Tropomyosin and actin isoforms modulate the localization of tropomyosin strands on actin filaments

Push and pull of tropomyosin's opposite effects on myosin attachment to actin. A chimeric tropomyosin host-guest study

Tropomyosin is a ubiquitous actin-binding protein with an extended coiled-coil structure. Tropomyosin-actin interactions are weak and loosely specific, but they potently influence myosin. One such influence is inhibitory and is due to tropomyosin's statistically preferred positions on actin that sterically interfere with actin's strong attachment site for myosin. Contrastingly, tropomyosin's other influence is activating. It increases myosin's overall actin affinity ∼4-fold. Stoichiometric considerations cause this activating effect to equate to an ∼4(7)-fold effect of myosin on the actin affinity of tropomyosin. These positive, mutual, myosin-tropomyosin effects are absent if Saccharomyces cerevisiae tropomyosin replaces mammalian tropomyosin. To investigate these phenomena, chimeric tropomyosins were generated in which 38-residue muscle tropomyosin segments replaced a natural duplication within S. cerevisiae tropomyosin TPM1. Two such chimeric tropomyosins were sufficiently folded coiled coils to allow functional study. The two chimeras differed from TPM1 but in opposite ways. Consistent with steric interference, myosin greatly decreased the actin affinity of chimera 7, which contained muscle tropomyosin residues 228-265. On the other hand, myosin S1 increased by an order of magnitude the actin affinity of chimera 3, which contained muscle tropomyosin residues 74-111. Similarly, myosin S1-ADP binding to actin was strengthened 2-fold by substitution of chimera 3 tropomyosin for wild-type TPM1. Thus, a yeast tropomyosin was induced to mimic the activating behavior of mammalian tropomyosin by inserting a mammalian tropomyosin sequence. The data were not consistent with direct tropomyosin-myosin binding. Rather, they suggest an allosteric mechanism, in which myosin and tropomyosin share an effect on the actin filament.

Tropomyosin isoform 3 promotes the formation of filopodia by regulating the recruitment of actin-binding proteins to actin filaments

Tropomyosins are believed to function in part by stabilizing actin filaments. However, accumulating evidence suggests that fundamental differences in function exist between tropomyosin isoforms, which contributes to the formation of functionally distinct filament populations. We investigated the functions of the high-molecular-weight isoform Tm3 and examined the molecular properties of Tm3-containing actin filament populations. Overexpression of the Tm3 isoform specifically induced the formation of filopodia and changes in actin solubility. We observed alterations in actin-binding protein recruitment to filaments, co-incident with changes in expression levels, which can account for this functional outcome. Tm3-associated filaments recruit active actin depolymerizing factor and are bundled into filopodia by fascin, which is both up-regulated and preferentially associated with Tm3-containing filaments in the Tm3 overexpressing cells. This study provides further insight into the isoform-specific roles of different tropomyosin isoforms. We conclude that variation in the tropomyosin isoform composition of microfilaments provides a mechanism to generate functionally distinct filament populations.

Tropomyosin isoform modulation of focal adhesion structure and cell migration

Orderly cell migration is essential for embryonic development, efficient wound healing and a functioning immune system and the dysregulation of this process leads to a number of pathologies. The speed and direction of cell migration is critically dependent on the structural organization of focal adhesions in the cell. While it is well established that contractile forces derived from the acto-myosin filaments control the structure and growth of focal adhesions, how this may be modulated to give different outcomes for speed and persistence is not well understood. The tropomyosin family of actin-associating proteins are emerging as important modulators of the contractile nature of associated actin filaments. The multiple non-muscle tropomyosin isoforms are differentially expressed between tissues and across development and are thought to be major regulators of actin filament functional specialization. In the present study we have investigated the effects of two splice variant isoforms from the same alpha-tropomyosin gene, TmBr1 and TmBr3, on focal adhesion structure and parameters of cell migration. These isoforms are normally switched on in neuronal cells during differentiation and we find that exogenous expression of the two isoforms in undifferentiated neuronal cells has discrete effects on cell migration parameters. While both isoforms cause reduced focal adhesion size and cell migration speed, they differentially effect actin filament phenotypes and migration persistence. Our data suggests that differential expression of tropomyosin isoforms may coordinate acto-myosin contractility and focal adhesion structure to modulate cell speed and persistence.

Myofilament length dependent activation

The Frank-Starling law of the heart describes the interrelationship between end-diastolic volume and cardiac ejection volume, a regulatory system that operates on a beat-to-beat basis. The main cellular mechanism that underlies this phenomenon is an increase in the responsiveness of cardiac myofilaments to activating Ca(2+) ions at a longer sarcomere length, commonly referred to as myofilament length-dependent activation. This review focuses on what molecular mechanisms may underlie myofilament length dependency. Specifically, the roles of inter-filament spacing, thick and thin filament based regulation, as well as sarcomeric regulatory proteins are discussed. Although the "Frank-Starling law of the heart" constitutes a fundamental cardiac property that has been appreciated for well over a century, it is still not known in muscle how the contractile apparatus transduces the information concerning sarcomere length to modulate ventricular pressure development.

New insights into the regulation of the actin cytoskeleton by tropomyosin

The actin cytoskeleton is regulated by a variety of actin-binding proteins including those constituting the tropomyosin family. Tropomyosins are coiled-coil dimers that bind along the length of actin filaments. In muscles, tropomyosin regulates the interaction of actin-containing thin filaments with myosin-containing thick filaments to allow contraction. In nonmuscle cells where multiple tropomyosin isoforms are expressed, tropomyosins participate in a number of cellular events involving the cytoskeleton. This chapter reviews the current state of the literature regarding tropomyosin structure and function and discusses the evidence that tropomyosins play a role in regulating actin assembly.

What makes tropomyosin an actin binding protein? A perspective

Tropomyosin is a two-chained alpha-helical coiled coil that binds along the length of the actin filament and regulates its function. The paper addresses the question of how a "simple" coiled-coil sequence encodes the information for binding and regulating the actin filament, its universal target. Determination of the tropomyosin sequence confirmed Crick's predicted heptapeptide repeat of hydrophobic interface residues and revealed additional features that have been shown to be important for its function: a 7-fold periodicity predicted to correspond to actin binding sites and interruptions of the canonical interface with destabilizing residues, such as Ala. Evidence from published work is summarized, leading to the proposal of a paradigm that binding of tropomyosin to the actin filament requires local instability as well as regions of flexibility. The flexibility derives from bends and local unfolding at regions with a destabilized coiled-coil interface, as well as from the dynamic end-to-end complex. The features are required for tropomyosin to assume the form of the helical actin filament, and to bind to actin monomers along its length. The requirement of instability/flexibility for binding may be generalized to the binding of other coiled coils to their targets.

The C terminus of cardiac troponin I stabilizes the Ca2+-activated state of tropomyosin on actin filaments

RATIONALE

Ca(2+) control of troponin-tropomyosin position on actin regulates cardiac muscle contraction. The inhibitory subunit of troponin, cardiac troponin (cTn)I is primarily responsible for maintaining a tropomyosin conformation that prevents crossbridge cycling. Despite extensive characterization of cTnI, the precise role of its C-terminal domain (residues 193 to 210) is unclear. Mutations within this region are associated with restrictive cardiomyopathy, and C-terminal deletion of cTnI, in some species, has been associated with myocardial stunning.

OBJECTIVE

We sought to investigate the effect of a cTnI deletion-removal of 17 amino acids from the C terminus- on the structure of troponin-regulated tropomyosin bound to actin.

METHODS AND RESULTS

A truncated form of human cTnI (cTnI(1-192)) was expressed and reconstituted with troponin C and troponin T to form a mutant troponin. Using electron microscopy and 3D image reconstruction, we show that the mutant troponin perturbs the positional equilibrium dynamics of tropomyosin in the presence of Ca(2+). Specifically, it biases tropomyosin position toward an "enhanced C-state" that exposes more of the myosin-binding site on actin than found with wild-type troponin.

CONCLUSIONS

In addition to its well-established role of promoting the so-called "blocked-state" or "B-state," cTnI participates in proper stabilization of tropomyosin in the "Ca(2+)-activated state" or "C-state." The last 17 amino acids perform this stabilizing role. The data are consistent with a "fly-casting" model in which the mobile C terminus of cTnI ensures proper conformational switching of troponin-tropomyosin. Loss of actin-sensing function within this domain, by pathological proteolysis or cardiomyopathic mutation, may be sufficient to perturb tropomyosin conformation.

Curvature variation along the tropomyosin molecule

Complementarity between the tropomyosin supercoil and the helical contour of actin-filaments is required for the binding interaction of actin and tropomyosin (Li et al., 2010). Clusters of small alanine residues in place of canonical leucines along coiled-coil tropomyosin may be responsible for pre-shaping tropomyosin and promoting conformational complementarity to F-actin. A longitudinal displacement between the two chains of the tropomyosin coiled-coil induced by the alanine clusters could produce localized bending or limited flexibility along tropomyosin needed to shape tropomyosin (Brown and Cohen, 2005). To evaluate the influence of alanine clusters on tropomyosin curvature, we calculated the longitudinal displacement between amino acid residues on adjacent chains of the tropomyosin coiled-coil and related this "Z-displacement" to the position of the alanine clusters. Measurements were made on high-resolution crystal structures of tropomyosin fragments and on trajectories from molecular dynamics simulations of full-length alphaalpha-tropomyosin. We found no strict one-for-one spatial correlation between alanine cluster position and the Z-displacement. Neither did we find any direct correspondence between the clusters and the local curvature of tropomyosin. Rather than just causing specific local structural effects, the overall influence of alanine clusters is complex and delocalized, leading to a gradually changing bending pattern along the length of tropomyosin.

Striated muscle regulation of isometric tension by multiple equilibria

Cooperative activation of striated muscle by calcium is based on the movement of tropomyosin described by the steric blocking theory of muscle contraction. Presently, the Hill model stands alone in reproducing both myosin binding data and a sigmoidal-shaped curve characteristic of calcium activation (Hill TL (1983) Two elementary models for the regulation of skeletal muscle contraction by calcium. Biophys J 44: 383–396.). However, the free myosin is assumed to be fixed by the muscle lattice and the cooperative mechanism is based on calcium-dependent interactions between nearest neighbor tropomyosin subunits, which has yet to be validated. As a result, no comprehensive model has been shown capable of fitting actual tension data from striated muscle. We show how variable free myosin is a selective advantage for activating the muscle and describe a mechanism by which a conformational change in tropomyosin propagates free myosin given constant total myosin. This mechanism requires actin, tropomyosin, and filamentous myosin but is independent of troponin. Hence, it will work equally well with striated, smooth and non-muscle contractile systems. Results of simulations with and without data are consistent with a strand of tropomyosin composed of ∼20 subunits being moved by the concerted action of 3–5 myosin heads, which compares favorably with the predicted length of tropomyosin in the overlap region of thick and thin filaments. We demonstrate that our model fits both equilibrium myosin binding data and steady-state calcium-dependent tension data and show how both the steepness of the response and the sensitivity to calcium can be regulated by the actin-troponin interaction. The model simulates non-cooperative calcium binding both in the presence and absence of strong binding myosin as has been observed. Thus, a comprehensive model based on three well-described interactions with actin, namely, actin-troponin, actin-tropomyosin, and actin-myosin can explain the cooperative calcium activation of striated muscle.

Mutations in Troponin that cause HCM, DCM AND RCM: what can we learn about thin filament function?

Troponin (Tn) is a critical regulator of muscle contraction in cardiac muscle. Mutations in Tn subunits are associated with hypertrophic, dilated and restrictive cardiomyopathies. Improved diagnosis of cardiomyopathies as well as intensive investigation of new mouse cardiomyopathy models has significantly enhanced this field of research. Recent investigations have showed that the physiological effects of Tn mutations associated with hypertrophic, dilated and restrictive cardiomyopathies are different. Impaired relaxation is a universal finding of most transgenic models of HCM, predicted directly from the significant changes in Ca(2+) sensitivity of force production. Mutations associated with HCM and RCM show increased Ca(2+) sensitivity of force production while mutations associated with DCM demonstrate decreased Ca(2+) sensitivity of force production. This review spotlights recent advances in our understanding on the role of Tn mutations on ATPase activity, maximal force development and heart function as well as the correlation between the locations of these Tn mutations within the thin filament and myofilament function.

Differential binding of tropomyosin isoforms to actin modified with m-maleimidobenzoyl-N-hydroxysuccinimide ester and fluorescein-5-isothiocyanate

Differential interactions of tropomyosin (TM) isoforms with actin can be important for determination of the thin filament functions. A mechanism of tropomyosin binding to actin was studied by comparing interactions of five alphaTM isoforms with actin modified with m-maleimidobenzoyl-N-hydroxysuccinimide ester (MBS) and with fluorescein-5-isothiocyanate (FITC). MBS attachment sites were revealed with mass spectrometry methods. We found that the predominant actin fraction was cross-linked by MBS within subdomain 3. A smaller fraction of the modified actin was cross-linked within subdomain 2 and between subdomains 2 and 1. Moreover, investigated actins carried single labels in subdomains 1, 2, and 3. Such extensive modification caused a large decrease in actin affinity for skeletal and smooth muscle tropomyosins, nonmuscle TM2, and chimeric TM1b9a. In contrast, binding of nonmuscle isoform TM5a was less affected. Isoform's affinity for actin modified in subdomain 2 by binding of FITC to Lys61 was intermediate between the affinity for native actin and MBS-modified actin except for TM5a, which bound to FITC-actin with similar affinity as to actin modified with MBS. The analysis of binding curves according to the McGhee-von Hippel model revealed that binding to an isolated site, as well as cooperativity of binding to a contiguous site, was affected by both actin modifications in a TM isoform-specific manner.

Investigations into tropomyosin function using mouse models

Tropomyosin plays a key role in controlling calcium regulated sarcomeric contraction through its interactions with actin and the troponin complex. The focus of this review is on striated muscle tropomyosin isoforms and the in vivo approach we have taken to define the functional differences among these isoforms in regulating cardiac physiology. In addition, we address specific regions within tropomyosin that differ among the isoforms to impart differences in the physiological performance of muscle and the sarcomere itself. There is a high degree of amino acid identity among the three striated muscle alpha-, beta-, and gamma-tropomyosin isoforms; this identity ranges from 86% to 91%. We employ transgenic mouse model systems that express the different tropomyosin isoforms or chimeric tropomyosin molecules specifically in the myocardium. Results show that the three isoforms differentially regulate the rates of cardiac contraction and relaxation, along with conferring differences in myofilament calcium sensitivity and sarcomere tension development. We also found the putative troponin T binding regions of tropomyosin (amino acids 175-190 and 258-284) appear to a play significant role in imparting these physiological differences that are observed during cardiac and sarcomeric contraction/relaxation. In addition, we have successfully used chimeric tropomyosin molecules to rescue cardiomyopathic diseased mice by normalizing sarcomeric performance. These studies illustrate not only the importance of tropomyosin structure and function for understanding muscle physiology, but also demonstrate how this information can potentially be used for gene therapy.

Differences between cardiac and skeletal troponin interaction with the thin filament probed by troponin exchange in skeletal myofibrils

Troponin (Tn) is the calcium-sensing protein of the thin filament. Although cardiac troponin (cTn) and skeletal troponin (sTn) accomplish the same function, their subunit interactions within Tn and with actin-tropomyosin are different. To further characterize these differences, myofibril ATPase activity as a function of pCa and labeled Tn exchange in rigor myofibrils was used to estimate Tn dissociation rates from the nonoverlap and overlap region as a function of pCa. Measurement of ATPase activity showed that skeletal myofibrils containing >96% cTn had a higher pCa 9 ATPase activity than, but similar pCa 4 activity to, sTn-containing myofibrils. Analysis of the pCa-ATPase activity relation showed that cTn myofibrils were more calcium sensitive but less cooperative (pCa50 = 6.14, nH = 1.46) than sTn myofibrils (pCa50= 5.90, nH = 3.36). The time course of labeled Tn exchange at pCa 9 and 4 were quite different between cTn and sTn. The apparent cTn dissociation rates were approximately 2-10-fold faster than sTn under all the conditions studied. The apparent dissociation rates for cTn were 5 x 10(-3) min(-1), 150 x 10(-3) min(-1), and 260 x 10(-3) min(-1), whereas for sTn they were 0.6 x 10(-3) min(-1), 88 x 10(-3) min(-1), and 68 x 10(-3) min(-1) for the nonoverlap region at pCa 9, nonoverlap region at pCa 4, and overlap region at pCa 4, respectively. Normalization of the apparent dissociation rates gives 1:30:50 for cTn compared with 1:150:110 for sTn (nonoverlap at pCa 9:nonoverlap at pCa 4:overlap at pCa 4) suggesting that calcium has a smaller influence, whereas strong cross-bridges have a larger influence on cTn dissociation compared with sTn. The higher cTn dissociation rate in the nonoverlap region and ATPase activity at pCa 9 suggest that it gives a less off or inactive thin filament. Analysis of the intensity ratio (after a short time of exchange) as a function of pCa showed that cTn had greater calcium sensitivity but lower cooperativity than sTn. In addition, the magnitude of the change in intensity ratio going from pCa 9 to 4 was less for cTn than sTn. These data suggest that the influence of calcium on cTn exchange is less than sTn even though calcium can activate ATPase activity to a similar extent in cTn compared with sTn myofibrils. This may be explained partially by cTn being less off or inactive at pCa 9. Modeling of the intensity profiles obtained after Tn exchange at pCa 5.8 suggest that the profiles are best explained by a model that includes a long-range cross-bridge effect that grades with distance from the rigor cross-bridge for both cTn and sTn.

The effect of the dilated cardiomyopathy-causing mutation Glu54Lys of alpha-tropomyosin on actin-myosin interactions during the ATPase cycle

In order to understand how the Glu54Lys mutation of alpha-tropomyosin affects actomyosin interactions, we labeled SH1 helix of myosin subfragment-1 (S1) and the actin subdomain-1 with fluorescent probes. These proteins were incorporated into ghost muscle fibers and their conformational states were monitored during the ATPase cycle by measuring polarized fluorescence. The addition of wild-type alpha-tropomyosin to actin filaments increases the amplitude of the SH1 helix and subdomain-1 movements during the ATPase cycle, indicating the enhancement of the efficiency of work of each cross-bridge. The Glu54Lys mutation inhibits this effect. The Glu54Lys mutation also results in the coupling of the weak-binding sub-state of S1 to the strong-binding sub-state of actin thus altering the concerted conformational changes during the ATPase cycle. We suggest that these alterations will result in reduced force production, which is likely to underlie at least in part the contractile deficit observed in human dilated cardiomyopathy.

Modulation of the effects of tropomyosin on actin and myosin conformational changes by troponin and Ca2+

The molecular mechanisms by which troponin (TN)-tropomyosin (TM) regulates the myosin ATPase cycle were investigated using fluorescent probes specifically bound to Cys36 of TM, Cys707 of myosin subfragment-1, and Cys374 of actin incorporated into ghost muscle fibers. Intermediate states of actomyosin were simulated by using nucleotides and non-hydrolysable ATP analogs. Multistep changes in mobility and spatial arrangement of SH1 helix of myosin motor domain and actin subdomain-1 during the ATPase cycle were observed. Each intermediate state of actomyosin induced a definite conformational state and specific position of TM strands on the surface of thin filament. TM increased the amplitude of myosin SH1 helix and actin subdomain-1 movements at transition from weak- to strong-binding states shifting to the center of thin filament at strong-binding and to the periphery of thin filament at weak-binding states. TN modulated those movements in a capital ES, Cyrillicsmall a, Cyrillic(2+)-dependent manner. At high-Ca(2+), TN enhanced the effect of TM on SH1 helix and subdomain-1 movements by transferring TM further to the center of thin filament at strong-binding states. In contrast, at low-Ca(2+), TN inhibited the effect of TM movements, "freezing" actin structure in "OFF" state and TM in the position typical for weak-binding states, resulting in disturbing the interplay of actin and myosin.

The myosin C-loop is an allosteric actin contact sensor in actomyosin

Actin and myosin form the molecular motor in muscle. Myosin is the enzyme performing ATP hydrolysis under the allosteric control of actin such that actin binding initiates product release and force generation in the myosin power stroke. Non-equilibrium Monte Carlo molecular dynamics simulation of the power stroke suggested that a structured surface loop on myosin, the C-loop, is the actin contact sensor initiating actin activation of the myosin ATPase. Previous experimental work demonstrated C-loop binds actin and established the forward and reverse allosteric link between the C-loop and the myosin active site. Here, smooth muscle heavy meromyosin C-loop chimeras were constructed with skeletal (sCl) and cardiac (cCl) myosin C-loops substituted for the native sequence. In both cases, actin-activated ATPase inhibition is indicated mainly by the lower V(max). In vitro motility was also inhibited in the chimeras. Motility data were collected as a function of myosin surface density, with unregulated actin, and with skeletal and cardiac isoforms of tropomyosin-bound actin for the wild type, cCl, and sCl. Slow and fast subpopulations of myosin velocities in the wild-type species were discovered and represent geometrically unfavorable and favorable actomyosin interactions, respectively. Unfavorable interactions are detected at all surface densities tested. Favorable interactions are more probable at higher myosin surface densities. Cardiac tropomyosin-bound actin promotes the favorable actomyosin interactions by lowering the inhibiting geometrical constraint barriers with a structural effect on actin. Neither higher surface density nor cardiac tropomyosin-bound actin can accelerate motility velocity in cCl or sCl, suggesting the element initiating maximal myosin activation by actin resides in the C-loop.

Ca2+-dependent photocrosslinking of tropomyosin residue 146 to residues 157-163 in the C-terminal domain of troponin I in reconstituted skeletal muscle thin filaments

The Ca(2+)-dependent interaction of troponin I (TnI) with actin.tropomyosin (Tm) in muscle thin filaments is a critical step in the regulation of muscle contraction. Previous studies have suggested that, in the absence of Ca(2+), TnI interacts with Tm and actin in reconstituted muscle thin filaments, maintaining Tm at the outer domain of actin and blocking myosin-actin interaction. To obtain direct evidence for this Tm-TnI interaction, we performed photochemical crosslinking studies using Tm labeled with 4-maleimidobenzophenone at position 146 or 174 (Tm*146 or Tm*174, respectively), reconstituted with actin and troponin [composed of TnI, troponin T (TnT), and troponin C] or with actin and TnI. After near-UV irradiation, SDS gels of the Tm*146-containing thin filament showed three new high-molecular-weight bands determined to be crosslinked products Tm*146-TnI, Tm*146-troponin C, and Tm*146-TnT using fluorescence-labeled TnI, mass spectrometry, and Western blot analysis. While Tm*146-TnI was produced only in the absence of Ca(2+), the production of other crosslinked species did not show Ca(2+) dependence. Tm*174 mainly crosslinked to TnT. In the absence of actin, a similar crosslinking pattern was obtained with a much lower yield. A tryptic peptide from Tm*146-TnI with a molecular mass of 2601.2 Da that was not present in the tryptic peptides of Tm*146 or TnI was identified using HPLC and matrix-assisted laser desorption/ionization time-of-flight. This was shown, using absorption and fluorescence spectroscopy, to be the 4-maleimidobenzophenone-labeled peptide from Tm crosslinked to TnI peptide 157-163. These data, which show that a region in the C-terminal domain of TnI interacts with Tm in the absence of Ca(2+), support the hypothesis that a TnI-Tm interaction maintains Tm at the outer domain of actin and will help efforts to localize troponin in actin.Tm muscle thin filaments.

Some cardiomyopathy-causing troponin I mutations stabilize a functional intermediate actin state

We examined four cardiomyopathy-causing mutations of troponin I that appear to disturb function by altering the distribution of thin filament states. The R193H (mouse) troponin I mutant had greater than normal actin-activated myosin-S1 ATPase activity in both the presence and absence of calcium. The rate of ATPase activity was the same as that of the wild-type at near-saturating concentrations of the activator, N-ethylmaleimide-S1. This mutant appeared to function by stabilizing the active state of thin filaments. Mutations D191H, R146G, and R146W had lower ATPase activities in the presence of calcium, but higher activities in the absence of calcium. These effects were most pronounced with mutations at position 146. For all three mutants the rates were similar to those of the wild-type at near-saturating concentrations of N-ethylmaleimide-S1. These results, combined with previous results, show that any alteration in the normal distribution of actomyosin states is capable of producing cardiomyopathy. The results of the D191H, R146G, and R146W mutations are most readily explained if the intermediate state of regulated actin has a unique function. The intermediate state appears to have an ability to accelerate the rate of ATP hydrolysis by myosin that exceeds that of the inactive state.

In vivo and in vitro effects of two novel gamma-actin (ACTG1) mutations that cause DFNA20/26 hearing impairment

Here we report the functional assessment of two novel deafness-associated gamma-actin mutants, K118N and E241K, in a spectrum of different situations with increasing biological complexity by combining biochemical and cell biological analysis in yeast and mammalian cells. Our in vivo experiments showed that while the K118N had a very mild effect on yeast behaviour, the phenotype caused by the E241K mutation was very severe and characterized by a highly compromised ability to grow on glycerol as a carbon source, an aberrant multi-vacuolar pattern and the deposition of thick F-actin bundles randomly in the cell. The latter feature is consistent with the highly unusual spontaneous tendency of the E241K mutant to form bundles in vitro, although this propensity to bundle was neutralized by tropomyosin and the E241K filament bundles were hypersensitive to severing in the presence of cofilin. In transiently transfected NIH3T3 cells both mutant actins were normally incorporated into cytoskeleton structures, although cytoplasmic aggregates were also observed indicating an element of abnormality caused by the mutations in vivo. Interestingly, gene-gun mediated expression of these mutants in cochlear hair cells results in no gross alteration in cytoskeletal structures or the morphology of stereocilia. Our results provide a more complete picture of the biological consequences of deafness-associated gamma-actin mutants and support the hypothesis that the post-lingual and progressive nature of the DFNA20/26 hearing loss is the result of a progressive deterioration of the hair cell cytoskeleton over time.

Structural basis for the activation of muscle contraction by troponin and tropomyosin

The molecular regulation of striated muscle contraction couples the binding and dissociation of Ca(2+) on troponin (Tn) to the movement of tropomyosin on actin filaments. In turn, this process exposes or blocks myosin binding sites on actin, thereby controlling myosin crossbridge dynamics and consequently muscle contraction. Using 3D electron microscopy, we recently provided structural evidence that a C-terminal extension of TnI is anchored on actin at low Ca(2+) and competes with tropomyosin for a common site to drive tropomyosin to the B-state location, a constrained, relaxing position on actin that inhibits myosin-crossbridge association. Here, we show that release of this constraint at high Ca(2+) allows a second segment of troponin, probably representing parts of TnT or the troponin core domain, to promote tropomyosin movement on actin to the Ca(2+)-induced C-state location. With tropomyosin stabilized in this position, myosin binding interactions can begin. Tropomyosin appears to oscillate to a higher degree between respective B- and C-state positions on troponin-free filaments than on fully regulated filaments, suggesting that tropomyosin positioning in both states is troponin-dependent. By biasing tropomyosin to either of these two positions, troponin appears to have two distinct structural functions; in relaxed muscles at low Ca(2+), troponin operates as an inhibitor, while in activated muscles at high Ca(2+), it acts as a promoter to initiate contraction.

Isoform-specific variation in the intrinsic disorder of troponin I

Various intrinsic disorder (ID) prediction algorithms were applied to the three tissue isoforms of troponin I (TnI). The results were interpreted in terms of the known structure and dynamics of troponin. In line with previous results, all isoforms of TnI were predicted to have large stretches of ID. The predictions show that the C-termini of all isoforms are extensively disordered as is the N-terminal extension of the cardiac isoform. Cardiac TnI likely belongs to the group of intrinsically disordered signalling hub proteins. For a given portion of the protein sequence, most ID prediction approaches indicate isoform-dependent variations in the probability of disorder. Comparison of machine learning and physically based approaches suggests the ID variations are only partially attributable to local variations in the ratio of charged to hydrophobic residues. The VSL2B algorithm predicts the largest variations in ID across the isoforms, with the cardiac isoform having the highest probability of structured regions, and the fast-skeletal isoform having no intrinsic structure. The region corresponding to residues 57-95 of the fast-skeletal isoform, known to form a coiled coil substructure with troponin T, was highly variable between isoforms. The isoform-specific ID variations may have mechanistic significance, modulating the extent to which conformational fluctuations in tropomyosin are communicated to the troponin complex. We discuss structural mechanisms for this communication. Overall, the results motivate the development of predictors designed to address relative levels of disorder between highly similar proteins.

Mechanical and kinetic effects of shortened tropomyosin reconstituted into myofibrils

The effects of tropomyosin on muscle mechanics and kinetics were examined in skeletal myofibrils using a novel method to remove tropomyosin (Tm) and troponin (Tn) and then replace these proteins with altered versions. Extraction employed a low ionic strength rigor solution, followed by sequential reconstitution at physiological ionic strength with Tm then Tn. SDS-PAGE analysis was consistent with full reconstitution, and fluorescence imaging after reconstitution using Oregon-green-labeled Tm indicated the expected localization. Myofibrils remained mechanically viable: maximum isometric forces of myofibrils after sTm/sTn reconstitution (control) were comparable (~84%) to the forces generated by non-reconstituted preparations, and the reconstitution minimally affected the rate of isometric activation (k_act), calcium sensitivity (pCa₅₀), and cooperativity (n_H). Reconstitutions using various combinations of cardiac and skeletal Tm and Tn indicated that isoforms of both Tm and Tn influence calcium sensitivity of force development in opposite directions, but the isoforms do not otherwise alter cross-bridge kinetics. Myofibrils reconstituted with Δ23Tm, a deletion mutant lacking the second and third of Tm’s seven quasi-repeats, exhibited greatly depressed maximal force, moderately slower k_act rates and reduced n_H. Δ23Tm similarly decreased the cooperativity of calcium binding to the troponin regulatory sites of isolated thin filaments in solution. The mechanisms behind these effects of Δ23Tm also were investigated using P_i and ADP jumps. P_i and ADP kinetics were indistinguishable in Δ23Tm myofibrils compared to controls. The results suggest that the deleted region of tropomyosin is important for cooperative thin filament activation by calcium.

Gestalt-binding of tropomyosin to actin filaments

We argue that the overall behavior of tropomyosin on F-actin cannot be easily discerned by examining thin filaments reduced to their smallest interacting units. In isolation, the individual interactions of actin and tropomyosin, by themselves, are too weak to account for the specificity of the system. Instead the association of tropomyosin on actin can only be fully explained after considering the concerted action of the entire acto-tropomyosin system. We propose that the low K ( a ) describing tropomyosin:actin interaction, when taken together with the form-fitting complementarity of tropomyosin strands on F-actin and the tendency for tropomyosin to polymerize end-to-end, make possible unique thin filament functions both locally and at higher levels of filament organization.

Actin depolymerization factor/cofilin activation regulates actin polymerization and tension development in canine tracheal smooth muscle

The contractile activation of airway smooth muscle tissues stimulates actin polymerization, and the inhibition of actin polymerization inhibits tension development. Actin-depolymerizing factor (ADF) and cofilin are members of a family of actin-binding proteins that mediate the severing of F-actin when activated by dephosphorylation at serine 3. The role of ADF/cofilin activation in the regulation of actin dynamics and tension development during the contractile activation of smooth muscle was evaluated in intact canine tracheal smooth muscle tissues. Two-dimensional gel electrophoresis revealed that ADF and cofilin exist in similar proportions in the muscle tissues, and that approximately 40% of the total ADF/cofilin in unstimulated tissues is phosphorylated. Phospho-ADF/cofilin decreased concurrently with tension development in response to stimulation with acetylcholine (ACh) or potassium depolarization indicating the activation of ADF/cofilin. Expression of an inactive phospho-cofilin mimetic (cofilin S3E) but not wild type cofilin in the smooth muscle tissues inhibited endogenous ADF/cofilin dephosphorylation and ACh-induced actin polymerization. Expression of cofilin S3E in the tissues depressed tension development in response to ACh, but it did not affect myosin light chain phosphorylation. The ACh-induced dephosphorylation of ADF/cofilin required the Ca2+-dependent activation of calcineurin (PP2B). The results indicate that the activation of ADF/cofilin is regulated by contractile stimulation in tracheal smooth muscle and that cofilin activation is required for actin polymerization and tension development in response to contractile stimulation.

Direct association of calponin with specific domains of PKC-alpha

Calponin contributes to the regulation of smooth muscle contraction through its interaction with F-actin and inhibition of the actin-activated Mg-ATPase activity of phosphorylated myosin. Previous studies have shown that the contractile agonist acetylcholine induced a direct association of translocated calponin and PKC-alpha in the membrane. In the present study, we have determined the domain of PKC-alpha involved in direct association with calponin. In vitro binding assay was carried out by incubating glutathione S-transferase-calponin aa 92-229 with His-tagged proteins of individual domains and different combinations of domains of PKC-alpha. Calponin was found to bind directly to the full-length PKC-alpha. Calponin bound to C2 and C4 domains but not to C1 and C3 domains of PKC-alpha. When incubated with proteins of different combination of domains, calponin bound to C2-C3, C3-C4, and C2-C3-C4 but not to C1-C2 or C1-C2-C3. To determine whether these in vitro bindings mimic the in vivo associations, and in vivo binding assay was performed by transfecting colonic smooth muscle cells with His-tagged proteins of individual domains and different combinations of domains of PKC-alpha. Coimmunoprecipitation of calponin with His-tagged truncated forms of PKC-alpha showed that C1-C2, C1-C2-C3, C2-C3, and C3-C4 did not associate with calponin. Calponin associated only with full-length PKC-alpha and with C2-C3-C4 in cells in the resting state, and this association increased upon stimulation with acetylcholine. These data suggest that calponin bound to fragments that may mimic the active form of PKC-alpha and that the functional association of PKC-alpha with calponin requires both C2 and C4 domains during contraction of colonic smooth muscle cells.

Fast pressure jumps can perturb calcium and magnesium binding to troponin C F29W

We have used rapid pressure jump and stopped-flow fluorometry to investigate calcium and magnesium binding to F29W chicken skeletal troponin C. Increased pressure perturbed calcium binding to the N-terminal sites in the presence and absence of magnesium and provided an estimate for the volume change upon calcium binding (-12 mL/mol). We observed a biphasic response to a pressure change which was characterized by fast and slow reciprocal relaxation times of the order 1000/s and 100/s. Between pCa 8-5.4 and at troponin C concentrations of 8-28 muM, the slow relaxation times were invariant, indicating that a protein isomerization was rate-limiting. The fast event was only detected over a very narrow pCa range (5.6-5.4). We have devised a model based on a Monod-Wyman-Changeux cooperative mechanism with volume changes of -9 and +6 mL/mol for the calcium binding to the regulatory sites and closed to open protein isomerization steps, respectively. In the absence of magnesium, we discovered that calcium binding to the C-terminal sites could be detected, despite their position distal to the calcium-sensitive tryptophan, with a volume change of +25 mL/mol. We used this novel observation to measure competitive magnesium binding to the C-terminal sites and deduced an affinity in the range 200-300 muM (and a volume change of +35 mL/mol). This affinity is an order of magnitude tighter than equilibrium fluorescence data suggest based on a model of direct competitive binding. Magnesium thus indirectly modulates binding to the N-terminal sites, which may act as a fine-tuning mechanism in vivo.

Ca2+ -induced tropomyosin movement in scallop striated muscle thin filaments

Striated muscle contraction in most animals is regulated at least in part by the troponin-tropomyosin (Tn-Tm) switch on the thin (actin-containing) filaments. The only group that has been suggested to lack actin-linked regulation is the mollusks, where contraction is regulated through the myosin heads on the thick filaments. However, molluscan gene sequence data suggest the presence of troponin (Tn) components, consistent with actin-linked regulation, and some biochemical and immunological data also support this idea. The presence of actin-linked (in addition to myosin-linked) regulation in mollusks would simplify our general picture of muscle regulation by extending actin-linked regulation to this phylum as well. We have investigated this question structurally by determining the effect of Ca(2+) on the position of Tm in native thin filaments from scallop striated adductor muscle. Three-dimensional reconstructions of negatively stained filaments were determined by electron microscopy and single-particle image analysis. At low Ca(2+), Tm appeared to occupy the "blocking" position, on the outer domain of actin, identified in earlier studies of regulated thin filaments in the low-Ca(2+) state. In this position, Tm would sterically block myosin binding, switching off filament activity. At high Ca(2+), Tm appeared to move toward a position on the inner domain, similar to that induced by Ca(2+) in regulated thin filaments. This Ca(2+)-induced movement of Tm is consistent with the hypothesis that scallop thin filaments are Ca(2+) regulated.

Effect of actin C-terminal modification on tropomyosin isoforms binding and thin filament regulation

Tropomyosins, a family of actin-binding regulatory proteins, are present in muscle and non-muscle cells. Multiple tropomyosin (TM) isoforms differ in actin affinity and regulatory properties, but little is known about the molecular bases of these differences. The C-terminus of actin stabilizes contacts between actin subunits in the filament and interacts with myosin and regulatory proteins. The goal of this work was to reveal how structural changes in actin and differences between TM isoforms affect binding between these proteins and affect thin filament regulation. Actin proteolytically truncated by three C-terminal amino acids exhibited 1.2–1.5 fold reduced affinity for non-muscle and smooth muscle tropomyosin isoforms. The truncation increased the cooperativity of myosin S1-induced tropomyosin binding for short tropomyosins (TM5a and TM1b9a), but it was neutral for long isoforms (smTM and TM2). Actin modification affected regulation of actomyosin ATPase activity in the presence of all tropomyosins by shifting the filament into a more active state. We conclude that the integrity of the actin C-terminus is important for actin–tropomyosin interactions, however the increased affinity of tropomyosin binding in the S1-induced state of the filament appears not to be involved in the tropomyosin isoform-dependent mechanism of the actomyosin ATPase activation.

Invertebrate muscles: thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

This is the second in a series of canonical reviews on invertebrate muscle. We cover here thin and thick filament structure, the molecular basis of force generation and its regulation, and two special properties of some invertebrate muscle, catch and asynchronous muscle. Invertebrate thin filaments resemble vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, alternatively, are very different from vertebrate striated thick filaments and show great variation within invertebrates. Part of this diversity stems from variation in paramyosin content, which is greatly increased in very large diameter invertebrate thick filaments. Other of it arises from relatively small changes in filament backbone structure, which results in filaments with grossly similar myosin head placements (rotating crowns of heads every 14.5 nm) but large changes in detail (distances between heads in azimuthal registration varying from three to thousands of crowns). The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates this process is understood on the near atomic level. Invertebrate actomyosin is both thin (tropomyosin:troponin) and thick (primarily via direct Ca(++) binding to myosin) filament regulated, and most invertebrate muscles are dually regulated. These mechanisms are well understood on the molecular level, but the behavioral utility of dual regulation is less so. The phosphorylation state of the thick filament associated giant protein, twitchin, has been recently shown to be the molecular basis of catch. The molecular basis of the stretch activation underlying asynchronous muscle activity, however, remains unresolved.

Tropomyosin-based regulation of the actin cytoskeleton in time and space

Tropomyosins are rodlike coiled coil dimers that form continuous polymers along the major groove of most actin filaments. In striated muscle, tropomyosin regulates the actin-myosin interaction and, hence, contraction of muscle. Tropomyosin also contributes to most, if not all, functions of the actin cytoskeleton, and its role is essential for the viability of a wide range of organisms. The ability of tropomyosin to contribute to the many functions of the actin cytoskeleton is related to the temporal and spatial regulation of expression of tropomyosin isoforms. Qualitative and quantitative changes in tropomyosin isoform expression accompany morphogenesis in a range of cell types. The isoforms are segregated to different intracellular pools of actin filaments and confer different properties to these filaments. Mutations in tropomyosins are directly involved in cardiac and skeletal muscle diseases. Alterations in tropomyosin expression directly contribute to the growth and spread of cancer. The functional specificity of tropomyosins is related to the collaborative interactions of the isoforms with different actin binding proteins such as cofilin, gelsolin, Arp 2/3, myosin, caldesmon, and tropomodulin. It is proposed that local changes in signaling activity may be sufficient to drive the assembly of isoform-specific complexes at different intracellular sites.

Modulation of actin mechanics by caldesmon and tropomyosin

The ability of cells to sense and respond to physiological forces relies on the actin cytoskeleton, a dynamic structure that can directly convert forces into biochemical signals. Because of the association of muscle actin-binding proteins (ABPs) may affect F-actin and hence cytoskeleton mechanics, we investigated the effects of several ABPs on the mechanical properties of the actin filaments. The structural interactions between ABPs and helical actin filaments can vary between interstrand interactions that bridge azimuthally adjacent actin monomers between filament strands (i.e. by molecular stapling as proposed for caldesmon) or, intrastrand interactions that reinforce axially adjacent actin monomers along strands (i.e. as in the interaction of tropomyosin with actin). Here, we analyzed thermally driven fluctuations in actin's shape to measure the flexural rigidity of actin filaments with different ABPs bound. We show that the binding of phalloidin increases the persistence length of actin by 1.9-fold. Similarly, the intrastrand reinforcement by smooth and skeletal muscle tropomyosins increases the persistence length 1.5- and 2- fold respectively. We also show that the interstrand crosslinking by the C-terminal actin-binding fragment of caldesmon, H32K, increases persistence length by 1.6-fold. While still remaining bound to actin, phosphorylation of H32K by ERK abolishes the molecular staple (Foster et al. 2004. J Biol Chem 279;53387-53394) and reduces filament rigidity to that of actin with no ABPs bound. Lastly, we show that the effect of binding both smooth muscle tropomyosin and H32K is not additive. The combination of structural and mechanical studies on ABP-actin interactions will help provide information about the biophysical mechanism of force transduction in cells.

Cooperative binding of tropomyosin to actin

Tropomyosin molecules attach to the thin filament conjointly rather than separately, in a pattern indicating very high cooperativity. The equilibrium process drawing tropomyosins together on the actin filament can be measured by application ofa linear lattice model to bindingisotherm data and hypotheses on the mechanism of cooperativity can be tested. Each end of tropomyosin overlaps and attaches to the end ofa neighboring tropomyosin, facilitating the formation of continuous tropomyosin strands, without gaps between neighboring molecules along the thin filament. Interestingly, the overlap complexes vary greatly in size and composition among tropomyosin isoforms, despite consistently cooperative binding to actin. Also, the tendency of tropomyosin to bind to actin cooperatively rather than randomly does not correlate with the strength ofend-to-end binding.By implication, tropomyosin's actin-binding cooperativity likely involves effects on the actin filament, as well as direct interactions between adjacent tropomyosins.

Tropomyosins as discriminators of myosin function

Vertebrate nonmuscle cells express multiple tropomyosin isoforms that are sorted to subcellular compartments that have distinct morphological and dynamic properties. The creation of these compartments has a role in controlling cell morphology, cell migration and polarization of cellular components. There is increasing evidence that nonmuscle myosins are regulated by tropomyosin in these compartments via the regulation of actin attachment, ATPase kinetics, or by stabilization of cytoskeletal tracks for myosin-based transport. In this chapter, I review the literature describing the regulation of various myosins by tropomyosins and consider the mechanisms for this regulation.

Human tropomyosin isoforms in the regulation of cytoskeleton functions

Over the past two decades, extensive molecular studies have identified multiple tropomyosin isoforms existing in all mammalian cells and tissues. In humans, tropomyosins are encoded by TPM1 (alpha-Tm, 15q22.1), TPM2 (beta-Tm, 9p13.2-p13.1), TPM3 (gamma-Tm, 1q21.2) and TPM4 (delta-Tm, 19p13.1) genes. Through the use of different promoters, alternatively spliced exons and different sites of poly(A) addition signals, at least 22 different tropomyosin cDNAs with full-length open reading frame have been cloned. Compelling evidence suggests that these isoforms play important determinants for actin cytoskeleton functions, such as intracellular vesicle movement, cell migration, cytokinesis, cell proliferation and apoptosis. In vitro biochemical studies and in vivo localization studies suggest that different tropomyosin isoforms have differences in their actin-binding properties and their effects on other actin-binding protein functions and thus, in their specification ofactin microfilaments. In this chapter, we will review what has been learned from experimental studies on human tropomyosin isoforms about the mechanisms for differential localization and functions of tropomyosin. First, we summarize current information concerning human tropomyosin isoforms and relate this to the functions of structural homologues in rodents. We will discuss general strategies for differential localization oftropomyosin isoforms, particularly focusing on differential protein turnover and differential isoform effects on other actin binding protein functions. We will then review tropomyosin functions in regulating cell motility and in modulating the anti-angiogenic activity of cleaved high molecular weight kininogen (HKa) and discuss future directions in this area.

Role of tropomyosin in the regulation of contraction in smooth muscle

Smooth muscle contraction is due to the interaction ofmyosin filaments with thin filaments. Thin filaments are composed of actin, tropomyosin, caldesmon and calmodulin in ratios 14:2:1:1. Tissue specific isoforms of act and beta tropomyosin are expressed in smooth muscle. Compared with skeletal muscle tropomyosin, the cooperative activation of actomyosin is enhanced by smooth muscle tropomyosin: cooperative unit size is 10 and the equilibrium between on and off states is shifted towards the on state. The smooth muscle-specific actin-bindingprotein caldesmon, together with calmodulin regulates the activity of the thin filament in response to Ca2+. Caldesmon and calmodulin control the tropomyosin-mediated transition between on and offactivity states.

Tropomyosin and the steric mechanism of muscle regulation

Contraction in all muscles must be precisely regulated and requisite control systems must be able to adjust to changes in physiological and myopathic stimuli. In this chapter, we outline the structural evidence for a steric mechanism that governs muscle activity. The mechanism involves calcium and myosin induced changes in the position of tropomyosin along actin-based thin filaments. This process either blocks or uncovers myosin crossbridge binding sites on actin and consequently regulates crossbridge cycling on thin filaments, the sliding of thin and thick filaments and muscle shortening and force production.

The regulation of myosin binding to actin filaments by Lethocerus troponin

Lethocerus indirect flight muscle has two isoforms of troponin C, TnC-F1 and F2, which are unusual in having only a single C-terminal calcium binding site (site IV, isoform F1) or one C-terminal and one N-terminal site (sites IV and II, isoform F2). We show here that thin filaments assembled from rabbit actin and Lethocerus tropomyosin (Tm) and troponin (Tn) regulate the binding of rabbit myosin to rabbit actin in much the same way as the mammalian regulatory proteins. The removal of calcium reduces the rate constant for S1 binding to regulated actin about threefold, independent of which TmTn is used. This is consistent with calcium removal causing the TmTn to occupy the B or blocked state to about 70% of the total. The mid point pCa for the switch differed for TnC-F1 and F2 (pCa 6.9 and 6.0, respectively) consistent with the reported calcium affinities for the two TnCs. Equilibrium titration of S1 binding to regulated actin filaments confirms calcium regulated binding of S1 to actin and shows that in the absence of calcium the three actin filaments (TnC-F1, TnC-F2 and mammalian control) are almost indistinguishable in terms of occupancy of the B and C states of the filament. In the presence of calcium TnC-F2 is very similar to the control with approximately 80% of the filament in the C-state and 10-15% in the fully on M-State while TnC-F1 has almost 50% in each of the C and M states. This higher occupancy of the M-state for TnC-F1, which occurs above pCa 6.9, is consistent with this isoform being involved in the calcium activation of stretch activation. However, it leaves unanswered how a C-terminal calcium binding site of TnC can activate the thin filament.

Influence of enhanced troponin C Ca2+-binding affinity on cooperative thin filament activation in rabbit skeletal muscle

We studied how enhanced skeletal troponin C (sTnC) Ca2+-binding affinity affects cooperative thin filament activation and contraction in single demembranated rabbit psoas fibres. Three sTnC mutants were created and incorporated into skeletal troponin (sTn) for measurement of Ca2+ dissociation, resulting in the following order of rates: wild-type (WT) sTnC-sTn>sTnC(F27W)-sTn>M80Q sTnC-sTn>M80Q sTnCF27W-sTn. Reconstitution of sTnC-extracted fibres increased Ca2+ sensitivity of steady-state force (pCa(50)) by 0.08 for M80Q sTnC, 0.15 for sTnCF27W and 0.32 for M80Q sTnCF27W with minimal loss of slope (nH, degree of cooperativity). Near-neighbour thin filament regulatory unit (RU) interactions were reduced in fibres by incorporating mixtures of WT or mutant sTnC and D28A, D64A sTnC (xxsTnC) that does not bind Ca2+ at N-terminal sites. Reconstitution with sTnC: xxsTnC mixtures to 20% of pre-exchanged maximal force reduced pCa50 by 0.35 for sTnC: xxsTnC, 0.25 for M80Q sTnC: xxsTnC, and 0.10 for M80Q sTnCF27W: xxsTnC. It is interesting that pCa50 increased by approximately 0.1 for M80Q sTnC and approximately 0.3 for M80Q sTnCF27W when near-neighbour RU interactions were reduced; these values are similar in magnitude to those for fibres reconstituted with 100% mutant sTnC. After reconstitution with sTnC: xxsTnC mixtures, nH decreased to a similar value for all mutant sTnCs. Altered sTnC Ca2+-binding properties (M80Q sTnCF27W) did not affect strong crossbridge inhibition by 2,3-butanedione monoxime when near-neighbour thin filament RU interactions were reduced. Together these results suggest increased sTnC Ca2+ affinity strongly influences Ca2+ sensitivity of steady-state force without affecting near-neighbour thin filament RU cooperative activation or the relative contribution of crossbridges versus Ca2+ to thin filament activation.

Acetylation regulates tropomyosin function in the fission yeast Schizosaccharomyces pombe

Tropomyosin is an evolutionarily conserved alpha-helical coiled-coil protein that promotes and maintains actin filaments. In yeast, Tropomyosin-stabilised filaments are used by molecular motors to transport cargoes or to generate motile forces by altering the dynamics of filament growth and shrinkage. The Schizosaccharomyces pombe tropomyosin Cdc8 localises to the cytokinetic actomyosin ring during mitosis and is absolutely required for its formation and function. We show that Cdc8 associates with actin filaments throughout the cell cycle and is subjected to post-translational modification that does not vary with cell cycle progression. At any given point in the cell cycle 80% of Cdc8 molecules are acetylated, which significantly enhances their affinity for actin. Reconstructions of electron microscopic images of actin-Cdc8 filaments establish that the majority of Cdc8 strands sit in the 'closed' position on actin filaments, suggesting a role in the regulation of myosin binding. We show that Cdc8 regulates the equilibrium binding of myosin to actin without affecting the rate of myosin binding. Unacetylated Cdc8 isoforms bind actin, but have a reduced ability to regulate myosin binding to actin. We conclude that although acetylation of Cdc8 is not essential, it provides a regulatory mechanism for modulating actin filament integrity and myosin function.

Effects of a R133W beta-tropomyosin mutation on regulation of muscle contraction in single human muscle fibres

A novel R133W beta-tropomyosin (beta-Tm) mutation, associated with muscle weakness and distal limb deformities, has recently been identified in a woman and her daughter. The muscle weakness was not accompanied by progressive muscle wasting or histopathological abnormalities in tibialis anterior muscle biopsy specimens. The aim of the present study was to explore the mechanisms underlying the impaired muscle function in patients with the beta-Tm mutation. Maximum force normalized to fibre cross-sectional area (specific force, SF), maximum velocity of unloaded shortening (V0), apparent rate constant of force redevelopment (ktr) and force-pCa relationship were evaluated in single chemically skinned muscle fibres from the two patients carrying the beta-Tm mutation and from healthy control subjects. Significant differences in regulation of muscle contraction were observed in the type I fibres: a lower SF (P<0.05) and ktr (P<0.01), and a faster V0 (P<0.05). The force-pCa relationship did not differ between patient and control fibres, indicating an unaltered Ca2+ activation of contractile proteins. Collectively, these results indicate a slower cross-bridge attachment rate and a faster detachment rate caused by the R133W beta-Tm mutation. It is suggested that the R133W beta-Tm mutation induces alteration in myosin-actin kinetics causing a reduced number of myosin molecules in the strong actin-binding state, resulting in overall muscle weakness in the absence of muscle wasting.

Investigation of thin filament near-neighbour regulatory unit interactions during force development in skinned cardiac and skeletal muscle

Ca(2+)-dependent activation of striated muscle involves cooperative interactions of cross-bridges and thin filament regulatory proteins. We investigated how interactions between individual structural regulatory units (RUs; 1 tropomyosin, 1 troponin, 7 actins) influence the level and rate of demembranated (skinned) cardiac muscle force development by exchanging native cardiac troponin (cTn) with different ratio mixtures of wild-type (WT) cTn and cTn containing WT cardiac troponin T/I + cardiac troponin C (cTnC) D65A (a site II inactive cTnC mutant). Maximal Ca(2+)-activated force (F(max)) increased in less than a linear manner with WT cTn. This contrasts with results we obtained previously in skeletal fibres (using sTnC D28A, D65A) where F(max) increased in a greater than linear manner with WT sTnC, and suggests that Ca(2+) binding to each functional Tn activates < 7 actins of a structural regulatory unit in cardiac muscle and > 7 actins in skeletal muscle. The Ca(2+) sensitivity of force and rate of force redevelopment (k(tr)) was leftward shifted by 0.1-0.2 -log [Ca(2+)] (pCa) units as WT cTn content was increased, but the slope of the force-pCa relation and maximal k(tr) were unaffected by loss of near-neighbour RU interactions. Cross-bridge inhibition (with butanedione monoxime) or augmentation (with 2 deoxy-ATP) had no greater effect in cardiac muscle with disruption of near-neighbour RU interactions, in contrast to skeletal muscle fibres where the effect was enhanced. The rate of Ca(2+) dissociation was found to be > 2-fold faster from whole cardiac Tn compared with skeletal Tn. Together the data suggest that in cardiac (as opposed to skeletal) muscle, Ca(2+) binding to individual Tn complexes is insufficient to completely activate their corresponding RUs, making thin filament activation level more dependent on concomitant Ca(2+) binding at neighbouring Tn sites and/or crossbridge feedback effects on Ca(2+) binding affinity.

Myosin Surface Loop 4 Modulates Inhibition of Actomyosin 1b ATPase Activity by Tropomyosin

Structural studies of the class I myosin, MyoE, led to the predictions that loop 4, a surface loop near the actin-binding region that is longer in class I myosins than in other myosin subclasses, might limit binding of myosins I to actin when actin-binding proteins, like tropomyosin, are present, and might account for the exclusion of myosin I from stress fibers. To test these hypotheses, mutant molecules of the related mammalian class I myosin, Myo1b, in which loop 4 was truncated (from an amino acid sequence of RMNGLDES to NGLD) or replaced with the shorter and distinct loop 4 found in Dictyostelium myosin II (GAGEGA), were expressed in vitro and their interaction with actin and with actin-tropomyosin was tested. Saturating amounts of expressed fibroblast tropomyosin-2 resulted in a decrease in the maximum actin-activated Mg2+-ATPase activity of wild-type Myo1b but had little or no effect on the actin-activated Mg2+-ATPase activity of the two mutants. In motility assays, few actin filaments bound tightly to Myo1b-WT-coated cover slips when tropomyosin-2 was present, whereas actin filaments both bound and were translocated by Myo1b-NGLD or Myo1b-GAGEGA in both the presence and absence of tropomyosin-2. When expressed in mammalian cells, like the wild type, the mutant myosins were largely excluded from tropomyosin-containing actin filaments, indicating that in the cell additional factors besides loop 4 determine targeting of myosins I to specific subpopulations of actin filaments.

Mutations in fast skeletal troponin I, troponin T, and β‐tropomyosin that cause distal arthrogryposis all increase contractile function

Distal arthrogryposes (DAs) are a group of disorders characterized by congenital contractures of distal limbs without overt neurological or muscle disease. Unexpectedly, mutations in genes encoding the fast skeletal muscle regulatory proteins troponin T (TnT), troponin I (TnI), and beta-tropomyosin (beta-TM) have been shown to cause autosomal dominant DA. We tested how these mutations affect contractile function by comparing wild-type (WT) and mutant proteins in actomyosin ATPase assays and in troponin-replaced rabbit psoas fibers. We have analyzed all four reported mutants: Arg63His TnT, Arg91Gly beta-TM, Arg174Gln TnI, and a TnI truncation mutant (Arg156ter). Thin filaments, reconstituted using actin and WT troponin and beta-TM, activated myosin subfragment-1 ATPase in a calcium-dependent, cooperative manner. Thin filaments containing either a troponin or beta-TM DA mutant produced significantly enhanced ATPase rates at all calcium concentrations without alternating calcium-sensitivity or cooperativity. In troponin-exchanged skinned fibers, each mutant caused a significant increase in Ca2+ sensitivity, and Arg156ter TnI generated significantly higher maximum force. Arg91Gly beta-TM was found to have a lower actin affinity than WT and form a less stable coiled coil. We propose the mutations cause increased contractility of developing fast-twitch skeletal muscles, thus causing muscle contractures and the development of the observed limb deformities.