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PubMed1998

Regulation of scallop myosin by calcium. Cooperativity and the "off" state.

Scallop subfragment 1 (S1) is an unregulated molecule; it differs from heavy meromyosin (HMM) and myosin in that it has no "off" state, although it contains the full complement of light chains and the triggering calcium binding site. S1 differs from myosin by lacking the head-rod junction and being single-headed. The contribution of the head-rod junction was evaluated by studying single-headed myosin. Isolated single-headed myosins show some regulation; their actin activated ATPase is stimulated about 3-fold by calcium. However, in contrast to HMM and myosin, the calcium dependence of ATPase activation of single-headed myosin is non-cooperative. The single ATP turnover rate of single-headed myosin in the absence of calcium is less than 30 seconds (our experimental resolution) compared to the approximately 5 minute turnover rate of myosin. HMM and myosin exhibit several cooperative features not shown by S1. Calcium binding becomes cooperative in the presence of nucleotide analogues in HMM and myosin, but not in S1. Nucleotide analogues are bound cooperatively by myosin and HMM in the absence of calcium; the introduction of calcium to the system reduces the affinity and abolishes the cooperative binding of nucleotide in the double headed molecules. Conversely, S1 shows normal binding curves for nucleotide analogues both in the presence and absence of calcium. Therefore, there is direct communication between the calcium binding sites and nucleotide binding sites in regulated molecules that is mediated by interaction between the two heads. .

Animals↗
PubMed1975

An ultrastructural study of the microfilaments in rat brain by means of heavy meromyosin labeling. I. The perikaryon, the dendrites and the axon.

Nervous tissue pieces from the caudate nucleus and the substantia nigra of the rat were incubated in cold glycerol solutions of decreasing concentrations and then transferred into standard phosphate buffer (pH 7.0) or into tris-K+-Mg++-Ca++ buffer (pH 7.9) containing HMM, prepared from rabbit skeletal muscle by tryptic digestion. As controls, pieces were immersed for an identical period in the same buffers (1) without HMM or (2) with HMM to which had been added 2.5 mM Na+ pyrophosphate or 5 mM ATP. In control neurons smooth-surfaced microfilaments, about 50 A in diameter, were observed. After reaction with HMM, the microfilaments were increased in number and density and in width to 180-200 A. A meshwork was formed. Arrowheads pointing in the same direction were spaced at regular intervals (300-350 A) among short segments of the surfaces of the microfilaments, depending upon the plane of section. More often, however, typical arrowheads were not observed, and the surfaces of the microfilaments were seen coated with polarized side-arms cross-bridging the spaces between adjacent elements at more or less regular intervals. When cross-sectioned, the microfilaments appeared as dense dots from which a material of lesser electron density radiated. Following incubation in HMM solutions containing Na+ pyrophosphate or ATP, no arrowhead structures were seen. Of particular interest was the structural relation of the actin-like filaments with occasional, tapered myosin-like filaments, and with the plasma membrane, which served as anchor points. Mitochondria and smooth ER membranes were observed to be attached to the actin-like filaments or enmeshed in the network. The microtubules, as well as most of the neurofilaments, were disrupted by the glycerination procedure at 4 degrees, and thus no precision about the structural relationship of the actin-like filaments with the latter elements could be added. The role of the actin-like filaments in the transport of material, by a mechanism of chemomechanical transduction, throughout the neuron from sites of synthesis to functional locations, and between several functional locations, is discussed.

Actins↗
PubMed1975

An ultrastructural study of the microfilaments in rat brain by means of E-PTA staining and heavy meromyosin labeling. II. The synapses.

To identify structures involved in the translocation of the synaptic vesicles towards the presynaptic membrane, an ultrastructural study has been undertaken by means of (1) the E-PTA stain and (2) the HMM-labeling procedure. Using serial sections of E-PTA stained nervous tissue, especially those made in transversal and tangential planes, the geometric order of the presynaptic grid and of its constituents has been described in detail. It consisted of dense projections having the shape of small truncated pyramids cut parallel to their hexagonal bases which rested on the electron-lucent presynaptic membrane. The dense projections were arranged at the points of equilateral triangles. Around each dense projection, six asymmetric hexagonal holes were seen to be arrayed in an hexagonal pattern, forming thus the presynaptic sieve. From the spiny tops of the dense projections, which appeared as specialized structures of the dense material coating the inner surface of the plasma membrane at the level of the synaptic cleft, fine filaments, 40--60 A in diameter, radiated and formed a three-dimensional meshwork pervading the presynaptic bag. The dense cytoplasmic coating delineating the plasma membrane served as anchor points for these microfilaments. Upon incubation with rabbit skeletal muscle HMM the microfilaments underwent specific structural changes, consisting of: (1) a striking increase in diameter; (2) the association of periodic and polarized substructures with their surfaces. The synaptic vesicles and mitochondria were seen to be attached to the numerous HMM-decorated filaments or enmeshed in the network formed by these filaments. The actin-like filaments were anchored to the plasma membrane at many points and to the presynaptic dense projections. Following incubation in the buffer alone or in buffer HMM solutions containing Na+ pyrophosphate or ATP, no arrowheaded structures were observed. Thus, a network consisting of actin-like filaments was demonstrated in the presynaptic bag. Of particular interest was the structural relation of the actin-like filaments with the occasional, tapered myosin-like filaments. The role of the presynaptic actin-like network in the transport of synaptic vesicles towards the presynaptic membrane by a mechanism of chemomechanical transduction is discussed. In the postsynaptic dendrite or dendritic spine, a filamentous network was observed to be attached to the subsynaptic web by means of the E-PTA stain and of the HMM-labeling procedure. The occurrence of an actin-like meshwork in the postsynaptic region is suggested to produce changes in the macromolecular configuration of the postsynaptic membrane by a "mechanoenzyme" system similar to that described in the mitochondrial membrane.

Actins↗
PubMed1992

Caldesmon binds to smooth muscle myosin and myosin rod and crosslinks thick filaments to actin filaments.

It is well established that caldesmon binds to actin (Kb = 10(7) - 10(-8) M-1) and to tropomyosin (Kb = 10(6) M-1) and that it is a potent inhibitor of actomyosin ATPase. Caldesmon can also bind tightly to myosin. We investigated the binding of smooth muscle and nonmuscle caldesmon isoforms (CDh and CDl respectively) to myosin using proteins from sheep aorta. Both caldesmon isoforms bind to myosin with indistinguishable affinity. The affinity is about 10(6) M-1 in low salt buffer, but is weakened by increasing [KCl] reaching 10(5) M-1 in 100 mM KCl. The stoichiometry of binding is about three caldesmon per myosin molecule. Stoichiometry and affinity are not dependent on whether myosin is phosphorylated nor on the presence of Mg2+ and ATP, provided the ionic strength is maintained constant. The caldesmon binding site of smooth muscle myosin is located in the S-2 region, consequently both HMM and myosin rod bind to caldesmon. Over a range of conditions myosin and myosin rod binding to caldesmon were indistinguishable. Skeletal muscle myosin has no caldesmon binding site. Smooth muscle myosin rods form side-polar filaments in low salt buffer in which the backbone packing of LMM into the filament shaft is clearly visible in negatively-stained electron microscopic images. Sometimes the S-2 portions can be seen 'frayed' from the filament shaft. When caldesmon is bound the filament shaft appears to be about 20% thicker and the frayed effect is dramatically increased; long filamentous 'whiskers' are often seen curving out from the filament shaft. Similar structures are observed with smooth muscle and with non-muscle caldesmon. Myosin also binds to caldesmon when it is incorporated into the thin filament; however, this interaction is qualitatively different. Measurements of smooth muscle HMM binding to native thin filaments in the presence of 3 mM MgATP shows there is a high affinity binding (Kb = 10(6) M-1) which is independent of [Ca2+] and of the level of myosin phosphorylation. The stoichiometry is one HMM molecule per actin monomer which is equivalent to up to 14 HMM bound at high affinity per caldesmon. Negatively stained electron microscopic images of the HMM.ADP.Pi-thin filament complex have failed to show any attachment of HMM to the thin filaments. When rod filaments are added to actin plus caldesmon or to native thin filaments the rod filaments are strongly associated with the actin filament bundles. The majority of rod filaments are lined up parallel and in close proximity to actin filaments.(ABSTRACT TRUNCATED AT 400 WORDS)

Actin Cytoskeleton↗
PubMed2000

Comparative study on three chemotherapeutic regimens for the treatment of advanced epithelial ovarian cancer.

To investigate the best first-line chemotherapy regimen for the treatment of advanced epithelial ovarian cancer (AEOC), the efficacy of three chemotherapy regimens for treatment of the patients with AEOC in our hospital during Jan. 1992-Jan. 1999 was retrospectively analyzed. The therapeutic effects were compared with the supplement of Melphalan + Hexamethylme (PAM + HMM), cisplatin + adriamycin + cyclophosphamide or isofamide (PAC) or cisplatin + cyclophosphamide or isofamide (PC), Taxol + cisplatin (TP) combined chemotherapy after cytoreductive surgery. The results showed that the overall effective rate of TP was significantly higher than that of PAM + HMM (P < 0.05); The complete remission rate of TP was significantly higher than that of PAM + HMM and PAC or PC (all P < 0.05); The 2-year survival rate free of tumor of TP was obviously higher than that of PAM + HMM and PAC or PC (all P < 0.05). It was concluded that the therapeutic effect of TP regimen in the treatment of AEOC was better than PAM + HMM and PAC or PC and TP regimen could be recommended currently as the preferred first-line one for the treatment of AEOC.

Adult↗
PubMed2003

Secreted protein prediction system combining CJ-SPHMM, TMHMM, and PSORT.

To increase the coverage of secreted protein prediction, we describe a combination strategy. Instead of using a single method, we combine Hidden Markov Model (HMM)-based methods CJ-SPHMM and TMHMM with PSORT in secreted protein prediction. CJ-SPHMM is an HMM-based signal peptide prediction method, while TMHMM is an HMM-based transmembrane (TM) protein prediction algorithm. With CJ-SPHMM and TMHMM, proteins with predicted signal peptide and without predicted TM regions are taken as putative secreted proteins. This HMM-based approach predicts secreted protein with Ac (Accuracy) at 0.82 and Cc (Correlation coefficient) at 0.75, which are similar to PSORT with Ac at 0.82 and Cc at 0.76. When we further complement the HMM-based method, i.e., CJ-SPHMM + TMHMM with PSORT in secreted protein prediction, the Ac value is increased to 0.86 and the Cc value is increased to 0.81. Taking this combination strategy to search putative secreted proteins from the International Protein Index (IPI) maintained at the European Bioinformatics Institute (EBI), we constructed a putative human secretome with 5235 proteins. The prediction system described here can also be applied to predicting secreted proteins from other vertebrate proteomes.

Computational Biology↗
PubMed1979

Hexamethylmelamine: an evaluation of its role in the treatment of ovarian cancer.

Hexamethylmelamine (HMM), NSC 13875, a synthetic agent structurally related to triethylenemelamine, has clinical antitumor activity and a role in the treatment of ovarian cancers of epithelial origin. Fifty-four patients, with International Federation of Gynecology and Obstetrics Stage III or IV carcinomas, previously untreated with chemotherapy or irradiation therapy, were treated with HMM (8 mg/kg/day) as a single agent at the M. D. Anderson Hospital and Tumor Institute in Houston, Texas, between January, 1973, and May, 1976. The response end points analyzed were complete plus partial response rate, duration of remission, and survival time. The complete and partial responses were verified whenever possible by "second-look" operation. Seventeen patients (31.8%) responded to HMM and three had no evidence of cancer, determined by multiple biopsies at second-look operation. Gastrointestinal, hematologic, and nervous system toxic effects were severe in 10 patients, requiring discontinuation of HMM. This study shows that HMM can induce a complete response and provide an extended disease-free interval without maintenance chemotherapy.

Adult↗
PubMed1982

Studies of the mode of action of antitumour triazenes and triazines-III. Metabolism studies on hexamethylmelamine.

These is good evidence that the antitumour agent hexamethylmelamine (HMM) undergoes oxidative metabolic activation which might occur in the liver and/or extrahepatically. The hepatic microsomal N-methylmelamine metabolizing enzymes were investigated in mice and exhibited different affinities for different melamine derivatives. The apparent Km values are 0.09 mM for HMM, 0.23 mM for pentamethylmelamine, 0.91 mM for 2,2,4,6-tetramethylmelamine and 1.7 mM for trimethylmelamine. HMM inhibited its own metabolism in vitro at substrate concentrations greater than 0.05 mM. Its hepatic microsomal N-demethylation rate was reduced when the mice were pretreated with the hepatic glutathione depleting agent methyliodide. Injection of hexaethylmelamine, a derivative of HMM without antineoplastic properties against the M5076 sarcoma in mice, lead to plasma concentrations of drug and metabolite pentaethylmelamine which were only a fraction of the drug and metabolite levels achieved after a similar dose of HMM.

Altretamine↗
PubMed1987

Crosslinking of trypsin digested acto-heavy meromyosin as a probe of the affinity of the two myosin heads to actin.

The interaction of the two heads of the myosin molecule with actin was studied by tryptic digestion of HMM in the presence of actin, followed by crosslinking the two nicked heavy chains with Nbs2 at the S2 region. In view of the protection by actin of the 50/60 kDa junction against proteolysis, the percentage of the heads interacting with actin was estimated from the proportion of the 110 kDa to the 60 kDa digestion product. Under conditions such that about 50% of HMM heads were protected by actin (at an actin to HMM head molar ratio of 1:1 in the absence of nucleotide, or 3:1 in the presence of 5 mM ADP), the crosslinking of the digestion products yielded a 230 kDa (110 + 110 kDa), 125 kDa (60 + 60 kDa) and 175 kDa (60 + 110 kDa) species. Since the latter should be the only crosslinking product when only one head of HMM molecule is protected by actin, it is concluded that there is no preferential binding of one of the two HMM heads to actin in the presence of ADP or at equimolar actin to myosin heads ratio.

Actins↗
PubMed1990

Use of hidden Markov models for electrocardiographic signal analysis.

Hidden Markov modelling (HMM) is a powerful stochastic modelling technique that has been successfully applied to automatic speech recognition problems. We are currently investigating the application of HMM to electrocardiographic signal analysis with the goal of improving ambulatory ECG analysis. The HMM approach specifies a Markov chain to model a "hidden" sequence that in this case is the underlying state of the heart. Each state of the Markov chain has an associated output function that describes the statistical characteristics of measurement samples generated during that state. Given a measurement sequence and HMM parameter estimates, the most likely underlying state sequence can be determined and used to infer beat classification. Advantages of this approach include resistance to noise, ability to model low-amplitude waveforms such as the P wave, and availability of an algorithm for automatically estimating model parameters from training data. We have applied the HMM approach to QRS complex detection and to arrhythmia analysis with encouraging results.

Algorithms↗
PubMed1990

Inhibition of sliding movement of F-actin by crosslinking emphasizes the role of actin structure in the mechanism of motility.

The effects of crosslinking of monomeric and polymeric actin with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC), disuccinimidyl suberate (DSS) and glutaraldehyde on the interaction with heavy meromyosin (HMM) in solution and on the sliding movement on glass-attached HMM were examined. The Vmax values of actin-activated HMM ATPase decreased in the following order: intact actin = EDC F-actin greater than DSS actin greater than glutaraldehyde F-actin = glutaraldehyde G-actin greater than EDC G-actin. The affinity of actin for HMM in the presence of ATP decreased in the following order: DSS actin greater than glutaraldehyde F-actin = glutaraldehyde G-actin greater than intact actin greater than EDC F-actin greater than EDC G-actin. However, sliding movement was inhibited only in the case of glutaraldehyde-crosslinked F and G-actin and EDC-crosslinked G-actin. Interestingly, after copolymerization of "non-motile" glutaraldehyde or EDC-crosslinked monomers with "motile" monomers of intact actin sliding of the copolymers was observed and its rate was independent of the type of crosslinked monomer, i.e. of the manner of their interaction with HMM. These data strongly indicate that inhibition of the sliding of actin by crosslinking cannot be explained entirely by changes in the Vmax value or affinity for myosin heads. We conclude that movement is generated by interaction of myosin with segments of F-actin containing a number of intact monomers, and the mechanism of inhibition involves an effect of the crosslinkers on the structure of F-actin itself.

Actins↗
PubMed1984

Phenotypic characterization and functional activity of human milk macrophages.

A large population (about 80%) of the cells obtained from colostrum and early human milk were considered to be macrophages by the following criteria: nonspecific esterase stain, adherence, phagocytosis and IgG-Fc receptor expression. The majority of freshly isolated human milk macrophages (HMM phi) stain for the monocyte antigen OKM1. Another monocyte antigen, 61D3, was expressed only by 30% of HMM phi. Class II antigens were expressed by HMM phi. About 85% of the cells were DR-positive whereas 50% were DS-positive as assessed with a panel of monoclonal antibodies directed against class II antigens. Monocyte and class II antigens were gradually lost during in vitro culture. HMM phi can support proliferative response to antigens and mitogens when cocultured with autologous peripheral T cells. The proliferative response was significantly reduced when monoclonal antibodies to DR or DS were added to the assay. These results indicate that HMM phi have the phenotype and functional characteristics of antigen presenting cells.

Antigen-Presenting Cells↗
PubMed1988

Evaluation of mechanical parameters of the mechanochemically 'active state' of actin filaments on the basis of purely chemical data.

Oosawa and his collaborators (cf. F. Oosawa, Biophys. Chem. 11 (1980) 443), employing various optical techniques, have shown that the flexibility of actin filaments increases upon interacting with the enzymatically active myosin fragments, particularly heavy meromyosin (HMM). It has been reported (S. Hitchock, L. Carlsson and U. Lindberg, Cell 7 (1976) 53) that HMM can accelerate the DNase 1-induced depolymerization of F-actin, provided MgATP is also present. Since, as we have demonstrated (cf. J. Borejdo myosin, is endowed with mechanochemical capability, we made an attempt to correlate the enhanced rate of depolymerization with the decrease in rigidity of the G-G bonds in F-actin. On the basis of the chemical kinetic data of Hitchcock et al. we could derive the approximate value of the HMM-MgATP-induced change in rigidity which is a mechanical molecular parameter. Since interaction between HMM or HMM subfragment-1 and F-actin in the presence of MgATP leads to the movement of the myosin heads along the actin filaments, it is argued that the enzymic behavior of this system should not be analyzed on the basis of simple, equilibrium, complex formation.

Actins↗
PubMed1991

Analysis of prognostic factors and survival in patients with ovarian cancer treated with second-line hexamethylmelamine (altretamine).

Hexamethylmelamine (altretamine, HMM) 260 mg/m2/day p.o. for 14 days followed by a 14-day drug-free interval was administered to 52 outpatients with advanced ovarian cancer who had previously been treated with chemotherapy. Prior to HMM, 92% (48/52) of these patients had received cisplatin and cyclophosphamide with or without doxorubicin. Two more patients received other cisplatin-based regimens. At the completion of HMM therapy, 15% (8/52) displayed no evidence of disease (NED). Of these eight patients, five are still alive 32 to 82 months after altretamine therapy (median follow-up of 46 months). At 41 months, one patient died of intercurrent illness with no clinical evidence of recurrence; the other two patients died of their disease at 21 and 31 months following HMM therapy. The median survival of the total group was 11 months: nine months for patients who did not respond to altretamine and 46+ months for patients with NED after altretamine (p less than 0.05). Intermittent oral administration of single-agent altretamine was well tolerated: eight patients reported moderate gastrointestinal symptoms, and only one patient reported severe gastrointestinal symptoms. Moderate neurologic toxicity was reported by five patients. No WBC fell below 2000 mm3 and platelet counts fell below 100,000 mm3 in only three patients; no patient experienced severe hematologic toxicity. In this series of patients, the overall response (15%) was comparable to or better than those reported for more toxic chemotherapeutic regimens. On the basis of these data and those reported by other investigators, HMM warrants consideration as a reasonable option in the management of recurrent or persistent ovarian cancer.

Altretamine↗
PubMed1991

Heavy-meromyosin-decorated actin filaments: a simple method to preserve actin filaments for rotary shadowing.

It has become accepted that deep-freeze-drying at or below -90 degrees C is necessary to preserve the structure of supramolecular assemblies such as actin filaments (AFs) for metal shadowing. This has kept the metal shadowing technique from widespread use in the study of proteins complexed with AFs because of the limited availability of the apparatus for deep-freeze-drying. I report here that adsorption to freshly cleaved mica, treatment with buffered uranyl acetate in glycerol solution, rinsing, and removal of liquid eliminate the need of freeze-drying to preserve the structure of AFs. This technique, in combination with metal shadowing, was applied to the study of AFs decorated with heavy meromyosin (HMM). It was observed that (1) when HMM molecules are associated with single AFs in the majority of cases only one head of each HMM molecule makes contact at the point furthest from the neck region; (2) binding of HMM causes bundling of AFs, probably by the two heads of each molecule binding different filaments; and (3) the binding of HMM to the bundled AFs appears to be more stable than that to a single AF. This method of specimen preparation requires no freeze-drying and is therefore easily applicable to other large protein complexes.

Actins↗
PubMed2004

Transformation of positively charged aluminium-species in unstable mixing zones following liming.

Liming is widely used to counteract chronic toxicity of positively charged monomeric aluminium species (Ali). Immediately after liming, unstable mixing zones are formed due to the sudden increase in pH. Transformation of monomeric Ali species takes place instantaneously and transient positively charged Al polymers, being acute toxic to fish, are formed in the mixing zones. Using in situ hollow fibre ultrafiltration interfaced with ion chromatography in unstable mixing zone field experiments performed in two river systems situated south and southwest of Norway, information on time-dependent transformations of low molecular mass (LMM) and high molecular mass (HMM) positively charged Ali-species has been followed. The formation of HMM Ali species from LMM Ali occurred rapidly following liming. HMM Ali species have a certain lifetime and are transformed to high molecular mass neutral Al-species (HMM Alo) and then to non-reactive colloidal Al species (HMM Alc). Concentration levels of transient Al-species formed in the mixing zone and the rate of transformation depend on the concentrations of LMM Ali species, Al complexing ligands (DOC and Si) in the input water and on pH in the mixing zone after liming. A dynamic model describing transformation processes influencing the Al speciation in mixing zones following a sudden increase in pH is suggested. Based on the experimental results, associated rate constants and half-lives for transient Al-species were estimated.

Journal Article↗
PubMed2005

Gill reactivity of aluminium-species following liming.

In acidified aluminium (Al) rich freshwater positively charged Al-species (Al(i)) are the key toxic components due to the accumulation in fish gills. As a countermeasure, liming is used to increase the pH and reduce the concentration of Al(i)-species; in particular low molecular mass (LMM) Al(i)-species by hydrolyses. However, very toxic high molecular mass (HMM) Al polymers can form in the unstable mixing zone immediately after liming. In the present work gill reactivity of LMM and HMM Al-species was studied under controlled conditions in eight channel-tank mixing zone systems in the field where Atlantic salmon (Salmo salar L.) kept in cages were exposed to defined mixing zone water. Mixing zones were created by continually liming acid river water (pH 5.0-5.7) high in LMM Al(i) to pH 6.0 and 6.4, respectively. Transformation processes affecting the Al-speciation as a function of time after liming were documented by in situ hollow fibre ultrafiltration interfaced with ion chromatography, while the Al accumulation in fish gills was used as bioindicator. For fish exposed to mixing zone water immediately after liming (1 min) the Al accumulation in gills (mug Al g(-1)) was higher (factor of 2) than for fish exposed to acid water prior to liming, due to the formation of gill reactive HMM Al(i)-species. The Al accumulation in gills followed a first order kinetic expression reaching steady-state conditions after 24-h exposures. The deposition rate of Al in gills (mug Al g(-1) h(-1)) correlated with the water concentrations of HMM Al(i) (R(2)=0.80) in the mixing zones, and for LMM Al(i) in the acid water (R(2)=0.92). Due to the transient nature of HMM Al(i) the deposition rate of Al decreased from the point of liming with a factor of 10 downstream the channel-tank system (i.e. 100 min after liming). The concentration of gill accumulated Al was higher (factor of 3) immediately after high level liming (pH 6.4) than following low level liming (pH 6.0). However, high level liming was more efficient in detoxifying Al in downstream waters. Furthermore, the bioavailability of a given LMM Al(i) concentration as well as bioreactivity following liming was dependent on the TOC and silicon concentration in the acid water. Increased concentration of TOC (1.5 to 4.3 mg l(-1)) and silicon (0.3 to 1.0 mg l(-1)) reduced the gill reactivity of Al(i) by approximately 50%.

Aluminum↗
PubMed2000

Characterization of single actomyosin rigor bonds: load dependence of lifetime and mechanical properties.

Load dependence of the lifetime of the rigor bonds formed between a single myosin molecule (either heavy meromyosin, HMM, or myosin subfragment-1, S1) and actin filament was examined in the absence of nucleotide by pulling the barbed end of the actin filament with optical tweezers. For S1, the relationship between the lifetime (tau) and the externally imposed load (F) at absolute temperature T could be expressed as tau(F) = tau(0).exp(-F.d/k(B)T) with tau(0) of 67 s and an apparent interaction distance d of 2.4 nm (k(B) is the Boltzmann constant). The relationship for HMM was expressed by the sum of two exponentials, with two sets of tau(0) and d being, respectively, 62 s and 2.7 nm, and 950 s and 1.4 nm. The fast component of HMM coincides with tau(F) for S1, suggesting that the fast component corresponds to single-headed binding and the slow component to double-headed binding. These large interaction distances, which may be a common characteristic of motor proteins, are attributed to the geometry for applying an external load. The pulling experiment has also allowed direct estimation of the number of myosin molecules interacting with an actin filament. Actin filaments tethered to a single HMM molecule underwent extensive rotational Brownian motion, indicating a low torsional stiffness for HMM. From these results, we discuss the characteristics of interaction between actin and myosin, with the focus on the manner of binding of myosin.

Actins↗