Modulation of myosin function by isoform-specific properties of Saccharomyces cerevisiae and muscle tropomyosins

Skeletal muscle transcriptome in healthy aging

Age-associated changes in gene expression in skeletal muscle of healthy individuals reflect accumulation of damage and compensatory adaptations to preserve tissue integrity. To characterize these changes, RNA was extracted and sequenced from muscle biopsies collected from 53 healthy individuals (22-83 years old) of the GESTALT study of the National Institute on Aging-NIH. Expression levels of 57,205 protein-coding and non-coding RNAs were studied as a function of aging by linear and negative binomial regression models. From both models, 1134 RNAs changed significantly with age. The most differentially abundant mRNAs encoded proteins implicated in several age-related processes, including cellular senescence, insulin signaling, and myogenesis. Specific mRNA isoforms that changed significantly with age in skeletal muscle were enriched for proteins involved in oxidative phosphorylation and adipogenesis. Our study establishes a detailed framework of the global transcriptome and mRNA isoforms that govern muscle damage and homeostasis with age.

A role for actin flexibility in thin filament-mediated contractile regulation and myopathy

Striated muscle contraction is regulated by the translocation of troponin-tropomyosin strands over the thin filament surface. Relaxation relies partly on highly-favorable, conformation-dependent electrostatic contacts between actin and tropomyosin, which position tropomyosin such that it impedes actomyosin associations. Impaired relaxation and hypercontractile properties are hallmarks of various muscle disorders. The α-cardiac actin M305L hypertrophic cardiomyopathy-causing mutation lies near residues that help confine tropomyosin to an inhibitory position along thin filaments. Here, we investigate M305L actin in vivo, in vitro, and in silico to resolve emergent pathological properties and disease mechanisms. Our data suggest the mutation reduces actin flexibility and distorts the actin-tropomyosin electrostatic energy landscape that, in muscle, result in aberrant contractile inhibition and excessive force. Thus, actin flexibility may be required to establish and maintain interfacial contacts with tropomyosin as well as facilitate its movement over distinct actin surface features and is, therefore, likely necessary for proper regulation of contraction.

The α-cardiac actin M305L hypertrophic cardiomyopathy-causing mutation is located near residues that help confine tropomyosin to an inhibitory position along thin filaments. Here the authors assessed M305L actin in vivo, in vitro, and in silico to characterize emergent pathological properties and define the mechanistic basis of disease.

The actin 'A-triad's' role in contractile regulation in health and disease

Striated muscle contraction is regulated by Ca²⁺ -dependent modulation of myosin cross-bridge binding to F-actin by the thin filament troponin (Tn)-tropomyosin (Tm) complex. In the absence of Ca²⁺ , Tn binds to actin and constrains Tm to an azimuthal location where it sterically occludes myosin binding sites along the thin filament surface. This limits force production and promotes muscle relaxation. In addition to Tn-actin interactions, inhibitory Tm positioning requires associations between other thin filament constituents. For example, the actin 'A-triad', composed of residues K326, K328 and R147, forms numerous, highly favourable electrostatic contacts with Tm that are critical for establishing its inhibitory azimuthal binding position. Here, we review recent findings, including the identification and interrogation of modifications within and proximal to the A-triad that are associated with disease and/or altered muscle behaviour, which highlight the surface feature's role in F-actin-Tm interactions and contractile regulation.

A molecular evolution approach to study the roles of tropomyosin in fission yeast

Tropomyosin, a coiled-coil protein that binds along the length of the actin filament, is a universal regulator of the actin cytoskeleton. We have taken a bioinformatics/proteomic approach to studying structure-function relationships in this protein. The presence of a single, essential tropomyosin gene, cdc8, in fission yeast, Schizosaccharomyces pombe, enables a systems-based approach to define the residues that are important for cellular functions. Using molecular evolution methodologies we identified the most conserved residues and related them to the coiled coil structure. Mutants in which one or more of 21 of the most conserved surface residues was mutated to Ala were tested for the ability to rescue growth of a temperature-sensitive cdc8 mutant when overexpressed at the restrictive temperature. Based on altered morphology of the septum and actin cytoskeleton, we selected three sets of mutations for construction of mutant cdc8 strains using marker reconstitution mutagenesis and analysis of recombinant protein in vitro: D16A.K30A, V114S.E117A.H118A and R121A.D131A.E138A. The mutations have sequence-specific effects on cellular morphology including cell length, organization of cytoskeletal structures (actin patches, actin cables and contractile rings), and in vitro actin affinity, lending credence to the proteomic approach introduced here. We propose that bioinformatics is a valid analysis tool for defining structure-function relationships in conserved proteins in this model organism.

Periodicities designed in the tropomyosin sequence and structure define its functions

Tropomyosin is an actin binding protein that regulates actin filament dynamics and its interactions with actin binding proteins such as myosin, tropomodulin, formin, Arp2/3 and ADF-cofilin in most eukaryotic cells. Tropomyosin is the prototypical two-chained, α-helical coiled coil protein that associates end-to-end and binds to both sides of the actin filament. Each tropomyosin molecule spans four to seven actin monomers in the filament, depending on the size of the tropomyosin. Tropomyosins have a periodic heptad repeat sequence that is characteristic of coiled coil proteins as well as additional periodicities required for its interaction with the actin filament, where each periodic repeat interacts with one actin molecule. This review addresses the role of periodic features of the Tm molecule in carrying out its universal functions of binding to the actin filament and its regulation and the specific features that may determine the isoform specificity of tropomyosins.

Tropomyosin is essential for processive movement of a class V myosin from budding yeast

Myosin V is an actin-based motor protein involved in intracellular cargo transport [1]. Given this physiological role, it was widely assumed that all class V myosins are processive, able to take multiple steps along actin filaments without dissociating. This notion was challenged when several class V myosins were characterized as nonprocessive in vitro, including Myo2p, the essential class V myosin from S. cerevisiae [2-6]. Myo2p moves cargo including secretory vesicles and other organelles for several microns along actin cables in vivo. This demonstrated cargo transporter must therefore either operate in small ensembles or behave processively in the cellular context. Here we show that Myo2p moves processively in vitro as a single motor when it walks on an actin track that more closely resembles the actin cables found in vivo. The key to processivity is tropomyosin: Myo2p is not processive on bare actin but highly processive on actin-tropomyosin. The major yeast tropomyosin isoform, Tpm1p, supports the most robust processivity. Tropomyosin slows the rate of MgADP release, thus increasing the time the motor spends strongly attached to actin. This is the first example of tropomyosin switching a motor from nonprocessive to processive motion on actin.

Tropomyosin: double helix from the protein world

This review concerns the structure and functions of tropomyosin (TM), an actin-binding protein that plays a key role in the regulation of muscle contraction. The TM molecule is a dimer of α-helices, which form a coiled-coil. Recent views on the TM structure are analyzed, and special attention is concentrated on those structural traits of the TM molecule that distinguish it from the other coiled-coil proteins. Modern data are presented on TM functional properties, such as its interaction with actin and ability to move on the surface of actin filaments, which underlies the regulation of the actin-myosin interaction upon contraction of skeletal and cardiac muscles. Also, part of the review is devoted to analysis of the effects of mutations in TM genes associated with muscle diseases (myopathies) on the structure and functions of TM.

Tropomyosin isoforms and reagents

Tropomyosins are rod-like dimers which form head-to-tail polymers along the length of actin filaments and regulate the access of actin binding proteins to the filaments.1 The diversity of tropomyosin isoforms, over 40 in mammals, and their role in an increasing number of biological processes presents a challenge both to experienced researchers and those new to this field. The increased appreciation that the role of these isoforms expands beyond that of simply stabilizing actin filaments has lead to a surge of reagents and techniques to study their function and mechanisms of action. This report is designed to provide a basic guide to the genes and proteins and the availability of reagents which allow effective study of this family of proteins. We highlight the value of combining multiple techniques to better evaluate the function of different tm isoforms and discuss the limitations of selected reagents. Brief background material is included to demystify some of the unfortunate complexity regarding this multi-gene family of proteins including the unconventional nomenclature of the isoforms and the evolutionary relationships of isoforms between species. Additionally, we present step-by-step detailed experimental protocols used in our laboratory to assist new comers to the field and experts alike.

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.

Dual regulatory functions of the thin filament revealed by replacement of the troponin I inhibitory peptide with a linker

Striated muscles are relaxed under low Ca(2+) concentration conditions due to actions of the thin filament protein troponin. To investigate this regulatory mechanism, an 11-residue segment of cardiac troponin I previously termed the inhibitory peptide region was studied by mutagenesis. Several mutant troponin complexes were characterized in which specific effects of the inhibitory peptide region were abrogated by replacements of 4-10 residues with Gly-Ala linkers. The mutations greatly impaired two of troponin's actions under low Ca(2+) concentration conditions: inhibition of myosin subfragment 1 (S1)-thin filament MgATPase activity and cooperative suppression of myosin S1-ADP binding to thin filaments with low myosin saturation. Inhibitory peptide replacement diminished but did not abolish the Ca(2+) dependence of the ATPase rate; ATPase rates were at least 2-fold greater when Ca(2+) rather than EGTA was present. This residual regulation was highly cooperative as a function of Ca(2+) concentration, similar to the degree of cooperativity observed with WT troponin present. Other effects of the mutations included 2-fold or less increases in the apparent affinity of the thin filament regulatory Ca(2+) sites, similar decreases in the affinity of troponin for actin-tropomyosin regardless of Ca(2+), and increases in myosin S1-thin filament ATPase rates in the presence of saturating Ca(2+). The overall results indicate that cooperative myosin binding to Ca(2+)-free thin filaments depends upon the inhibitory peptide region but that a cooperatively activating effect of Ca(2+) binding does not. The findings suggest that these two processes are separable and involve different conformational changes in the thin filament.

Tropomyosin and myosin-II cellular levels promote actomyosin ring assembly in fission yeast

A combination of in vivo and in vitro approaches were used to show how tropomyosin and myosin-II contribute to contractile ring assembly in fission yeast. Ring assembly is sensitive to changes in the cellular levels of myosin-II, and tropomyosin works to maximize myosin-II motor function during this process by stabilizing actomyosin interactions.

Myosin-II (Myo2p) and tropomyosin are essential for contractile ring formation and cytokinesis in fission yeast. Here we used a combination of in vivo and in vitro approaches to understand how these proteins function at contractile rings. We find that ring assembly is delayed in Myo2p motor and tropomyosin mutants, but occurs prematurely in cells engineered to express two copies of myo2. Thus, the timing of ring assembly responds to changes in Myo2p cellular levels and motor activity, and the emergence of tropomyosin-bound actin filaments. Doubling Myo2p levels suppresses defects in ring assembly associated with a tropomyosin mutant, suggesting a role for tropomyosin in maximizing Myo2p function. Correspondingly, tropomyosin increases Myo2p actin affinity and ATPase activity and promotes Myo2p-driven actin filament gliding in motility assays. Tropomyosin achieves this by favoring the strong actin-bound state of Myo2p. This mode of regulation reflects a role for tropomyosin in specifying and stabilizing actomyosin interactions, which facilitates contractile ring assembly in the fission yeast system.

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.

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.

Differential interaction of cardiac, skeletal muscle, and yeast tropomyosins with fluorescent (pyrene235) yeast actin

To monitor binding of tropomyosin to yeast actin, we mutated S235 to C and labeled the actin with pyrene maleimide at both C235 and the normally reactive C374. Saturating cardiac tropomyosin (cTM) caused about a 20% increase in pyrene fluorescence of the doubly labeled F-actin but no change in WT actin C374 probe fluorescence. Skeletal muscle tropomyosin caused only a 7% fluorescence increase, suggesting differential binding modes for the two tropomyosins. The increased cTM-induced fluorescence was proportional to the extent of tropomyosin binding. Yeast tropomyosin (TPM1) produced less increase in fluorescence than did cTM, whereas that caused by yeast TPM2 was greater than either TPM1 or cTM. Cardiac troponin largely reversed the cTM-induced fluorescence increase, and subsequent addition of calcium resulted in a small fluorescence recovery. An A230Y mutation, which causes a Ca(+2)-dependent hypercontractile response of regulated thin filaments, did not change probe235 fluorescence of actin alone or with tropomyosin +/- troponin. However, addition of calcium resulted in twice the fluorescence recovery observed with WT actin. Our results demonstrate isoform-specific binding of different tropomyosins to actin and suggest allosteric regulation of the tropomyosin/actin interaction across the actin interdomain cleft.

The essential role of PP1beta in Drosophila is to regulate nonmuscle myosin

Reversible phosphorylation of myosin regulatory light chain (MRLC) is a key regulatory mechanism controlling myosin activity and thus regulating the actin/myosin cytoskeleton. We show that Drosophila PP1beta, a specific isoform of serine/threonine protein phosphatase 1 (PP1), regulates nonmuscle myosin and that this is the essential role of PP1beta. Loss of PP1beta leads to increased levels of phosphorylated nonmuscle MRLC (Sqh) and actin disorganisation; these phenotypes can be suppressed by reducing the amount of active myosin. Drosophila has two nonmuscle myosin targeting subunits, one of which (MYPT-75D) resembles MYPT3, binds specifically to PP1beta, and activates PP1beta's Sqh phosphatase activity. Expression of a mutant form of MYPT-75D that is unable to bind PP1 results in elevation of Sqh phosphorylation in vivo and leads to phenotypes that can also be suppressed by reducing the amount of active myosin. The similarity between fly and human PP1beta and MYPT genes suggests this role may be conserved.

The DNase-I binding loop of actin may play a role in the regulation of actin-myosin interaction by tropomyosin/troponin

Various lines of evidence suggest that communication between tropomyosin and myosin in the regulation of vertebrate-striated muscle contraction involves yet unknown changes in actin conformation. Possible participation of loop 38-52 in this communication has recently been questioned based on unimpaired Ca(2+) regulation of myosin interaction, in the presence of the tropomyosin-troponin complex, with actin cleaved by subtilisin between Met(47) and Gly(48). We have compared the effects of actin cleavage by subtilisin and by protease ECP32, between Gly(42) and Val(43), on its interaction with myosin S1 in the presence and absence of tropomyosin or tropomyosin-troponin. Both individual modifications reduced activation of S1 ATPase by actin to a similar extent. The effect of ECP cleavage, but not of subtilisin cleavage, was partially reversed by stabilization of interprotomer contacts with phalloidin, indicating different pathways of signal transmission from the N- and C-terminal parts of loop 38-52 to myosin binding sites. ECP cleavage diminished the affinity to tropomyosin and reduced its inhibition of acto-S1 ATPase at low S1 concentrations, but increased the tropomyosin-mediated cooperative enhancement of the ATPase by S1 binding to actin. These effects were reversed by phalloidin. Subtilisin-cleaved actin more closely resembled unmodified actin than the ECP-modified actin. Limited proteolysis of the modified and unmodified F-actins revealed an allosteric effect of ECP cleavage on the conformation of the actin subdomain 4 region that is presumably involved in tropomyosin binding. Our results point to a possible role of the N-terminal part of loop 38-52 of actin in communication between tropomyosin and myosin through changes in actin structure.

N Terminus Is Essential for Tropomyosin Functions

Down-regulation of several key actin-binding proteins, such as alpha-actinin, vinculin, gelsolin, and tropomyosins (TMs), is considered to contribute to the disorganized cytoskeleton present in many neoplastic cells. TMs stabilize actin filaments against the gel severing actions of proteins such as cofilin. Among multiple TMs expressed in non-muscle cells, tropomyosin-1 (TM1) isoform induces stress fibers and functions as a suppressor of malignant transformation. However, the molecular mechanisms of TM1-mediated cytoskeletal effects and tumor suppression remain poorly understood. We have hypothesized that the ability of TM1 to stabilize microfilaments is crucial for tumor suppression. In this study, by employing a variant TM1, which contains an N-terminal hemagglutinin epitope tag, we demonstrate that the N terminus is a key determinant of tropomyosin-1 function. Unlike the wild type TM1, the modified protein fails to restore stress fibers and inhibit anchorage-independent growth in transformed cells. Furthermore, the N-terminal modification of TM1 disorganizes the cytoskeleton and delays cytokinesis in normal cells, abolishes binding to F-actin, and disrupts the dimeric associations in vivo. The functionally defective TM1 allows the association of cofilin to stress fibers and disorganizes the microfilaments, whereas wild type TM1 appears to restrict the binding of cofilin to stress fibers. TM1-induced cytoskeletal reorganization appears to be mediated through preventing cofilin interaction with microfilaments. Our studies provide in vivo functional evidence that the N terminus is a critical determinant of TM1 functions, which in turn determines the organization of stress fibers.

Local Destabilization of the Tropomyosin Coiled Coil Gives the Molecular Flexibility Required for Actin Binding†

Tropomyosin, a coiled coil protein that binds along the length of actin filaments, contains 40 uninterrupted heptapeptide repeats characteristic of coiled coils. Yet, it is flexible. Regions of tropomyosin that may be important for binding to the filament and for interacting with troponin deviate from canonical coiled coil structure in subtle ways, altering the local conformation or energetics without interrupting the coiled coil. In a region rich in interface alanines (an Ala cluster), the chains pack closer than in canonical coiled coils, and are staggered, resulting in a bend [Brown et al. (2001) Proc. Natl. Acad. Sci. U.S.A. 98, 8496-8501]. Brown et al. suggested that bends at alanine clusters allow tropomyosin to wind on the actin filament helix. Another explanation is that local destabilization of the coiled coil, rather than close packing of the chains at Ala clusters per se, allows flexibility. Changing three Ala residues to canonical interface residues, A74L-A78V-A81L, greatly stabilized tropomyosin, measured using circular dichroism and differential scanning calorimetry, and reduced actin affinity >10-fold. Normal actin affinity and stability were restored in a mutant A74Q-A78N-A81Q that mimicked the stability of the Ala cluster but not the close packing of the chains. Analysis and modeling of comparable mutations introduced closer to the N-terminus show that the effects on stability and function depend on context. Models based on tropomyosin crystal structures give insight into possible effects of the mutations on the structure. We conclude that the significance of the Ala clusters in allowing flexibility of tropomyosin is stability-driven.

Polarization of specific tropomyosin isoforms in gastrointestinal epithelial cells and their impact on CFTR at the apical surface

Microfilaments have been reported to be polarized in a number of cell types based both on function and isoform composition. There is evidence that microfilaments are involved in the movement of vesicles and the polarized delivery of proteins to specialized membrane domains. We have investigated the composition of actin microfilaments in gastrointestinal epithelial cells and their role in the delivery of the cystic fibrosis transmembrane conductance regulator (CFTR) into the apical membrane using cultured T84 cells as a model. We identified a specific population of microfilaments containing the tropomyosin (Tm) isoforms Tm5a and/or Tm5b, which are polarized in T84 cell monolayers. Polarization of this microfilament population occurs very rapidly in response to cell-cell and cell-substratum contact and is not inhibited by jasplakinolide, suggesting this involves the movement of intact filaments. Colocalization of Tm5a and/or Tm5b and CFTR was observed in long-term cultures. A reduction in Tm5a and Tm5b expression, induced using antisense oligonucleotides, resulted in an increase in both CFTR surface expression and chloride efflux in response to cAMP stimulation. We conclude that Tm isoforms Tm5a and/or Tm5b mark an apical population of microfilaments that can regulate the insertion and/or retention of CFTR into the plasma membrane.

Cardiomyopathic tropomyosin mutations that increase thin filament Ca2+ sensitivity and tropomyosin N-domain flexibility

The relationship between tropomyosin thermal stability and thin filament activation was explored using two N-domain mutants of alpha-striated muscle tropomyosin, A63V and K70T, each previously implicated in familial hypertrophic cardiomyopathy. Both mutations had prominent effects on tropomyosin thermal stability as monitored by circular dichroism. Wild type tropomyosin unfolded in two transitions, separated by 10 degrees C. The A63V and K70T mutations decreased the melting temperature of the more stable of these transitions by 4 and 10 degrees C, respectively, indicating destabilization of the N-domain in both cases. Global analysis of all three proteins indicated that the tropomyosin N-domain and C-domain fold with a cooperative free energy of 1.0-1.5 kcal/mol. The two mutations increased the apparent affinity of the regulatory Ca2+ binding sites of thin filament in two settings: Ca2+-dependent sliding speed of unloaded thin filaments in vitro (at both pH 7.4 and 6.3), and Ca2+ activation of the thin filament-myosin S1 ATPase rate. Neither mutation had more than small effects on the maximal ATPase rate in the presence of saturating Ca2+ or on the maximal sliding speed. Despite the increased tropomyosin flexibility implied by destabilization of the N-domain, neither the cooperativity of thin filament activation by Ca2+ nor the cooperative binding of myosin S1-ADP to the thin filament was altered by the mutations. The combined results suggest that a more dynamic tropomyosin N-domain influences interactions with actin and/or troponin that modulate Ca2+ sensitivity, but has an unexpectedly small effect on cooperative changes in tropomyosin position on actin.

Targeting of a tropomyosin isoform to short microfilaments associated with the Golgi complex

A growing body of evidence suggests that the Golgi complex contains an actin-based filament system. We have previously reported that one or more isoforms from the tropomyosin gene Tm5NM (also known as gamma-Tm), but not from either the alpha- or beta-Tm genes, are associated with Golgi-derived vesicles (Heimann et al., (1999). J. Biol. Chem. 274, 10743-10750). We now show that Tm5NM-2 is sorted specifically to the Golgi complex, whereas Tm5NM-1, which differs by a single alternatively spliced internal exon, is incorporated into stress fibers. Tm5NM-2 is localized to the Golgi complex consistently throughout the G1 phase of the cell cycle and it associates with Golgi membranes in a brefeldin A-sensitive and cytochalasin D-resistant manner. An actin antibody, which preferentially reacts with the ends of microfilaments, newly reveals a population of short actin filaments associated with the Golgi complex and particularly with Golgi-derived vesicles. Tm5NM-2 is also found on these short microfilaments. We conclude that an alternative splice choice can restrict the sorting of a tropomyosin isoform to short actin filaments associated with Golgi-derived vesicles. Our evidence points to a role for these Golgi-associated microfilaments in vesicle budding at the level of the Golgi complex.

Regulatory proteins alter nucleotide binding to acto-myosin of sliding filaments in motility assays

The sliding speed of unregulated thin filaments in motility assays is only about half that of the unloaded shortening velocity of muscle fibers. The addition of regulatory proteins, troponin and tropomyosin, is known to increase the sliding speed of thin filaments in the in vitro motility assay. To learn if this effect is related to the rate of MgADP dissociation from the acto-S1 cross-bridge head, the effects of regulatory proteins on nucleotide binding and release in motility assays were measured in the presence and absence of regulatory proteins. The apparent affinity of acto-heavy meromyosin (acto-HMM) for MgATP was reduced by the presence of regulatory proteins. Similarly, the regulatory proteins increase the concentration of MgADP required to inhibit sliding. These results suggest that regulatory proteins either accelerate the rate of MgADP release from acto-HMM-MgADP or slow its binding to acto-HMM. The reduction of temperature also altered the relationship between thin filament sliding speed and the regulatory proteins. At lower temperatures, the regulatory proteins lost their ability to increase thin filament sliding speed above that of unregulated thin filaments. It is hypothesized that structural changes in the actin portion of the acto-myosin interface are induced by regulatory protein binding to actin.

High-molecular-weight tropomyosins localize to the contractile rings of dividing CNS cells but are absent from malignant pediatric and adult CNS tumors

Tropomyosin has been implicated in the control of actin filament dynamics during cell migration, morphogenesis, and cytokinesis. In order to gain insight into the role of tropomyosins in cell division, we examined their expression in developing and neoplastic brain tissue. We found that the high-molecular-weight tropomyosins are downregulated at birth, which correlates with glial cell differentiation and withdrawal of most cells from the cell cycle. Expression of these isoforms was restricted to proliferative areas in the embryonic brain and was absent from the adult, where the majority of cells are quiescent. However, they were induced under conditions where glial cells became proliferative in response to injury. During cytokinesis, these tropomyosin isoforms were associated with the contractile ring. We also investigated tropomyosin expression in neoplastic CNS tissues. Low-grade astrocytic tumors expressed high-molecular-weight tropomyosins, while highly malignant CNS tumors of diverse origin did not (P </= 0.001). Furthermore, high-molecular-weight tropomyosins were absent from the contractile ring in highly malignant astrocytoma cells. Our findings suggest a role for high-molecular-weight tropomyosins in astrocyte cytokinesis, although highly malignant CNS tumors are still able to undergo cell division in their absence. Additionally, the correlation between high-molecular-weight tropomyosin expression and tumor grade suggests that tropomyosins are potentially useful as indicators of CNS tumor grade.

Folding and function of the troponin tail domain. Effects of cardiomyopathic troponin T mutations

Troponin contains a globular Ca(2+)-binding domain and an elongated tail domain composed of the N terminus of subunit troponin T (TnT). The tail domain anchors troponin to tropomyosin and actin, modulates myosin function, and is a site of cardiomyopathy-inducing mutations. Critical interactions between tropomyosin and troponin are proposed to depend on tail domain residues 112-136, which are highly conserved across phyla. Most cardiomyopathy mutations in TnT flank this region. Three such mutations were examined and had contrasting effects on peptide TnT-(1-156), promoting folding and thermal stability assessed by circular dichroism (F110I) or weakening folding and stability (T104V and to a small extent R92Q). Folding of both TnT-(1-156) and whole troponin was promoted by replacing bovine TnT Thr-104 with human TnT Ala-104, further indicating the importance of this cardiomyopathy site residue for protein folding. Mutation F110I markedly stabilized the troponin tail but weakened binding of holo-troponin to actin-tropomyosin 8-fold, suggesting that loss of flexibility impairs troponin tail function. The effect of the F110I mutation on troponin-tropomyosin binding to actin was much less, indicating this flexibility is particularly important for the interactions of troponin with tropomyosin. We suggest that most cardiomyopathic mutations in the troponin tail alter muscle function indirectly, by perturbing interactions between troponin and tropomyosin requisite for the complex effects of these proteins on myosin.

Tropomyosin 3 expression leads to hypercontractility and attenuates myofilament length-dependent Ca(2+) activation

Tropomyosin (TM), an integral component of the thin filament, is encoded by three striated muscle isoforms: alpha-TM, beta-TM, and TPM 3. Although the alpha-TM and beta-TM isoforms are well characterized, less is known about the function of the TPM 3 isoform, which is predominantly found in the slow-twitch musculature of mammals. To determine its functional significance, we ectopically expressed this isoform in the hearts of transgenic mice. We generated six transgenic mouse lines that produce varying levels of TPM 3 message with ectopic TPM 3 protein accounting for 40-60% of the total striated muscle tropomyosin. The transgenic mice have normal life spans and exhibit no morphological abnormalities in their sarcomeres or hearts. However, there are significant functional alterations in cardiac performance. Physiological assessment of these mice by using closed-chest analyses and a work-performing model reveals a hyperdynamic effect on systolic and diastolic function. Analysis of detergent-extracted fiber bundles demonstrates a decreased sensitivity to Ca(2+) in force generation and a decrease in length-dependent Ca(2+) activation with no detectable change in interfilament spacing as determined by using X-ray diffraction. Our data are the first to demonstrate that TM isoforms can affect sarcomeric performance by decreasing sensitivity to Ca(2+) and influencing the length-dependent Ca(2+) activation.

The troponin tail domain promotes a conformational state of the thin filament that suppresses myosin activity

In cardiac and skeletal muscles tropomyosin binds to the actin outer domain in the absence of Ca(2+), and in this position tropomyosin inhibits muscle contraction by interfering sterically with myosin-actin binding. The globular domain of troponin is believed to produce this B-state of the thin filament (Lehman, W., Hatch, V., Korman, V. L., Rosol, M., Thomas, L. T., Maytum, R., Geeves, M. A., Van Eyk, J. E., Tobacman, L. S., and Craig, R. (2000) J. Mol. Biol. 302, 593-606) via troponin I-actin interactions that constrain the tropomyosin. The present study shows that the B-state can be promoted independently by the elongated tail region of troponin (the NH(2) terminus (TnT-(1-153)) of cardiac troponin T). In the absence of the troponin globular domain, TnT-(1-153) markedly inhibited both myosin S1-actin-tropomyosin MgATPase activity and (at low S1 concentrations) myosin S1-ADP binding to the thin filament. Similarly, TnT-(1-153) increased the concentration of heavy meromyosin required to support in vitro sliding of thin filaments. Electron microscopy and three-dimensional reconstruction of thin filaments containing TnT-(1-153) and either cardiac or skeletal muscle tropomyosin showed that tropomyosin was in the B-state in the complete absence of troponin I. All of these results indicate that portions of the troponin tail domain, and not only troponin I, contribute to the positioning of tropomyosin on the actin outer domain, thereby inhibiting muscle contraction in the absence of Ca(2+).

Current awareness on yeast

In order to keep subscribers up-to-date with the latest developments in their field, this current awareness service is provided by John Wiley & Sons and contains newly-published material on yeasts. Each bibliography is divided into 10 sections. 1 Books, Reviews & Symposia; 2 General; 3 Biochemistry; 4 Biotechnology; 5 Cell Biology; 6 Gene Expression; 7 Genetics; 8 Physiology; 9 Medical Mycology; 10 Recombinant DNA Technology. Within each section, articles are listed in alphabetical order with respect to author. If, in the preceding period, no publications are located relevant to any one of these headings, that section will be omitted. (3 weeks journals - search completed 5th. Dec. 2001)