nanoparticles within GPMVs ranged from 32 to 434 cP. The nanoparticle diffusion was commonly affected by relatively large, macromolecular structures within the bleb.

Anti-inflammatory effects of a fluorovinyloxyacetamide compound KT-15087 in microglia cells

The microglial activation plays an important role in the progression of neurodegenerative diseases by secreting various proinflammatory cytokines and neurotoxic factors. Inhibition of microglial activation may alleviate neurodegenerative processes. To search for novel therapeutic agents against neuroinflammatory diseases, several fluorovinyloxyacetamide derivatives were screened for anti-inflammatory effects in lipopolysaccharide (LPS)-stimulated microglial cells. From cell-based screening, it was found that a novel synthetic compound KT-15087 markedly attenuated the production of nitric oxide (NO) and tumor necrosis factor (TNF)-alpha in microglial cells. KT-15087 also suppressed the gene expression of inducible nitric oxide synthase (iNOS), TNF-alpha and interleukin (IL)-1beta. The compound inhibited the nuclear translocation and DNA binding of NF-kappaB as well as the phosphorylation of p38 mitogen-activated protein kinases (MAPK) and c-jun N-terminal kinase (JNK). Moreover, KT-15087 showed a neuroprotective activity by reducing the cytotoxicity of LPS-stimulated microglia toward B35 neuroblastoma cells in the coculture. The neuroprotective activity of the compound was most effective when microglia were pretreated with the compound prior to LPS challenge. Taken collectively, KT-15087 has an anti-inflammatory activity in microglia, and might have a therapeutic potential for the treatment of neuroinflammatory diseases.

Metallization of surface-attached actin networks

Several biopolymers have been used as templates for nanowire construction however the metallization procedures typically occur in solution and do not address the issue of wire placement on a surface. Among the biopolymers, actin has a large array of regulatory handles and the potential for point-to-point self-assembly of nanocircuitry. As a step toward the use of actin in more sophisticated constructs, we report here on methods for the successful metallization of surface-attached actin networks.

Rho GTPases and hypoxia in pulmonary vascular endothelial cells

The pulmonary endothelium is a single-cell layer forming the inner lining of a vast network of arteries, veins, and capillaries in the lung. Its main function is to regulate the contractility of underlying vascular smooth muscle cells (SMCs), which determines vascular tone and allows adaptation of blood flow to oxygenative conditions. Low oxygen tension (hypoxia) causes vasoconstriction of pulmonary vasculature and, depending on the duration of hypoxia, this effect may be reversed by reoxygenation. The key role of the pulmonary endothelium in the regulation of vascular tone has focused considerable attention on the effects of hypoxia/reoxygenation on pulmonary endothelial barrier function. Hypoxia increases endothelial permeability, which is believed to promote vasoconstriction by facilitating the leakage of vasoactive agents from the blood to the underlying SMCs. Data show that Rho GTPases RhoA and Rac1 regulate pulmonary endothelial barrier function in response to changes in oxygen tension. This chapter describes methods to isolate and culture primary pulmonary artery endothelial cells, to measure changes in endothelial barrier function and reactive oxygen species production, and to study the role of Rho GTPases in endothelial responses to hypoxia and reoxygenation.

Caveolin-1alpha and -1beta perform nonredundant roles in early vertebrate development

Caveolins are integral membrane proteins that localize predominantly to lipid rafts, where they oligomerize to form flask-shaped organelles termed caveolae and play important roles in membrane trafficking, signal transduction, and other cellular processes. To investigate potential roles for caveolin-1 (cav-1) in development, cav-1alpha and -1beta cDNAs were functionally characterized in the zebrafish. Cav-1alpha and -1beta mRNAs exhibited overlapping but distinct expression patterns throughout embryogenesis. Targeted depletion of either Cav-1 isoform, using antisense morpholino oligomers, resulted in a substantial loss of caveolae and dramatic neural, eye, and somite defects by 12 hours after fertilization, the time at which mRNA levels of both isoforms substantially increased in wild-type animals. Morphant phenotypes were rescued by injection of homotypic (cav-1alpha/cav-1alpha) but not heterotypic (cav-1alpha/cav-1beta) zebrafish and human cav-1 cRNAs, revealing nonredundant and evolutionarily conserved functions for the individual Cav-1 isoforms. Mutation of a known Cav-1 phosphorylation site unique to Cav-1alpha (Y14-->F) resulted in a failure to rescue the cav-1alpha morphant phenotype, verifying an essential role for Cav-1alpha specifically and implicating this residue in early developmental functions. Cav-1alpha and -1beta morphants also exhibited disruption in the actin cytoskeleton. These results support important and previously unanticipated roles for the Caveolin-1 isoforms in vertebrate organogenesis.

Position and orientation of phalloidin in F-actin determined by X-ray fiber diffraction analysis

Knowledge of the phalloidin binding position in F-actin and the relevant understanding of the mechanism of F-actin stabilization would help to define the structural characteristics of the F-actin filament. To determine the position of bound phalloidin experimentally, x-ray fiber diffraction data were obtained from well-oriented sols of F-actin and the phalloidin-F-actin complex. The differences in the layer-line intensity distributions, which were clearly observed even at low resolution (8 A), produced well-resolved peaks corresponding to interphalloidin vectors in the cylindrically averaged difference-Patterson map, from which the radial binding position was determined to be approximately 10 A from the filament axis. Then, the azimuthal and axial positions were determined by single isomorphous replacement phasing and a cross-Patterson map in radial projection to be approximately 84 degrees and 0.5 A relative to the actin mass center. The refined position was close to the position found by prior researchers. The position of rhodamine attached to phalloidin in the rhodamine-phalloidin-F-actin complex was also determined, in which the conjugated Leu(OH)(7) residue was found to face the outside of the filament. The position and orientation of the bound phalloidin so determined explain the increase in the interactions between long-pitch strands of F-actin and would also account for the inhibition of phosphate release, which might also contribute to the F-actin stabilization. The method of analysis developed in this study is applicable for the determination of binding positions of other drugs, such as jasplakinolide and dolastatin 11.

3-D in vitro model of early skeletal muscle development

An understanding of the mechanical and mechano-molecular responses that occur during the differentiation of mouse C2C12 [corrected] myoblasts in 3-D culture is critical for understanding growth, which is important for progress towards producing a tissue-engineered muscle construct. We have established the main differences in force generation between skeletal myoblasts, dermal fibroblasts, and smooth muscle cells in a 3-D culture model in which cells contract a collagen gel construct. This model was developed to provide a reproducible 3-D muscle organoid in which differences in force generation could be measured, as the skeletal myoblasts fused to form myotubes within a collagen gel. Maintenance of the 3-D culture under sustained uni-axial tension, was found to promote fusion of myoblasts to form aligned multi-nucleate myotubes. Gene expression of both Insulin Like Growth Factor (IGF-1 Ea) and an isoform of IGF-1 Ea, Mechano-growth factor (IGF-1 Eb, also termed MGF), was monitored in this differentiating collagen construct over the time course of fusion and maturation (0-7 days). This identified a transient surge in both IGF-1 and MGF expression on day 3 of the developing construct. This peak of IGF-1 and MGF expression, just prior to differentiation, was consistent with the idea that IGF-1 stimulates differentiation through a Myogenin pathway [Florini et al., 1991: Mol. Endocrinol. 5:718-724]. MGF gene expression was increased 77-fold on day 3, compared to a 36-fold increase with IGF-1 on day 3. This indicates an important role for MGF in either differentiation or, more likely, a response to mechanical or tensional cues.

Conformational and dynamic differences between actin filaments polymerized from ATP- or ADP-actin monomers

Conformational and dynamic properties of actin filaments polymerized from ATP- or ADP-actin monomers were compared by using fluorescence spectroscopic methods. The fluorescence intensity of IAEDANS attached to the Cys(374) residue of actin was smaller in filaments from ADP-actin than in filaments from ATP-actin monomers, which reflected a nucleotide-induced conformational difference in subdomain 1 of the monomer. Radial coordinate calculations revealed that this conformational difference did not modify the distance of Cys(374) from the longitudinal filament axis. Temperature-dependent fluorescence resonance energy transfer measurements between donor and acceptor molecules on Cys(374) of neighboring actin protomers revealed that the inter-monomer flexibility of filaments assembled from ADP-actin monomers were substantially greater than the one of filaments from ATP-actin monomers. Flexibility was reduced by phalloidin in both types of filaments.

Synthesis of proto-oncogene proteins and cyclins depends on intact microfilaments

It is well established that microfilament disintegration by cytochalasin D (CD) as well as latrunculin (LAT)-A and LAT-B causes an inhibition of S phase entry of various nontransformed cell lines. Our experiments extended these observations to human embryonal diploid fibroblasts (Wi-38). To investigate the question whether this stop of DNA synthesis is due to a decline of the synthesis of proteins that are necessary for G1 progression and S phase entry, we examined the expression of two proto-oncogenes (c-fos, c-jun) and three cyclins (D1, E, A) after altering the microfilament system. Disintegration of microfilaments by CD, LAT-A, or LAT-B of asynchronously growing fibroblasts caused a strong dose-dependent and time-dependent inhibition of total protein synthesis. Expression of c-jun, cyclins D1, E, and A decreased by about the same percentage as total protein synthesis. The strong induction of total protein synthesis after reactivating serum-starved fibroblasts by adding fetal calf serum was suppressed, when CD or LAT-A were added to the culture medium during this reactivation process. While expression of cyclin E as well as cyclin A decreased by about the same percentage as total protein synthesis, cyclin D1 was more suppressed after microfilament disintegration. After reactivating growth-arrested Wi-38 fibroblasts, cultured in suspension for 12 h, by transferring them to a rigid substratum they could adhere to, total protein synthesis was strongly induced. Again alteration of microfilaments by CD suppressed that increase. The expression of cyclin D1 was slightly more suppressed than total protein synthesis after addition of CD during that reactivation process. Our results suggest that alteration of microfilaments causes a strong decline of total protein synthesis accompanied by a decrease of the expression of proteins that are required for G1 progression and S phase entry. The diminished presence of proteins that are important for cell cycle progression could explain the inhibition of DNA synthesis after microfilament disintegration by various drugs.

Influence of tightly bound Mg2+ and Ca2+, nucleotides, and phalloidin on the microsecond torsional flexibility of F-actin

To better understand the relationship between structure and molecular dynamics in F-actin, we have monitored the torsional flexibility of actin filaments as a function of the type of tightly bound divalent cation (Ca2+ or Mg2+) or nucleotide (ATP or ADP), the level of inorganic phosphate and analogues, KCl concentration, and the level of phalloidin. Torsional flexibility on the microsecond time scale was monitored by measuring the steady-state phosphorescence emission anisotropy (rFA) of the triplet probe erythrosin-5-iodoacetamide covalently bound to Cys-374 of skeletal muscle actin; extrapolations to an infinite actin concentration corrected the measured anisotropy values for the influence of variable amounts of rotationally mobile G-actin in solution. The type of tightly bound divalent cation modulated the torsional flexibility of F-actin polymerized in the presence of ATP; filaments with Mg2+ bound (rFA = 0.066) at the active site cleft were more flexible than those with Ca2+ bound (rFA = 0.083). Filaments prepared from G-actin in the presence of MgADP were more flexible (rFA = 0.051) than those polymerized with MgATP; the addition of exogenous inorganic phosphate or beryllium trifluoride to ADP filaments, however, decreased the filament flexibility (increased the anisotropy) to that seen in the presence of MgATP. While variations in KCl concentration from 0 to 150 mM did not modulate the torsional flexibility of the filament, the binding of phalloidin decreased the torsional flexibility of all filaments regardless of the type of cation or nucleotide bound at the active site. These results emphasize the dynamic malleability of the actin filament, the role of the cation-nucleotide complex in modulating the torsional flexibility, and suggest that the structural differences that have previously been seen in electron micrographs of actin filaments manifest themselves as differences in torsional flexibility of the filament.

Dethiophalloidin increases Ca2+ responsiveness of skinned cardiac muscle

Phalloidin, an F-actin stabilizing peptide, is known to enhance Ca2+ responsiveness in skinned cardiac muscle. Here we studied the effects of dethiophalloidin (DTPH), a phalloidin derivative which, binding much more weakly to F-actin, on skinned bovine left ventricle muscle. When added to activated skinned muscle, DTPH (15-80 micron), similarly to phalloidin, caused a rapid (within several minutes) enhancement of active force; the relative force enhancement by DTPH became greater as Ca2+ concentration was decreased. Unlike phalloidin, DTPH effects were reversible. Using a value of the force enhancement at 15 micron DTPH (76% of maximum), an apparent equilibrium constant for DTPH binding to myofilaments was estimated at about 5 micron. Force-pCa plots showed that DTPH (80 micron) brought about a 10% increase in the maximal Ca(2+)-activated force and a 0.34 pCa units increase in the Ca2+ sensitivity. Both changes are stronger than those caused by phalloidin in similar conditions (6% and 0.2 pCa units, respectively). As with phalloidin, DTPH did not change the value of the Hill coefficient in the fit tothe force-pCa curve. DTPH and phalloidin interacted as follows: (1) pre-treatment with phalloidin entirely prevented the response to DTPH, indicating the absence of any non-specific DTPH action; and (2) when added after DTPH, phalloidin decreased the force enhancement due to DTPH, reflecting a stronger effect of DTPH to increase force. In conclusion, the stabilization of F-actin structure is not a major factor in the mechanism by which phalloidin modifies contraction.

Bleb formation and F-actin distribution during mitosis and tumor necrosis factor-induced apoptosis

The murine cell line C3HA has been used extensively in studies of the cytopathology that accompanies TNF-induced cytolysis. This cell line undergoes an apoptic form of cell death characterized by plasma membrane blebbing and cytoplasmic boiling. Since plasma membrane blebs also appear on C3HA cells during mitosis, in this report we have compared these blebs with those that appear during apoptosis to determine whether they represent related structures. Our results reveal several differences. During mitosis, the blebs that appear are smaller and more heterogeneous in size than are those that appear during apoptosis. In addition, during mitosis bleb formation is preceded by the appearance of microvilli on the cell surface. No microvilli are observed during apoptosis. The staining pattern with rhodamine phalloidin also differed between mitotic and apoptic blebs, indicating a difference in their content of f-actin. The blebs that form during mitosis stained in a bright, uniform manner, suggesting an association with f-actin. In contrast, apoptic blebs were stained only at their base, the bleb itself being devoid of f-actin-associated staining. This difference may help explain why mitotic blebs are reintegrated into the cell surface, while the blebs that form during apoptosis are not.

Cross-link between cys 374 and cys 10 of actin abolishes polymerizability and allows study of the properties of the "F-actin monomer"

Actin cross-linked between cys 374 and cys 10 via a disulfide-containing bridge, c-A, is completely unpolymerizable even in the presence of phalloidin. Upon the addition of dithiothreitol, c-A polymerizes with high yield, indicating that denaturation due to the modification was almost absent. In the present study we show that cross-linked actin is a useful model for studying the properties of monomeric actin under polymerization conditions. Addition of salt, for example, produced fluorescence changes possibly reflecting conformational transitions but did not lead to the development of phalloidin binding capacity. Cross-linking of the two cysteine residues also caused a decrease in the nucleotide exchange rate by a factor of ca. 3, an effect that was fully reversed by the addition of KCl. Cross-linked actin inhibits DNase I to the same extent as G-actin and binds thymosin beta 4 and profilin as shown by cross-linking studies. Capping capacity for the barbed end of the filament was not observed, although it might have been expected from the fact that both ends of the cross-link are anchored to subdomain 1. Using the 61-FITC derivative of c-A we showed that c-A is able to bind to myosin S1 with a KD in the microM range. In agreement with this, c-A shows actomyosin ATPase activity with a Kapp comparable to that of F-actin, but a Vmax decreased by a factor of ca. 11. The c-A myosin S1 complex provides the hitherto smallest model of actomyosin, which appears promising for crystallization and X-ray analysis.

Thin filament activation by phalloidin in skinned cardiac muscle

Phalloidin binds very tightly and specifically to actin and brings about a marked stabilization of the F-actin filament. In this study the effects of phalloidin on force generation and Ca2+ sensitivity of skinned bovine ventricular muscle were investigated. At all free Ca2+ concentrations addition of phalloidin to activated fibers caused an enhancement of active force. At full Ca2+ activation the force increase was about 6% and the relative force enhancement became greater as the Ca2+ concentration was decreased. Force-pCa plots obtained with fibers pre-treated with phalloidin showed that phalloidin produced an approximately 0.2 pCa unit increase in Ca2+ sensitivity without significant changes in cooperativity of activation. These results suggest that interactions between G-actin subunits may play an important role in cardiac force development.

Activation of alpha-2-adrenoceptors results in an increase in F-actin formation in HIT-T15 pancreatic B-cells

1. Alpha-2-adrenoceptor agonists, such as noradrenaline, are potent inhibitors of insulin secretion, and it has been suggested that they control a late step in the pathway of exocytosis. We have investigated whether this could be related to a change in the extent of actin polymerization in the pancreatic B-cell, since actin microfilaments are implicated in regulating the access of secretory granules to the plasma membrane prior to exocytosis. 2. Cultured HIT-T15 pancreatic B-cells responded to noradrenaline with an increase in F-actin content, as judged by a rise in the fluorescence output after probing of the cells with phalloidin (a toxin which binds specifically to F-actin) conjugated to rhodamine. The response to noradrenaline was rapid, dose-dependent and sustained and could be reproduced by the highly selective alpha-2-agonist UK14,304. Examination of HIT-T15 cells by fluorescence microscopy after treatment with rhodamine-phalloidin, revealed a significant localization of F-actin immediately adjacent to the plasma membrane. The pattern of F-actin distribution in the cells was not altered dramatically by noradrenaline, although the intensity of staining close to the plasma membrane appeared to be slightly reduced. 3. The increase in F-actin content induced by noradrenaline and UK14,304 was inhibited significantly by the alpha-2-antagonist idazoxan but not by the alpha-1-selective antagonist prazosin. Pretreatment of HIT-T15 cells with pertussis toxin did not lead to any direct alteration in F-actin content, although the toxin significantly modified the responses induced by noradrenaline and UK14,304. In each case, cells incubated for 24 h with pertussis toxin responded to the alpha-2-agonist with an enhanced fluorescence output, indicating that F-actin levels had increased still further. This did not correlate with any gross change in the distribution of F-actin as judged by fluorescence microscopy. 4. The results demonstrate that alpha-2-adrenoceptors are coupled to control of actin polymerization in HIT-T15 cells. They suggest that regulation of F-actin formation could be a component of the mechanism by which alpha-2-agonists mediate inhibition of insulin secretion.

Actin and the actomyosin interface: a review

This review deals with the structure of the actin monomer, its assembly into filaments and the loci on F-actin involved in binding myosin. Two distinctly different arrangements of monomers have been suggested for actin filaments. One model proposed by Holmes et al. is well developed. It places the so-called 'large' domain close to the filament axis and the so-called 'small' domain out near the surface of the filament. A second, less-well developed, model proposed by Schutt et al. locates the 'small' domain close to the filament axis and they rotate the monomer so that 'bottom' of the 'large' domain is at the highest radius. We analyze the available evidence for the models of F-actin derived from X-ray diffraction, reconstructions from electron micrographs, fluorescence resonance energy transfer spectroscopy, chemical cross-linking, antibody probes, limited proteolysis, site-directed and natural mutations, nuclear magnetic resonance spectroscopy and other techniques. The result is an actin-centered view of the loci on actin which are probably involved in its interaction with the myosin 'head'. From these multiple contacts we speculate on the sequence of steps between the initial weak-binding state of S-1 to the actin filament through to the stable strong-binding state seen in the absence of free Mg-ATP, i.e., the rigor state.

Effect of phalloidin on the ATPase activity of striated muscle myofibrils

Phalloidin was shown to increase the ATPase activity and Ca2+ sensitivity of both bovine cardiac and rabbit psoas myofibrils when assayed in a solution containing 50 mM KCl, 100 mM MOPS (pH 7.0), 2 mM MgCl2, 1 mM ATP, 2 mM EGTA, and varying concentrations of Ca2+ (temperature 21-22 degrees C). The phalloidin effect in cardiac myofibrils developed over a time course of several minutes in the presence of 50 microM phalloidin. Relative increase of ATPase activity was maximal at pCa 8 and decreased with decrease in pCa. In cardiac myofibrils the increase was about 70% at pCa 8 and 20% at pCa 4 following 20-30 min pre-incubation with 2 microM or 50 microM phalloidin. The effect persisted after excess phalloidin was washed out. The increase in Ca2+ sensitivity was approximately 0.15 pCa units. For skeletal myofibrils treated with 2 microM phalloidin all changes were considerably less than those seen with cardiac myofibrils and the changes were even less when the myofibrils were exposed to 50 microM phalloidin. These results show that when specifically bound to actin, phalloidin can change the kinetic parameters of the cross-bridge cycle and may also alter the Ca2+ sensitivity of the contractile system. The effects of phalloidin seem to vary with muscle type.

Actin structure and function: roles in mitochondrial organization and morphogenesis in budding yeast and identification of the phalloidin-binding site

To further elucidate the functions of actin in budding yeast and to relate actin structure to specific roles and interactions in vivo, we determined the phenotypes caused by 13 charged-to-alanine mutations isolated previously in the single Saccharomyces cerevisiae actin gene. Defects in actin organization, morphogenesis, budding pattern, chitin deposition, septation, nuclear segregation, and mitochondrial organization were observed. In wild-type cells, mitochondria were found to be aligned along actin cables. Many of the amino acid substitutions that had the most severe effects on mitochondrial organization are located under the myosin "footprint" on the actin monomer, suggesting that actin-myosin interactions might underlie mitochondrial organization in yeast. In addition, one mutant (act1-129; R177A, D179A) produced an actin that assembled into cables and patches that could be visualized by anti-actin immunofluorescence in situ and that assembled into microfilaments of normal appearance in vitro as judged by electron microscopy but which could not be labeled by rhodamine-phalloidin in situ or in vitro. Rhodamine-phalloidin could label actin filaments assembled from all of the other mutant actins, including one (act1-119; R116A, E117A, K118A) that is altered at a residue (E117) that can be chemically cross-linked to phalloidin. The implication of residues R177 and/or D179 in phalloidin binding is in close agreement with a recently reported molecular model in which the phalloidin-binding site is proposed to be at the junction of two or three actin monomers in the filament.