Actin residue glu(93) is identified as an amino acid affecting myosin binding

Lysine acetylation of F-actin decreases tropomyosin-based inhibition of actomyosin activity

Recent proteomics studies of vertebrate striated muscle have identified lysine acetylation at several sites on actin. Acetylation is a reversible post-translational modification that neutralizes lysine's positive charge. Positively charged residues on actin, particularly Lys³²⁶ and Lys³²⁸, are predicted to form critical electrostatic interactions with tropomyosin (Tpm) that promote its binding to filamentous (F)-actin and bias Tpm to an azimuthal location where it impedes myosin attachment. The troponin (Tn) complex also influences Tpm's position along F-actin as a function of Ca²⁺ to regulate exposure of myosin-binding sites and, thus, myosin cross-bridge recruitment and force production. Interestingly, Lys³²⁶ and Lys³²⁸ are among the documented acetylated residues. Using an acetic anhydride-based labeling approach, we showed that excessive, nonspecific actin acetylation did not disrupt characteristic F-actin-Tpm binding. However, it significantly reduced Tpm-mediated inhibition of myosin attachment, as reflected by increased F-actin-Tpm motility that persisted in the presence of Tn and submaximal Ca²⁺ Furthermore, decreasing the extent of chemical acetylation, to presumptively target highly reactive Lys³²⁶ and Lys³²⁸, also resulted in less inhibited F-actin-Tpm, implying that modifying only these residues influences Tpm's location and, potentially, thin filament regulation. To unequivocally determine the residue-specific consequences of acetylation on Tn-Tpm-based regulation of actomyosin activity, we assessed the effects of K326Q and K328Q acetyl (Ac)-mimetic actin on Ca²⁺-dependent, in vitro motility parameters of reconstituted thin filaments (RTFs). Incorporation of K328Q actin significantly enhanced Ca²⁺ sensitivity of RTF activation relative to control. Together, our findings suggest that actin acetylation, especially Lys³²⁸, modulates muscle contraction via disrupting inhibitory Tpm positioning.

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.

Evaluation of the expression stability of β-actin under bacterial infection in Macrobrachium nipponense

The selection of a suitable reference gene is an important prerequisite for the precise analysis of target gene expression by real-time quantitative PCR (qPCR). The present study aims to explore the expression pattern of the Macrobrachium nipponense (M. nipponense) β-actin gene under Aeromonas hydrophila bacterial infection conditions. The complete sequence of the β-actin gene from M. nipponense was cloned by PCR. Identified and named β-actin genes were searched in the NCBI database, and the characteristics of the β-actin gene were analyzed using bioinformatics methods. The expression profiles of β-actin under stresses challenged by bacteria after 3, 6, 12, 24 and 48 h were investigated by measuring Ct values by qPCR. The prokaryotic expression vector pET-30a-actin was constructed by PCR and recombinant DNA techniques. Fused protein was induced by IPTG in the transformed Escherichia coli BL21 (DE3). Recombinant rActin was purified by nickel column. The bioinformatics analysis result revealed that the deduced protein encoded by the β-actin gene from M. nipponense had the highest homology with other prawns in the homologous assay (99%). The phylogenetic tree indicates that the β-actin from M. nipponense and other crustaceans have a single cluster. The qPCR results revealed that a stable expression of β-actin was observed in response to the A. hydrophila challenge for 3-48 h, and the Ct value was 22 ± 1.5. β-actin was ranked as a stable gene after the bacterial challenge, which was selected as the appropriate reference gene in M. nipponense.

Distortion of the Actin A-Triad Results in Contractile Disinhibition and Cardiomyopathy

Striated muscle contraction is regulated by the movement of tropomyosin over the thin filament surface, which blocks or exposes myosin binding sites on actin. Findings suggest that electrostatic contacts, particularly those between K326, K328, and R147 on actin and tropomyosin, establish an energetically favorable F-actin-tropomyosin configuration, with tropomyosin positioned in a location that impedes actomyosin associations and promotes relaxation. Here, we provide data that directly support a vital role for these actin residues, termed the A-triad, in tropomyosin positioning in intact functioning muscle. By examining the effects of an A295S α-cardiac actin hypertrophic cardiomyopathy-causing mutation, over a range of increasingly complex in silico, in vitro, and in vivo Drosophila muscle models, we propose that subtle A-triad-tropomyosin perturbation can destabilize thin filament regulation, which leads to hypercontractility and triggers disease. Our efforts increase understanding of basic thin filament biology and help unravel the mechanistic basis of a complex cardiac disorder.

Actin genes and their expression in pacific white shrimp, Litopenaeus vannamei

Actin is a multi-functional gene family that can be divided into muscle-type actins and non-muscle-type actins. In this study, 37 unigenes encoding actins were identified from RNA-Seq data of Pacific white shrimp, Litopenaeus vannamei. According to phylogenetic analysis, four and three cDNAs belong to cytoplasmic- and heart-type actins and were named LvActinCT and LvActinHT, respectively. 10 cDNAs belong to the slow-type skeletal muscle actins, and 18 belong to the fast-type skeletal muscle actins; they were designated LvActinSSK and LvActinFSK, respectively. Some muscle actin genes formed gene clusters in the genome. Multiple alternative transcription starts sites (ATSSs) were found for LvActinCT1. Based on the early developmental expression profile, almost all LvActins were highly expressed between the early limb bud and post-larval stages. Using LvActinSSK5 as probes, slow-type muscle was localized in pleopod muscle and superficial ventral muscle. We also found three actin genes that were down-regulated in the hemocytes of white spot syndrome virus (WSSV)- and Vibrio parahaemolyticus-infected L. vannamei. This study provides valuable information on the actin gene structure of shrimp, furthers our understanding of the shrimp muscle system and helps us develop strategies for disease control and sustainable shrimp farming.

Non-Straub type actin from molluscan catch muscle

We have developed a method of obtaining natural actin from smooth muscles of the bivalves on the example of the Сrenomytilus grayanus catch muscle. The muscles were previously rigorized to prevent a loss of thin filaments during homogenization and washings. Thin filaments were isolated with a low ionic strength solution in the presence of ATP and sodium pyrophosphate. Surface proteins of thin filaments-tropomyosin, troponin, calponin and some minor actin-binding proteins-were dissociated from actin filaments by increasing the ionic strength to 0.6 M KCL. Natural fibrillar actin obtained in that way depolymerizes easily in low ionic strength solutions commonly used for the extraction of Straub-type actin from acetone powder. Purification of natural actin was carried out by the polymerization-depolymerization cycle. The content of inactivated actin remaining in the supernatant is much less than at a similar purification of Straub-type actin. A comparative investigation was performed between the natural mussel actin and the Straub-type rabbit skeletal actin in terms of the key properties of actin: polymerization, activation of Mg-ATPase activity of myosin, and the electron-microscopic structure of actin polymers.

Pseudo-acetylation of K326 and K328 of actin disrupts Drosophila melanogaster indirect flight muscle structure and performance

In striated muscle tropomyosin (Tm) extends along the length of F-actin-containing thin filaments. Its location governs access of myosin binding sites on actin and, hence, force production. Intermolecular electrostatic associations are believed to mediate critical interactions between the proteins. For example, actin residues K326, K328, and R147 were predicted to establish contacts with E181 of Tm. Moreover, K328 also potentially forms direct interactions with E286 of myosin when the motor is strongly bound. Recently, LC-MS/MS analysis of the cardiac acetyl-lysine proteome revealed K326 and K328 of actin were acetylated, a post-translational modification (PTM) that masks the residues' inherent positive charges. Here, we tested the hypothesis that by removing the vital actin charges at residues 326 and 328, the PTM would perturb Tm positioning and/or strong myosin binding as manifested by altered skeletal muscle function and structure in the Drosophila melanogaster model system. Transgenic flies were created that permit tissue-specific expression of K326Q, K328Q, or K326Q/K328Q acetyl-mimetic actin and of wild-type actin via the UAS-GAL4 bipartite expression system. Compared to wild-type actin, muscle-restricted expression of mutant actin had a dose-dependent effect on flight ability. Moreover, excessive K328Q and K326Q/K328Q actin overexpression induced indirect flight muscle degeneration, a phenotype consistent with hypercontraction observed in other Drosophila myofibrillar mutants. Based on F-actin-Tm and F-actin-Tm-myosin models and on our physiological data, we conclude that acetylating K326 and K328 of actin alters electrostatic associations with Tm and/or myosin and thereby augments contractile properties. Our findings highlight the utility of Drosophila as a model that permits efficient targeted design and assessment of molecular and tissue-specific responses to muscle protein modifications, in vivo.

Skeletal muscle α-actin diseases (actinopathies): pathology and mechanisms

Mutations in the skeletal muscle α-actin gene (ACTA1) cause a range of congenital myopathies characterised by muscle weakness and specific skeletal muscle structural lesions. Actin accumulations, nemaline and intranuclear bodies, fibre-type disproportion, cores, caps, dystrophic features and zebra bodies have all been seen in biopsies from patients with ACTA1 disease, with patients frequently presenting with multiple pathologies. Therefore increasingly it is considered that these entities may represent a continuum of structural abnormalities arising due to ACTA1 mutations. Recently an ACTA1 mutation has also been associated with a hypertonic clinical presentation with nemaline bodies. Whilst multiple genes are known to cause many of the pathologies associated with ACTA1 mutations, to date actin aggregates, intranuclear rods and zebra bodies have solely been attributed to ACTA1 mutations. Approximately 200 different ACTA1 mutations have been identified, with 90 % resulting in dominant disease and 10 % resulting in recessive disease. Despite extensive research into normal actin function and the functional consequences of ACTA1 mutations in cell culture, animal models and patient tissue, the mechanisms underlying muscle weakness and the formation of structural lesions remains largely unknown. Whilst precise mechanisms are being grappled with, headway is being made in terms of developing therapeutics for ACTA1 disease, with gene therapy (specifically reducing the proportion of mutant skeletal muscle α-actin protein) and pharmacological agents showing promising results in animal models and patient muscle. The use of small molecules to sensitise the contractile apparatus to Ca(2+) is a promising therapeutic for patients with various neuromuscular disorders, including ACTA1 disease.

In vitro motility of native thin filaments from Drosophila indirect flight muscles reveals that the held-up 2 TnI mutation affects calcium activation

A procedure for the isolation of regulated native thin filaments from the indirect flight muscles (IFM) of Drosophila melanogaster is described. These are the first striated invertebrate thin filaments to show Ca-regulated in vitro motility. Regulated native thin filaments from wild type and a troponin I mutant, held-up-2, were compared by in vitro motility assays that showed that the mutant troponin I caused activation of motility at pCa values higher than wild type. The held-up2 mutation, in the sole troponin I gene (wupA) in the Drosophila genome, is known to cause hypercontraction of the IFM and other muscles in vivo leading to their eventual destruction. The mutation causes substitution of alanine by valine at a homologous and completely conserved troponin I residue (A25) in the vertebrate skeletal muscle TnI isoform. The effects of the held-up 2 mutation on calcium activation of thin filament in vitro motility are discussed with respect to its effects on hypercontraction and dysfunction. Previous electron microscopy and 3-dimensional reconstruction studies showed that the tropomyosin of held-up 2 thin filaments occupies positions associated with the so-called 'closed' state, but independently of calcium concentration. This is discussed with respect to calcium dependent regulation of held-up-2 thin filaments in in vitro motility.

Direct visualisation and kinetic analysis of normal and nemaline myopathy actin polymerisation using total internal reflection microscopy

Actin filaments were formed by elongation of pre-formed nuclei (short crosslinked actin-HMM complexes) that were attached to a microscope cover glass. By using TIRF illumination we could see actin filaments at high contrast despite the presence of 150 nM TRITC-phalloidin in the solution. Actin filaments showed rapid bending and translational movements due to Brownian motion but the presence of the methylcellulose polymer network constrained lateral movement away from the surface. Both the length and the number of filaments increased with time. Some filaments did not change length at all and some filaments joined up end-to-end (annealing). We did not see any decrease in filament length or filament breakage. For quantitative analysis of polymerisation time course we measured the contour length of all the filaments in a frame at a series of time points and also tracked the length of individual filaments over time. Elongation rate was the same measured by both methods (0.23 microm/min at 0.1 microM actin) and was up to 10 times faster than previously published measurements. The annealed filament population reached 30% of the total after 40 min. Polymerisation rate increased linearly with actin concentration. K(on) was 2.07 microm min(-1) microM(-1) (equivalent to 34.5 monomers s(-1) microM(-1)) and critical concentration was less than 20 nM. This technique was used to study polymerisation of a mutant actin (D286G) from a transgenic mouse model. D286G actin elongated at a 40% lower rate than non-transgenic actin.

Twelve actin-encoding cDNAs from the American lobster, Homarus americanus: cloning and tissue expression of eight skeletal muscle, one heart, and three cytoplasmic isoforms

Lobster muscles express a diverse array of myofibrillar protein isoforms. Three fiber types (fast, slow-twitch or S1, and slow-tonic or S2) differ qualitatively and quantitatively in myosin heavy and light chains, troponin-T, -I, and -C, paramyosin, and tropomyosin variants. However, little is known about the diversity of actin isoforms present in crustacean tissues. In this report we characterized cDNAs that encode twelve actin isoforms in the American lobster, Homarus americanus: eight from skeletal muscle (Ha-ActinSK1-8), one from heart (Ha-ActinHT1), and three cytoplasmic type actins from hepatopancreas (Ha-ActinCT1-3). All twelve cDNAs were products of distinct genes, as indicated by differences in the 3'-untranslated regions (UTRs). The open reading frames specified polypeptides 376 or 377 amino acids in length. Although key amino residues are conserved in the lobster actins, variations in nearby sequences may affect actin polymerization and/or interactions with other myofibrillar proteins. Quantitative reverse transcription-polymerase chain reaction showed muscle fiber type- and tissue-specific expression patterns. Ha-Actin-HT1 was expressed exclusively in heart (87% of the total; 12% of the total was Ha-ActinCT1). Ha-ActinCT1 was expressed in all tissues, while CT2 and CT3 were expressed only in hepatopancreas, with Ha-ActinCT2 as the major isoform (93% of the total). Ha-ActinSK1 and SK2 were the major isoforms (88% and 12% of the total, respectively) in the S1 fibers of crusher claw closer muscle. Fast fibers in the cutter claw closer and deep abdominal muscles differed in SK isoforms. Ha-ActinSK3, SK4, and SK5 were the major isoforms in cutter claw closer muscle (12%, 48%, and 37% of the total, respectively). Ha-ActinSK5 and SK8 were the major isoforms in deep abdominal flexor (31% and 65% of the total, respectively) and extensor (46% and 53% of the total, respectively) muscles, with SK6 and SK7 expressed at low levels. These data indicate that fast fibers in cutter claw and abdominal muscles show a phenotypic plasticity with respect to the expression of actin isoforms and may constitute discrete subtypes that differ in contractile properties.

Close proximity of myosin loop 3 to troponin determined by triangulation of resonance energy transfer distance measurements

Cooperative activation of the thin filament is known to be influenced by the tight binding of myosin to actin, but the molecular mechanism underlying this contribution of myosin is not well understood. To better understand the structural relationship of myosin with the regulatory troponin complex, resonance energy transfer measurements were used to map the location of troponin relative to a neighboring myosin bound to actin using atomic models. Using a chicken troponin T isoform that contains a single cysteine near the binding interface between troponins T, I, and C, this uniquely labeled cysteine on troponin was found to be remarkably near loop 3 of myosin. This loop has previously been localized near the actin and myosin interface by chemical cross-linking methods, but its functional contributions have not been established. The implications of this close proximity are examined by molecular modeling, which suggests that only restricted conformations of actomyosin can accommodate the presence of troponin at this location near the cross-bridge. This potential for interaction between troponin and myosin heads that bind near it along the thin filament raises the possibility of models in which direct myosin and troponin interactions may play a role in the regulatory mechanism.

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.

Kinetic characterization of the function of myosin loop 4 in the actin-myosin interaction

Myosin interacts with actin during its enzymatic cycle, and actin stimulates myosin's ATPase activity. There are extensive interaction surfaces on both actin and myosin. Several surface loops of myosin play different roles in actomyosin interaction. However, the functional role of loop 4 in actin binding is still ambiguous. We explored the role of loop 4 by either mutating its conserved acidic group, Glu-365, to Gln (E365Q), or by replacing the entire loop with three glycines (DeltaAL) in a Dictyostelium discoideum myosin II motor domain (MD) containing a single tryptophan residue. This native tryptophan (Trp-501) is located in the relay loop and is sensitive to nucleotide binding and lever-arm movement. Fluorescence and fast kinetic measurements showed that the mutations in loop 4 do not alter the enzymatic steps of the ATPase cycle in the absence of actin. By contrast, actin binding was significantly weakened in the absence and presence of ADP and ATP in both mutants. Because the strength of actin-myosin interaction increases in the order of rigor, ADP, and ATP complex, we conclude that loop 4 is a functional actin-binding region that stabilizes actomyosin complex, particularly in weak actin-binding states.

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.

A closer look at energy transduction in muscle

Muscular force is the sum of unitary force interactions generated as filaments of myosins move forcibly along parallel filaments of actins, understanding that the free energy required comes from myosin-catalyzed ATP hydrolysis. Using results from conventional biochemistry, our own mutational studies, and diffraction images from others, we attempt, in molecular detail, an account of a unitary interaction, i.e., what happens after a traveling myosin head, bearing an ADP-P(i), reaches the next station of an actin filament in its path. We first construct a reasonable model of the myosin head and actin regions that meet to form the "weakly bound state". Separately, we consider Holmes' model of the rigor state [Holmes, K. C., Angert, I., Kull, F. J., Jahn, W. & Schröder, R. R. (2003) Nature 425, 423-427], supplemented with several heretofore missing residues, thus realizing the "strongly bound state." Comparing states suggests how influences initiated at the interface travel elsewhere in myosin to discharge various functions, including striking the actins. Overall, state change seems to occur by attachment of a hydrophobic triplet (Trp-546, Phe-547, and Pro-548) of myosin to an actin conduit with a hydrophobic guiding rail (Ile-341, Ile-345, Leu-349, and Phe-352) and the subsequent linear movement of the triplet along the rail.

Cargo Transport: Two Motors Are Sometimes Better Than One

Molecular motor proteins are crucial for the proper distribution of organelles and vesicles in cells. Much of our current understanding of how motors function stems from studies of single motors moving cargos in vitro. More recently, however, there has been mounting evidence that the cooperation of multiple motors in moving cargos and the regulation of motor-filament affinity could be key mechanisms that cells utilize to regulate cargo transport. Here, we review these recent advances and present a picture of how the different mechanisms of regulating the number of motors moving a cargo could facilitate cellular functions.

The pathogenesis ofACTA1-related congenital fiber type disproportion

OBJECTIVE

Mutations in ACTA1 have been associated with a variety of changes in muscle histology that likely result from fundamental differences in the way that ACTA1 mutations disrupt muscle function. Recently, we reported three patients with congenital fiber type disproportion (CFTD) caused by novel heterozygous missense mutations in ACTA1 (D292V, L221P, P332S) with marked type 1 fiber hypotrophy as the only pathological finding on muscle biopsy. We have investigated the basis for the histological differences between these CFTD patients and patients with ACTA1 nemaline myopathy (NM).

METHODS AND RESULTS

Mass spectrometry and two-dimensional gel electrophoresis demonstrate that mutant actin accounts for 25 and 50% of alpha-skeletal actin in the skeletal muscle of patients with the P332S and D292V mutations, respectively, consistent with a dominant-negative disease mechanism. In vitro motility studies indicate that abnormal interactions between actin and tropomyosin are the likely principal cause of muscle weakness for D292V, with tropomyosin stabilized in the "switched off" position. Both the D292V and P322S CFTD mutations are associated with normal sarcomeric structure on electron microscopy, which is atypical for severe NM. In contrast, we found no clear difference between ACTA1 mutations associated with NM and CFTD in tendency to polymerize or aggregate in C2C12 expression models.

INTERPRETATION

These data suggest that ACTA1 CFTD mutations cause weakness by disrupting sarcomere function rather than structure. We raise the possibility that the presence or absence of structural disorganization when mutant actin incorporates into sarcomeres may be an important determinant of whether the histological patterns of CFTD or NM develop in ACTA1 myopathy.

Functional Consequences of a Mutation in an Expressed Human α-Cardiac Actin at a Site Implicated in Familial Hypertrophic Cardiomyopathy

Point mutations in human alpha-cardiac actin cause familial hypertrophic cardiomyopathy. Functional characterization of these actin mutants has been limited by the lack of a high level expression system for human cardiac actin. Here, wild-type (WT) human alpha-cardiac actin and a mutant E99K actin have been expressed and purified from the baculovirus/insect cell expression system. Glu-99 in subdomain 1 of actin is thought to interact with a positively charged cluster located in the lower 50-kDa domain of the myosin motor domain. Actin-activated ATPase measurements using the expressed actins and beta-cardiac myosin showed that the mutation increased the K(m) for actin 4-fold (4.7 +/- 0.7 mum for WT versus 19.1 +/- 3.0 mum for the mutant), whereas the V(max) values were similar. The mutation slightly decreased the affinity of actin for S1 in the absence of nucleotide, which can partly be accounted for by a slower rate of association. The in vitro motility for the E99K mutant was consistently lower than WT over a range of ionic strengths, which is likely related to the lower average force supported by the mutant actin. The thermal stability of the E99K was comparable to that of WT-actin, implying no folding defects. The lower density of negative charge in subdomain 1 of actin therefore weakens the actomyosin interaction sufficiently to decrease the force and motion generating capacity of E99K actin, thus providing the primary insult that ultimately leads to the disease phenotype.

An alternative domain near the nucleotide-binding site of Drosophila muscle myosin affects ATPase kinetics

In Drosophila melanogaster expression of muscle myosin heavy chain isoforms occurs by alternative splicing of transcripts from a single gene. The exon 7 domain is one of four variable regions in the catalytic head and is located near the nucleotide-binding site. To ascribe a functional role to this domain, we created two chimeric myosin isoforms (indirect flight isoform-exon 7a and embryonic-exon 7d) that differ from the native indirect flight muscle and embryonic body-wall muscle isoforms only in the exon 7 region. Germline transformation and subsequent expression of the chimeric myosins in the indirect flight muscle of myosin-null Drosophila allowed us to purify the myosin for in vitro studies and to assess in vivo structure and function of transgenic muscles. Intriguingly, in vitro experiments show the exon 7 domain modulates myosin ATPase activity but has no effect on actin filament velocity, a novel result compared to similar studies with other Drosophila variable exons. Transgenic flies expressing the indirect flight isoform-exon 7a have normal indirect flight muscle structure, and flight and jump ability. However, expression of the embryonic-exon 7d chimeric isoform yields flightless flies that show improvements in both the structural stability of the indirect flight muscle and in locomotor abilities as compared to flies expressing the embryonic isoform. Overall, our results suggest the exon 7 domain participates in the regulation of the attachment of myosin to actin in order to fine-tune the physiological properties of Drosophila myosin isoforms.

Invertebrate Muscles: Muscle Specific Genes and Proteins

This is the first of a projected series of canonic reviews covering all invertebrate muscle literature prior to 2005 and covers muscle genes and proteins except those involved in excitation-contraction coupling (e.g., the ryanodine receptor) and those forming ligand- and voltage-dependent channels. Two themes are of primary importance. The first is the evolutionary antiquity of muscle proteins. Actin, myosin, and tropomyosin (at least, the presence of other muscle proteins in these organisms has not been examined) exist in muscle-like cells in Radiata, and almost all muscle proteins are present across Bilateria, implying that the first Bilaterian had a complete, or near-complete, complement of present-day muscle proteins. The second is the extraordinary diversity of protein isoforms and genetic mechanisms for producing them. This rich diversity suggests that studying invertebrate muscle proteins and genes can be usefully applied to resolve phylogenetic relationships and to understand protein assembly coevolution. Fully achieving these goals, however, will require examination of a much broader range of species than has been heretofore performed.

E93K charge reversal on actin perturbs steric regulation of thin filaments

Contraction in striated muscles is regulated by Ca2+-dependent movement of tropomyosin-troponin on thin filaments. Interactions of charged amino acid residues between the surfaces of tropomyosin and actin are believed to play an integral role in this steric mechanism by influencing the position of tropomyosin on the filaments. To investigate this possibility further, thin filaments were isolated from troponin-regulated, indirect flight muscles of Drosophila mutants that express actin with an amino acid charge reversal at residue 93 located at the interface between actin subdomains 1 and 2, in which a lysine residue is substituted for a glutamic acid. Electron microscopy and 3D helical reconstruction were employed to evaluate the structural effects of the mutation. In the absence of Ca2+, tropomyosin was in a position that blocked the myosin-binding sites on actin, as previously found with wild-type filaments. However, in the presence of Ca2+, tropomyosin position in the mutant filaments was much more variable than in the wild-type ones. In most cases (approximately 60%), tropomyosin remained in the blocking position despite the presence of Ca2+, failing to undergo a normal Ca2+-induced change in position. Thus, switching of a negative to a positive charge at position 93 on actin may stabilize negatively charged tropomyosin in the Ca2+-free state regardless of Ca2+ levels, an alteration that, in turn, is likely to interfere with steric regulation and consequently muscle activation. These results highlight the importance of actin's surface charges in determining the distribution of tropomyosin positions on thin filaments derived from troponin-regulated striated muscles.

Age-Related Decline in Actomyosin Function

To understand the molecular basis of the functional decline in aging muscle, we examined the functional (actomyosin ATPase) and chemical (cysteine content) changes in actin and myosin purified from the muscles of young (4- to 12-month-old) and old (27- to 35-month-old) Fisher 344 rats. Using the soluble, catalytically active myosin fragment, heavy meromyosin (HMM), we determined the maximum rate (V(max)) and actin concentration at half V(max) (K(m)) of the actomyosin ATPase, using four combinations of actin and HMM from old and young rats. V(max) and K(m) were significantly lower when both actin and HMM were obtained from old rats than when both proteins were obtained from young rats. The number of reactive cysteines in HMM significantly decreased with age, but no change was detected in the number of reactive cysteines in actin. We conclude that aging results in chemical changes in myosin (probably oxidation of cysteines) that have inhibitory effects on the actin-activated myosin ATPase.

Expression of a nonpolymerizable actin mutant in Sf9 cells

We have succeeded in expressing actin in the baculovirus/Sf9 cell system in high yield. The wild-type (WT) actin is functionally indistinguishable from tissue-purified actin in its ability to activate ATPase activity and to support movement in an in vitro motility assay. Having achieved this feat, we used a mutational strategy to express a monomeric actin that is incapable of polymerization. Native actin requires actin binding proteins or chemical modification to maintain it in a monomeric state. The mutant actin sediments in the analytical ultracentrifuge as a homogeneous monomeric species of 3.2 S in 100 mM KCl and 2 mM MgCl(2), conditions that cause WT actin to polymerize. The two point mutations that render actin nonpolymerizable are in subdomain 4 (A204E/P243K; "AP-actin"), distant from the myosin binding site. AP-actin binds to skeletal myosin subfragment 1 (S1) and forms a homogeneous complex as demonstrated by analytical ultracentrifugation. The ATPase activity of a cross-linked AP-actin.S1 complex is higher than that of S1 alone, although less than that supported by filamentous actin (F-actin). AP-Actin is an excellent candidate for structural studies of complexes of actin with motor proteins and other actin-binding proteins.

Structural dynamics of actin during active interaction with myosin: different effects of weakly and strongly bound myosin heads

We have used optical spectroscopy (transient phosphorescence anisotropy, TPA, and fluorescence resonance energy transfer, FRET) to detect the effects of weakly bound myosin S1 on actin during the actomyosin ATPase cycle. The changes in actin were reported by (a) a phosphorescent probe (ErIA) attached to Cys 374 and (b) a FRET donor-acceptor pair, IAEDANS attached to Cys 374 and a nucleotide analogue (TNPADP) in the nucleotide-binding cleft. Strong interactions were detected in the absence of ATP, and weak interactions were detected in the presence of ATP or its slowly hydrolyzed analogue ATP-gamma-S, under conditions where a significant fraction of weakly bound acto-S1 complex was present and the rate of nucleotide hydrolysis was low enough to enable steady-state measurements. The results show that actin in the weakly bound complex with S1 assumes a new structural state in which (a) the actin filament has microsecond rotational dynamics intermediate between that of free actin and the strongly bound complex and (b) S1-induced changes are not propagated along the actin filament, in contrast to the highly cooperative changes due to the strongly bound complex. We propose that the transition on the acto-myosin interface from weak to strong binding is accompanied by transitions in the structural dynamics of actin parallel to transitions in the dynamics of interacting myosin heads.

Functional characterisation of a mutant actin (Met132Val) from a patient with nemaline myopathy

The mutation Met132Val in the ACTA1 gene was identified in a patient with mild nemaline myopathy (NM). We examined actin mRNA and protein from biopsy samples. Sixty-one percent of the mRNA from the biopsy was not cleaved with BstX1, indicating the presence of mutant messenger in vivo. Monomeric actin was extracted from 2.5 mg of mutant muscle and wild type muscle. A proportion of the NM actin did not polymerise in 50 mM KCl, 2.5 mM MgCl2 but all the wild-type actin did. NM actin was fully polymerised by 50 mM KCl, 2.5 mM MgCl2, 150 nM rhodamine-phalloidin. Thin filaments reconstituted with this co-polymer were different from wild-type. The NM actin produces faster sliding of thin filaments at pCa5 and higher relative isometric force. We conclude that the mutant mRNA and protein is expressed and that the mutation reduces polymerisability and alters thin filament function.

Structural Plasticity of Functional Actin

Actin is one of the most conserved and versatile proteins capable of forming homopolymers and interacting with numerous other proteins in the cell. We performed an alanine mutagenesis scan covering the entire beta-actin molecule. Somewhat surprisingly, the majority of the mutants were capable of reaching a stable conformation. We tested the ability of these mutants to bind to various actin binding proteins, thereby mapping different interfaces with actin. Additionally, we tested their ability to copolymerize with alpha-actin in order to localize regions in actin that contact neighboring protomers in the filament. Hereby, we could discriminate between two existing models for filamentous actin and our data strongly support the right-handed double-stranded helix model. We present data corroborating this model in vivo. Mutants defective in copolymerization do not colocalize with the actin cytoskeleton and some impair its normal function, thereby disturbing cell shape.

Muscle disease caused by mutations in the skeletal muscle alpha-actin gene (ACTA1)

Mutations in the skeletal muscle alpha-actin gene (ACTA1) associated with congenital myopathy with excess of thin myofilaments, nemaline myopathy and intranuclear rod myopathy were first described in 1999. At that time, only 15 different missense mutations were known in ACTA1. More than 60 mutations have now been identified. This review analyses this larger spectrum of mutations in ACTA1. It investigates the molecular consequences of the mutations found to date, provides a framework for genotype-phenotype correlation and suggests future studies in light of results of investigation of normal and mutant actin in other systems, notably the actin specific to the indirect flight muscles of Drosophila. The larger series confirms that the majority of ACTA1 mutations are dominant, a small number are recessive and most isolated cases with no previous family history have de novo dominant mutations. The severity of the disease caused ranges from lack of spontaneous movements at birth requiring immediate mechanical ventilation, to mild disease compatible with life to adulthood. Overall, the mutations within ACTA1 are randomly distributed throughout the protein. However, the larger series of mutations now available indicates that there may be clustering of mutations associated with some phenotypes, e.g. actin myopathy. This would suggest that interference with certain actin functions may be more associated with certain phenotypes, though the exact pathophysiology of the actin mutations remains unknown.

Variable N-terminal regions of muscle myosin heavy chain modulate ATPase rate and actin sliding velocity

We integratively assessed the function of alternative versions of a region near the N terminus of Drosophila muscle myosin heavy chain (encoded by exon 3a or 3b). We exchanged the alternative exon 3 regions between an embryonic isoform and the indirect flight muscle isoform. Each chimeric myosin was expressed in Drosophila indirect flight muscle, in the absence of other myosin isoforms, allowing for purified protein analysis and whole organism locomotory studies. The flight muscle isoform generates higher in vitro actin sliding velocity and solution ATPase rates than the embryonic isoform. Exchanging the embryonic exon 3 region into the flight muscle isoform decreased ATPase rates to embryonic levels but did not affect actin sliding velocity or flight muscle ultrastructure. Interestingly, this swap only slightly impaired flight ability. Exchanging the flight muscle-specific exon 3 region into the embryonic isoform increased actin sliding velocity 3-fold and improved indirect flight muscle ultrastructure integrity but failed to rescue the flightless phenotype of flies expressing embryonic myosin. These results suggest that the two structural versions of the exon 3 domain independently influence the kinetics of at least two steps of the actomyosin cross-bridge cycle.

Suppression of muscle hypercontraction by mutations in the myosin heavy chain gene of Drosophila melanogaster

The indirect flight muscles (IFM) of Drosophila melanogaster provide a good genetic system with which to investigate muscle function. Flight muscle contraction is regulated by both stretch and Ca(2+)-induced thin filament (actin + tropomyosin + troponin complex) activation. Some mutants in troponin-I (TnI) and troponin-T (TnT) genes cause a "hypercontraction" muscle phenotype, suggesting that this condition arises from defects in Ca(2+) regulation and actomyosin-generated tension. We have tested the hypothesis that missense mutations of the myosin heavy chain gene, Mhc, which suppress the hypercontraction of the TnI mutant held-up(2) (hdp(2)), do so by reducing actomyosin force production. Here we show that a "headless" Mhc transgenic fly construct that reduces the myosin head concentration in the muscle thick filaments acts as a dose-dependent suppressor of hypercontracting alleles of TnI, TnT, Mhc, and flightin genes. The data suggest that most, if not all, mutants causing hypercontraction require actomyosin-produced forces to do so. Whether all Mhc suppressors act simply by reducing the force production of the thick filament is discussed with respect to current models of myosin function and thin filament activation by the binding of calcium to the troponin complex.

Random walks with thin filaments: application of in vitro motility assay to the study of actomyosin regulation

The in vitro motility devised by Kron and Spudich (Kron and Spudich, 1986; Kron et al., 1991) has proved a very valuable technique for studying the motor properties of myosin of all kinds but it is equally useful for the study of the thin filaments of muscle and their regulation. The movement of a population of thin filaments over immobilised myosin appears to be random but it does in fact yield a large amount of information about contractility and its regulation. The key to extracting useful information from in vitro motility assay experiments is the logical and comprehensive analysis of filament movements.

Dynamic docking of myosin and actin observed with resonance energy transfer

Atomic models of myosin subfragment-1 (S1) and the actin filament are docked together using resonance energy-transfer data from both pre- and postpowerstroke conditions. The quality of the resulting best fits discriminated between neck-region orientations of the S1 for a given set of experimental conditions. For measurements of the postpowerstroke states in the presence of ADP, resonance energy-transfer data alone are sufficient to dock the atomic models and provide evidence that S1 exists with at least two neck-region orientations under these conditions. To dock the prepowerstroke state, resonance energy-transfer data were used in combination with previous chemical cross-linking data to determine that a neck-region orientation similar to that of a proposed prepowerstroke state best fit the data. The resulting models determined independently from electron microscopy compare favorably with micrographs from the recent literature. The docking models by resonance energy transfer suggest that the larger movements in the light-chain binding domain are accompanied by twisting and rotating movements of the catalytic domain, causing a tilt of approximately 30 degrees during the weak-to-strong transition. This transition provides the displacement necessary to support motility and force generation.

Site-specific mutations in the myosin binding sites of actin affect structural transitions that control myosin binding

We have examined the effects of actin mutations on myosin binding, detected by cosedimentation, and actin structural dynamics, detected by spectroscopic probes. Specific mutations were chosen that have been shown to affect the functional interactions of actin and myosin, two mutations (4Ac and E99A/E100A) in the proposed region of weak binding to myosin and one mutation (I341A) in the proposed region of strong binding. In the absence of nucleotide and salt, S1 bound to both wild-type and mutant actins with high affinity (K(d) < microM), but either ADP or increased ionic strength decreased this affinity. This decrease was more pronounced for actins with mutations that inhibit functional interaction with myosin (E99A/E100A and I341A) than for a mutation that enhances the interaction (4Ac). The mutations E99A/E100A and I341A affected the microsecond time scale dynamics of actin in the absence of myosin, but the 4Ac mutation did not have any effect. The binding of myosin eliminated these effects of mutations on structural dynamics; i.e., the spectroscopic signals from mutant actins bound to S1 were the same as those from wild-type actin. These results indicate that mutations in the myosin binding sites affect structural transitions within actin that control strong myosin binding, without affecting the structural dynamics of the strongly bound actomyosin complex.

Genetics of the Drosophila flight muscle myofibril: a window into the biology of complex systems

This essay reviews the long tradition of experimental genetics of the Drosophila indirect flight muscles (IFM). It discusses how genetics can operate in tandem with multidisciplinary approaches to provide a description, in molecular terms, of the functional properties of the muscle myofibril. In particular, studies at the interface of genetics and proteomics address protein function at the cellular scale and offer an outstanding platform with which to elucidate how the myofibril works. Two generalizations can be enunciated from the studies reviewed. First, the study of mutant IFM proteomes provides insight into how proteins are functionally organized in the myofibril. Second, IFM mutants can give rise to structural and contractile defects that are unrelated, a reflection of the dual function that myofibrillar proteins play as fundamental components of the sarcomeric framework and biochemical "parts" of the contractile "engine".

Alternative exon-encoded regions of Drosophila myosin heavy chain modulate ATPase rates and actin sliding velocity

To investigate the molecular functions of the regions encoded by alternative exons from the single Drosophila myosin heavy chain gene, we made the first kinetic measurements of two muscle myosin isoforms that differ in all alternative regions. Myosin was purified from the indirect flight muscles of wild-type and transgenic flies expressing a major embryonic isoform. The in vitro actin sliding velocity on the flight muscle isoform (6.4 microm x s(-1) at 22 degrees C) is among the fastest reported for a type II myosin and was 9-fold faster than with the embryonic isoform. With smooth muscle tropomyosin bound to actin, the actin sliding velocity on the embryonic isoform increased 6-fold, whereas that on the flight muscle myosin slightly decreased. No difference in the step sizes of Drosophila and rabbit skeletal myosins were found using optical tweezers, suggesting that the slower in vitro velocity with the embryonic isoform is due to altered kinetics. Basal ATPase rates for flight muscle myosin are higher than those of embryonic and rabbit myosin. These differences explain why the embryonic myosin cannot functionally substitute in vivo for the native flight muscle isoform, and demonstrate that one or more of the five myosin heavy chain alternative exons must influence Drosophila myosin kinetics.

An actin subdomain 2 mutation that impairs thin filament regulation by troponin and tropomyosin

Striated muscle thin filaments adopt different quaternary structures, depending upon calcium binding to troponin and myosin binding to actin. Modification of actin subdomain 2 alters troponin-tropomyosin-mediated regulation, suggesting that this region of actin may contain important protein-protein interaction sites. We used yeast actin mutant D56A/E57A to examine this issue. The mutation increased the affinity of tropomyosin for actin 3-fold. The addition of Ca(2+) to mutant actin filaments containing troponin-tropomyosin produced little increase in the thin filament-myosin S1 MgATPase rate. Despite this, three-dimensional reconstruction of electron microscope images of filaments in the presence of troponin and Ca(2+) showed tropomyosin to be in a position similar to that found for muscle actin filaments, where most of the myosin binding site is exposed. Troponin-tropomyosin bound with comparable affinity to mutant and wild type actin in the absence and presence of calcium, and in the presence of myosin S1, tropomyosin bound very tightly to both types of actin. The mutation decreased actin-myosin S1 affinity 13-fold in the presence of troponin-tropomyosin and 2.6-fold in the absence of the regulatory proteins. The results suggest the importance of negatively charged actin subdomain 2 residues 56 and 57 for myosin binding to actin, for tropomyosin-actin interactions, and for regulatory conformational changes in the actin-troponin-tropomyosin complex.

Actomyosin interactions in a novel single muscle fiber in vitro motility assay

A novel in vitro motility assay has been developed to study the actomyosin interaction, in which the molecular motor protein myosin has been extracted and immobilized directly from 2-4 mm single rat skeletal muscle fiber segments. This method study was carried out to investigate: (1) the amount of myofibrillar proteins extracted from the fiber segment; (2) the effects of temperature on the speed at which actin is propelled by fast and slow myosin; and (3) the effects of myosin isoform expression on motility speed. Approximately 80% of the myosin and myosin-associated proteins were extracted from the fiber segments. while no significant extraction was shown of the thin filament proteins. Fluorescently labeled actin filaments moved with constant speed in a bi-directional motion over the high-density myosin region in the experimental chamber, and motility speed was highly dependent on the myosin heavy chain (MyHC) isoform extracted. At 25 degrees C, significant (P < 0.001) differences in motility speed were obtained between type I (1.31 +/- 0.23 microm/s, n = 11) and IIxb (5.81 +/- 0.35 microm/s, n = 6), or llb (6.07 +/- 0.33 microm/s, n = 8) MyHC isoforms. The motility speed and maximum velocity of unloaded shortening (V0) in single fibers were well correlated, indicating that filament speed is a good molecular analogue to contractile speed at the fiber level. The effects of temperature on filament motility speed were analyzed from 10 to 35 degrees C. The Q10 values, calculated in the 10-25 degrees C temperature range, differed between slow (4.20) and fast (2.38) myosin. In conclusion, this in vitro motility assay offers a unique possibility to compare the regulatory and modulatory influence of myosin isoforms and thin filament proteins on shortening velocity, at the cellular and molecular level in the same muscle fiber.

Drosophila ACT88F indirect flight muscle-specific actin is not N-terminally acetylated: a mutation in N-terminal processing affects actin function

Many eukaryotic proteins are co and post-translationally modified at their N termini by removal of one or two amino acid residues and N(alpha)-acetylation. Actins show two different forms of N-terminal processing dependent on their N-terminal sequence. In class II actins, which include muscle actins, the common primary sequence of Met-Cys-Asp-actin is processed to acetyl-Asp-actin. The functional significance of this in vivo is unknown. We have studied the indirect flight muscle-specific actin, ACT88F, of Drosophila melanogaster. Our results show that ACT88F is N-terminally processed in vivo as a class II actin by removal of the first two amino acid residues (Met and Cys), but that uniquely the N terminus is not acetylated. In addition we show that ACT88F is methylated, probably at His73. Flies carrying the mod(-) mutation fail to complete post-translational processing of ACT88F. We propose that the mod gene product is normally responsible for removing N-acetyl-cysteine from actin. The biological significance of this process is demonstrated by observations that retention of the N-acetyl-cysteine in ACT88F affects the flight muscle function of mod(-) flies. This suggests that the extreme N terminus affects actomyosin interactions in vivo, a proposal we have examined by in vitro motility assays of ACT88F F-actin from mod(-) flies. The mod(-) actin only moves in the presence of methylcellulose, a viscosity-enhancing agent, where it moves at velocities slightly, but significantly, reduced compared to wild-type. These data confirm that N-acetyl-cysteine at the N terminus affects actomyosin interactions, probably by reducing formation of the initial actomyosin collision complex, a process known to involve the actin N terminus.

Actomyosin: law and order in motility

The crystal structures of smooth muscle and scallop striated muscle myosin have both been completed in the past 18 months. Structural studies of unconventional myosins, in particular the stunning discovery that myosin VI moves backwards on actin, are starting to have deep impact on the field and have induced new ways of thinking about actin-based motility. Sophisticated genetic, biochemical and biophysical studies were used to test and refine hypotheses of the molecular mechanism of motility that were developed in the past. Although all these studies confirmed some aspects of these hypotheses, they also raised many new unresolved questions. Much of the evidence points to the importance of the actin-myosin binding process and an associated disorder-to-order transition.