Functional effects of nemaline myopathy mutations on human skeletal alpha-actin

A solution to the long-standing problem of actin expression and purification

Actin is the most abundant protein in the cytoplasm of eukaryotic cells and interacts with hundreds of proteins to perform essential functions, including cell motility and cytokinesis. Numerous diseases are caused by mutations in actin, but studying the biochemistry of actin mutants is difficult without a reliable method to obtain recombinant actin. Moreover, biochemical studies have typically used tissue-purified α-actin, whereas humans express six isoforms that are nearly identical but perform specialized functions and are difficult to obtain in isolation from natural sources. Here, we describe a solution to the problem of actin expression and purification. We obtain high yields of actin isoforms in human Expi293F cells. Experiments along the multistep purification protocol demonstrate the removal of endogenous actin and the functional integrity of recombinant actin isoforms. Proteomics analysis of endogenous vs. recombinant actin isoforms confirms the presence of native posttranslational modifications, including N-terminal acetylation achieved after affinity-tag removal using the actin-specific enzyme Naa80. The method described facilitates studies of actin under fully native conditions to determine differences among isoforms and the effects of disease-causing mutations that occur in all six isoforms.

Atomic view into Plasmodium actin polymerization, ATP hydrolysis, and fragmentation

Plasmodium actins form very short filaments and have a noncanonical link between ATP hydrolysis and polymerization. Long filaments are detrimental to the parasites, but the structural factors constraining Plasmodium microfilament lengths have remained unknown. Using high-resolution crystallography, we show that magnesium binding causes a slight flattening of the Plasmodium actin I monomer, and subsequent phosphate release results in a more twisted conformation. Thus, the Mg-bound monomer is closer in conformation to filamentous (F) actin than the Ca form, and this likely facilitates polymerization. A coordinated potassium ion resides in the active site during hydrolysis and leaves together with the phosphate, a process governed by the position of the Arg178/Asp180-containing A loop. Asp180 interacts with either Lys270 or His74, depending on the protonation state of the histidine, while Arg178 links the inner and outer domains (ID and OD) of the actin protomer. Hence, the A loop acts as a switch between stable and unstable filament conformations, the latter leading to fragmentation. Our data provide a comprehensive model for polymerization, ATP hydrolysis and phosphate release, and fragmentation of parasite microfilaments. Similar mechanisms may well exist in canonical actins, although fragmentation is much less favorable due to several subtle sequence differences as well as the methylation of His73, which is absent on the corresponding His74 in Plasmodium actin I.

A detailed mechanistic study of malaria parasite actins reveals at the atomic level how they polymerize, hydrolyze ATP, and are fragmented to keep actin filament lengths short enough for parasite survival.

K336I mutant actin alters the structure of neighbouring protomers in filaments and reduces affinity for actin-binding proteins

Mutation of the Lys-336 residue of actin to Ile (K336I) or Asp (K336E) causes congenital myopathy. To understand the effect of this mutation on the function of actin filaments and gain insight into the mechanism of disease onset, we prepared and biochemically characterised K336I mutant actin from Dictyostelium discoideum. Subtilisin cleavage assays revealed that the structure of the DNase-I binding loop (D-loop) of monomeric K336I actin, which would face the adjacent actin-protomer in filaments, differed from that of wild type (WT) actin. Although K336I actin underwent normal salt-dependent reversible polymerisation and formed apparently normal filaments, interactions of K336I filaments with alpha-actinin, myosin II, and cofilin were disrupted. Furthermore, co-filaments of K336I and WT actins also exhibited abnormal interactions with cofilin, implying that K336I actin altered the structure of the neighbouring WT actin protomers such that interaction between cofilin and the WT actin protomers was prevented. We speculate that disruption of the interactions between co-filaments and actin-binding proteins is the primary reason why the K336I mutation induces muscle disease in a dominant fashion.

Utilization of paramagnetic relaxation enhancements for structural analysis of actin-binding proteins in complex with actin

Actin cytoskeleton dynamics are controlled by various actin binding proteins (ABPs) that modulate the polymerization of the monomeric G-actin and the depolymerization of filamentous F-actin. Although revealing the structures of the actin/ABP complexes is crucial to understand how the ABPs regulate actin dynamics, the X-ray crystallography and cryoEM methods are inadequate to apply for the ABPs that interact with G- or F-actin with lower affinity or multiple binding modes. In this study, we aimed to establish the alternative method to build a structural model of G-actin/ABP complexes, utilizing the paramagnetic relaxation enhancement (PRE) experiments. Thymosin β4 (Tβ4) was used as a test case for validation, since its structure in complex with G-actin was reported recently. Recombinantly expressed G-actin, containing a cysteine mutation, was conjugated with a nitroxyl spin label at the specific site. Based on the intensity ratio of the ¹H-¹⁵N HSQC spectra of Tβ4 in the complex with G-actin in the paramagnetic and diamagnetic states, the distances between the amide groups of Tβ4 and the spin label of G-actin were estimated. Using the PRE-derived distance constraints, we were able to compute a well-converged docking structure of the G-actin/Tβ4 complex that shows great accordance with the reference structure.

Insights into the effects of disease-causing mutations in human actins

Mutations in all six actins in humans have now been shown to cause diseases. However, a number of factors have made it difficult to gain insight into how the changes in actin functions brought about by these pathogenic mutations result in the disease phenotype. These include the presence of multiple actins in the same cell, limited accessibility to pure mutant material, and complexities associated with the structures and their component cells that manifest the diseases. To try to circumvent these difficulties, investigators have turned to the use of model systems. This review describes these various approaches, the initial results obtained using them, and the insight they have provided into allosteric mechanisms that govern actin function. Although results so far have not explained a particular disease phenotype at the molecular level, they have provided valuable insight into actin function at the mechanistic level which can be utilized in the future to delineate the molecular bases of these different actinopathies.

Distinct underlying mechanisms of limb and respiratory muscle fiber weaknesses in nemaline myopathy

Nemaline myopathy is the most common congenital myopathy and is caused by mutations in various genes such as ACTA1 (encoding skeletal α-actin). It is associated with limb and respiratory muscle weakness. Despite increasing clinical and scientific interest, the molecular and cellular events leading to such weakness remain unknown, which prevents the development of specific therapeutic interventions. To unravel the potential mechanisms involved, we dissected lower limb and diaphragm muscles from a knock-in mouse model of severe nemaline myopathy expressing the ACTA1 His40Tyr actin mutation found in human patients. We then studied a broad range of structural and functional characteristics assessing single-myofiber contraction, protein expression, and electron microscopy. One of the major findings in the diaphragm was the presence of numerous noncontractile areas (including disrupted sarcomeric structures and nemaline bodies). This greatly reduced the number of functional sarcomeres, decreased the force generation capacity at the muscle fiber level, and likely would contribute to respiratory weakness. In limb muscle, by contrast, there were fewer noncontractile areas and they did not seem to have a major role in the pathogenesis of weakness. These divergent muscle-specific results provide new important insights into the pathophysiology of severe nemaline myopathy and crucial information for future development of therapeutic strategies.

Rapid nucleotide exchange renders Asp-11 mutant actins resistant to depolymerizing activity of cofilin, leading to dominant toxicity in vivo

Conserved Asp-11 of actin is a part of the nucleotide binding pocket, and its mutation to Gln is dominant lethal in yeast, whereas the mutation to Asn in human α-actin dominantly causes congenital myopathy. To elucidate the molecular mechanism of those dominant negative effects, we prepared Dictyostelium versions of D11N and D11Q mutant actins and characterized them in vitro. D11N and D11Q actins underwent salt-dependent reversible polymerization, although the resultant polymerization products contained small anomalous structures in addition to filaments of normal appearance. Both monomeric and polymeric D11Q actin released bound nucleotides more rapidly than the wild type, and intriguingly, both monomeric and polymeric D11Q actins hardly bound cofilin. The deficiency in cofilin binding can be explained by rapid exchange of bound nucleotide with ATP in solution, because cofilin does not bind ATP-bound actin. Copolymers of D11Q and wild type actins bound cofilin, but cofilin-induced depolymerization of the copolymers was slower than that of wild type filaments, which may presumably be the primary reason why this mutant actin is dominantly toxic in vivo. Purified D11N actin was unstable, which made its quantitative biochemical characterization difficult. However, monomeric D11N actin released nucleotides even faster than D11Q, and we speculate that D11N actin also exerts its toxic effects in vivo through a defective interaction with cofilin. We have recently found that two other dominant negative actin mutants are also defective in cofilin binding, and we propose that the defective cofilin binder is a major class of dominant negative actin mutants.

Functional characterization of the human α-cardiac actin mutations Y166C and M305L involved in hypertrophic cardiomyopathy

Inherited cardiomyopathies are caused by point mutations in sarcomeric gene products, including α-cardiac muscle actin (ACTC1). We examined the biochemical and cell biological properties of the α-cardiac actin mutations Y166C and M305L identified in hypertrophic cardiomyopathy (HCM). Untagged wild-type (WT) cardiac actin, and the Y166C and M305L mutants were expressed by the baculovirus/Sf9-cell system and affinity purified by immobilized gelsolin G4-6. Their correct folding was verified by a number of assays. The mutant actins also displayed a disturbed intrinsic ATPase activity and an altered polymerization behavior in the presence of tropomyosin, gelsolin, and Arp2/3 complex. Both mutants stimulated the cardiac β-myosin ATPase to only 50 % of WT cardiac F-actin. Copolymers of WT and increasing amounts of the mutant actins led to a reduced stimulation of the myosin ATPase. Transfection of established cell lines revealed incorporation of EGFP- and hemagglutinin (HA)-tagged WT and both mutant actins into cytoplasmic stress fibers. Adenoviral vectors of HA-tagged WT and Y166C actin were successfully used to infect adult and neonatal rat cardiomyocytes (NRCs). The expressed HA-tagged actins were incorporated into the minus-ends of NRC thin filaments, demonstrating the ability to form hybrid thin filaments with endogenous actin. In NRCs, the Y166C mutant led after 72 h to a shortening of the sarcomere length when compared to NRCs infected with WT actin. Thus our data demonstrate that a mutant actin can be integrated into cardiomyocyte thin filaments and by its reduced mode of myosin interaction might be the basis for the initiation of HCM.

Two single-headed myosin V motors bound to a tetrameric adapter protein form a processive complex

Myo4p, one of two class V myosins in budding yeast, continuously transports messenger RNA (mRNA) cargo in the cell but is nonprocessive when characterized in vitro. The adapter protein She3p tightly binds to the Myo4p rod, forming a single-headed motor complex. In this paper, we show that two Myo4p-She3p motors are recruited by the tetrameric mRNA-binding protein She2p to form a processive double-headed complex. The binding site for She3p was mapped to a single α helix that protrudes at right angles from She2p. Processive runs of several micrometers on yeast actin-tropomyosin filaments were observed only in the presence of She2p, and, thus, motor activity is regulated by cargo binding. While moving processively, each head steps ~72 nm in a hand-over-hand motion. Coupling two high-duty cycle monomeric motors via a common cargo-binding adapter protein creates a complex with transport properties comparable with a single dimeric processive motor such as vertebrate myosin Va.

Purification of cytoplasmic actin by affinity chromatography using the C-terminal half of gelsolin

A new rapid method of the cytoplasmic actin purification, not requiring the use of denaturants or high concentrations of salt, was developed, based on the affinity chromatography using the C-terminal half of gelsolin (G4-6), an actin filament severing and capping protein. When G4-6 expressed in Escherichia coli was added to the lysate of HeLa cells or insect cells infected with a baculovirus encoding the beta-actin gene, in the presence of Ca(2+) and incubated overnight at 4 degrees C, actin and G4-6 were both detected in the supernatant. Following the addition of Ni-Sepharose beads to the mixture, only actin was eluted from the Ni-NTA column by a Ca(2+)-chelating solution. The functionality of the cytoplasmic actins thus purified was confirmed by measuring the rate of actin polymerization, the gliding velocity of actin filaments in an in vitro motility assay on myosin V-HMM, and the ability to activate the ATPase activity of myosin V-S1.

Recombinant alpha-actin for specific fluorescent labeling

Until recently, actin was thought to act merely as a passive track for its motility partner, myosin, during actomyosin interactions. Yet a recent report having observed dynamical conformational changes in labeled skeletal muscle alpha-actin suggests that actin has a more active role. Because the labeling technique was still immature, however, conclusions regarding the significance of the different conformations are difficult to make. Here, we describe the preparation of fully active alpha-actin obtained from a baculovirus expression system. We developed alpha-actin recombinants, of which subdomains 1 and 2 have specific sites for fluorescent probes. This specific labeling technique offers to significantly expand the information acquired from actin studies.