Filamentous actin (F-actin) is the major protein of muscle thin filaments, and actin microfilaments are the main component of the eukaryotic cytoskeleton. Mutations in different actin isoforms lead to early-onset autosomal dominant non-syndromic hearing loss1, familial thoracic aortic aneurysms and dissections2, and multiple variations of myopathies3. In striated muscle fibres, the binding of myosin motors to actin filaments is mainly regulated by tropomyosin and troponin4,5. Tropomyosin also binds to F-actin in smooth muscle and in non-muscle cells and stabilizes and regulates the filaments there in the absence of troponin6. Although crystal structures for monomeric actin (G-actin) are available7, a high-resolution structure of F-actin is still missing, hampering our understanding of how disease-causing mutations affect the function of thin muscle filaments and microfilaments. Here we report the three-dimensional structure of F-actin at a resolution of 3.7 Å in complex with tropomyosin at a resolution of 6.5 Å, determined by electron cryomicroscopy. The structure reveals that the D-loop is ordered and acts as a central region for hydrophobic and electrostatic interactions that stabilize the F-actin filament. We clearly identify map density corresponding to ADP and Mg2+ and explain the possible effect of prominent disease-causing mutants. A comparison of F-actin with G-actin reveals the conformational changes during filament formation and identifies the D-loop as their key mediator. We also confirm that negatively charged tropomyosin interacts with a positively charged groove on F-actin. Comparison of the position of tropomyosin in F-actin–tropomyosin with its position in our previously determined F-actin–tropomyosin–myosin structure8 reveals a myosin-induced transition of tropomyosin. Our results allow us to understand the role of individual mutations in the genesis of actin- and tropomyosin-related diseases and will serve as a strong foundation for the targeted development of drugs.
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Electron Microscopy Data Bank
Protein Data Bank
The coordinates for the electron microscope structure has been deposited in the Electron Microscopy Data Bank under accession code EMD-6124. Coordinates of F-actin in complex with tropomyosin have been deposited in the Protein Data Bank with accession number 3J8A.
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We thank O. Hofnagel for excellent assistance in cryo sample preparation and electron microscopy, R. S. Goody for continuous support and for comments on the manuscript and I. Vetter for assistance in data processing. We gratefully acknowledge R. Matadeen and S. de Carlo (FEI Company) for image acquisition at the National Center for Electron Nanoscopy in Leiden (NeCEN), which is co-financed by grants from the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (project 175.010.2009.001) and by the European Union’s Regional Development Fund through ‘Kansen voor West’ (project 21Z.014). J.v.d.E. is a fellow of Studienstiftung des deutschen Volkes. This work was supported by the Behrens-Weise foundation (to S.R.), NIH U54 094598 and R01 60635 (to P.A.P.), DFG MA1081/19-1 (to D.J.M.) and R37HL036153 (to W.L.).
The authors declare no competing financial interests.
Extended data figures and tables
a–h, Representative digital micrographs and corresponding representative two-dimensional class averages of F-actin decorated with tropomyosin in amorphous ice (in total 300 class averages of randomly chosen 40,000 phase-flipped segments of in total 109,242 segments from 689 images) (a, b), negatively stained (in total 300 class averages of 27,926 segments from 111 images) (c, d), negatively stained after cross-linking with glutardialdehyde (in total 300 class averages of 27,011 segments from 81 images) (e, f), and a micrograph of negatively stained bare F-actin (in total 100 class averages of 8,371 segments from 40 images) (g, h). Scale bars, 50 nm. Each class average (‘Classifications’) contains 130–200 (cryo data set) or 70–90 (negatively stained data sets) single segments. Scale bars, 10 nm. A boxed region of the digital micrograph in a was band-pass filtered to allow a better visualization of the filaments. Insets in c, e and g show 2× magnified regions of the digital micrographs. i, Two of the class averages depicted in b that show a clear tropomyosin density. The tropomyosin density is indicated by black arrow heads. Scale bar, 10 nm. j, Comparison of representative class averages of the three negative-stain data sets. Class averages of the data sets with tropomyosin (top panels) show additional density and a larger diameter than bare F-actin (bottom panel). Scale bar, 10 nm.
a, FSC curves of different areas of interest (green) by masking (see Methods). The resolution of tropomyosin was estimated using the twice down-sampled data set (2.24 Å per pixel). The FSC0.5 criterion indicates that the tropomyosin density map has a resolution of 6.5 Å. The resolution of the final F-actin density map is estimated at a resolution of 3.7 Å. b, Surfaces of F-actin with B-factors (high is red, low is blue) estimated by the reciprocal space refinement in PHENIX42. A side view as well as top views on the inward- and outward-facing surfaces, that is, facing the adjacent F-actin subunit inside the filament and oriented to the periphery, respectively, are shown. c, d, Putative structures of the C and N termini (cyan), respectively. Map density is missing in these regions.
a, Overview of the atomic model of an F-actin subunit rainbow-coloured from the N terminus (blue) to the C terminus (red). ADP and the coordinated cation, probably Mg2+, are depicted inside their corresponding map densities. b–d, Side views. Interface between the D-loop (SD2) and the SD3 of the adjacent actin subunit (b), outer two helices of SD1 (c), and inner β-sheet with N terminus (d). e, f, Front views. Connection of SD1 and SD3 (e) and nucleotide binding cleft (f). g, h, Side views. Inner β-sheet of SD3 with one highlighted strand (g) and outer part of SD3 (h). i, j, Back views. SD4 separated in two parts.
a, Table of identified residues that are involved in intra- and intermolecular interactions and known mutations. Mutation data are from refs 3, 15, 20 and 52, 53, 54, 55, 56, 57, 58. b, Overview of the D-loop bound to the hydrophobic cleft in SD3 of the F-actin molecule on top. Surfaces and residues are coloured from high (yellow) to low (white) hydrophobicity. c, Involved hydrophobic residues of the D-loop. d, e, Modifications in the D-loop, like oxidation of methionine 44 (ref. 20) (d) or mutations of methionine 44 and glycine 46 (M44T, G46G)15,20 (e) change the polarity or insert charges and consequently weaken the hydrophobic interactions. Thus, this destabilization of the intrastrand contact is connected to anomalous actin filament assembly and nemaline myopathies.
a–e, The interface at the plug involves three residues (R39, E270, D286) that form salt bridges and mediate not only one interstrand contact but also one intrastrand contact (see also Fig. 2a). In addition, the orientation of residues 264–269 result in a negatively charged patch that electrostatically interacts with positively charged residues on the opposing actin. a, b, Front and back view of the interface at the plug, respectively. c–e, Surface representations (front views in c and d, back view in e) depicting the Coulomb potential (ranging from −10 kcal mol−1 (red) to +10 kcal mol−1 (blue) at pH 7.5), indicating that the interaction of the upper region of the plug with adjacent inter- or intrastrand molecules is mediated by electrostatic interactions. f, Another interstrand contact is formed by residues 110–115 of SD1 and residues 191–199 of SD4 of the adjacent actin. However, no prominent electrostatic or hydrophobic interactions could be identified at this interface.
a, Table of identified intramolecular interactions that result in a stabilization of the nucleotide binding cleft, coordination of ADP and a divalent cation. Mutation data are from refs 3, 15, 23, 53, 54, 56, 57 and 60, 61, 62, 63, 64. b, Coordination of ADP and Ca2+ in the nucleotide binding cleft in G-actin (PDB accession code 3EL2; ref. 59). c–e, Back views of the nucleotide binding cleft of F-actin (cyan) with bound ADP–Mg2+ or ADP–Ca2+ (c), ATP–Ca2+ (d, relative position taken from PDB accession code 3EL2; ref. 59) and conformational changes between the G-actin-ATP and F-actin-ADP state (e). Glutamine 137 is moved closer to ADP, coordinating not only the cation (as in G-actin) but also the nucleotide β-phosphate (c). The presence of ATP instead of ADP in the nucleotide-binding site would be sterically unfavourable, suggesting that a different intermediate conformation exists for F-actin-ATP (d). The shorter distance of glutamine 137 to the γ-phosphate probably induces ATP hydrolysis and then afterwards the cation takes the position of the γ-phosphate in the ADP-state (e). For comparison the position of glutamine 137 in G-actin is shown in yellow and the transition from G-actin to F-actin is depicted by arrows. f, Mutation of glutamine 137 to histidine results in hampered coordination of the ion and the nucleotide and is connected to nemaline myopathies23.
Extended Data Figure 7 Model of barbed-end and pointed-end binding and G- to F-actin transition based on a comparison of start point (G-actin) and end point (F-actin).
a, b, Binding of new G-actin (yellow, PDB accession code 3EL2; ref. 59) at the barbed end of F-actin (green) is initiated by the intrastrand binding of SD4 of G-actin to SD3 of F-actin (a, side view) and the interstrand binding of SD4 and SD1 (b, front view), respectively. G-actin is overlaid with a subunit of the structure found in F-actin (cyan). The main interstrand contacts are already present at the start of the transition and thereby guide the binding G-actin to its correct position, determining the symmetry of the filament. c, After initial binding the D-loop is trapped in the hydrophobic cleft of SD3 of F-actin and pulls on SD2. F-actin is depicted in surface representation (green). d, Finally, the SD2 and concomitantly SD1 are rotated and the final F-actin conformation is stabilized by various intramolecular interactions (see Figs 2c–f and 3c–f, Extended Data Fig. 6a). e, Owing to the relatively large distance from the SD1 and SD2 subdomains of the newly bound G-actin to the subunit of the opposite strand, the only influence of the opposite strand on the binding of G-actin is at the docking position of the D-loop. The transition of R39 illustrates well the docking of the D-loop. The residue is depicted in both states (G-actin and F-actin). f–h, Binding of new G-actin (yellow, PDB accession code 3EL2; ref. 59) at the pointed end of F-actin (green) is initiated by initial intrastrand binding (g) of SD3 of G-actin to SD4 of F-actin at the pointed end and interstrand binding of SD4 to SD3 (h), respectively. Again, the main interstrand contacts are available before the transition of G-actin (yellow) to F-actin (SD1 and SD2 in cyan, SD3 and SD4 in red). i, During binding to F-actin the transition from G-actin to F-actin is initiated by an induced fit of the F-actin D-loop to the hydrophobic cleft of the newly bound G-actin. This leads to a pulling down of the central β-sheet of SD3 of G-actin. j, The β-sheet is thereby straightened and pushes up two adjacent helices of SD1. k, The slight dislocation of these helices is transmitted to other regions of SD1 and thereby emphasized. This leads to a global rotation of SD1, which results in a considerable rotation of SD2 by an angle of 20° and a closure of the nucleotide binding cleft (Fig. 3; see Supplementary Video 3).
Extended Data Figure 8 Tropomyosin binding and comparison of reconstructions regarding the tropomyosin position on F-actin.
a, Table showing putative residues of F-actin involved in tropomyosin binding and known mutations. Mutation data are from refs 15 and 65. b–e, Reconstructions of F-actin decorated with tropomyosin calculated from: the cryo-EM data set filtered to 15 Å (b), from a negatively stained data set (c), from a negatively stained data set after cross-linking with glutardialdehyde (d), and from negatively stained bare F-actin (e). f, By calculating a difference map between tropomyosin–bare F-actin (blue) and glutardialdehyde–bare F-actin (green), differences in the tropomyosin position on bare F-actin (grey) are visualized. g, Overlay of difference maps showing that the position of tropomyosin in the cryo-EM reconstruction (yellow) corresponds to the tropomyosin position of the negatively stained data set with the cross-linked complex (green). h, Surface of F-actin and tropomyosin (pseudo-repeats 2–6) with the electrostatic Coulomb potentials at pH 7.5 and pH 4, ranging from −10 kcal mol−1 (red) to +10 kcal mol−1 (blue) (see also Fig. 4b). Tropomyosin was rotated by 180° and shifted to the right to allow a better view of the F-actin–tropomyosin interface. Difference maps of the glutardialdehyde–bare F-actin and the tropomyosin–bare F-actin map are shown on the F-actin surface at pH 7.5 (left) and pH 4.0 (middle), respectively.
a, Cryo-EM structure of the F-actin–tropomyosin complex with tropomyosin in the A-state. Tropomyosin (yellow, A-state), F-actin (green). b–d, Initial weak binding of myosin (magenta, PDB accession code 1LKX; ref. 66) to the F-actin–tropomyosin filament in the absence of troponin. Most of the myosin binding sites on F-actin are not occupied by tropomyosin and only loop 4 and the cardiomyopathy loop are sterically hindered from binding to the F-actin filament (b). Actin-induced closure of the 50-kDa cleft of myosin (c) results in a strong binding of myosin and tropomyosin moves to its M-state position (blue) (d). e–g, F-actin–tropomyosin–myosin complex in the rigor state (PDB accession code 4A7H; ref. 8). Myosin is shown in red. h–l, There are two possible ways for the transition of tropomyosin from the A-state to the M-state. Tropomyosin either rolls (i, j) or slides (k, l) from one to the other position. Rolling would involve an azimuthal rotation of ∼16° with respect to the F-actin axis (inset of j) and a left-handed rotation of ∼70° (indicated by solid arrows) or a right-handed rotation of ∼110° (indicated by dotted arrows) with respect to its own axis (i, j). Sliding would imply not only an azimuthal shift of ∼12 Å (indicated by white arrows in k), but also a tremendous shift of a half-tropomyosin repeat (that is, ∼35 Å) along the F-actin filament (k, l). The radius to the filament axis would be preserved in both situations. The inset of l depicts the vectors for a shifting transition of tropomyosin: an azimuthal and longitudinal shift of 12 Å and 35 Å, resulting in an overall shift of 37 Å.
This file contains a Supplementary Discussion and an additional reference. (PDF 179 kb)
a: Cryo-EM density of F-actin (grey, five central subunits: green, central subunit: cyan) in complex with tropomyosin (yellow) at an average resolution of 3.7 Å for F-actin and 6.5 Å for tropomyosin b: Zoom on the central five F-actin subunits. c: Zoom on the central F-actin subunit. (MOV 27110 kb)
a: Cryo-EM density of an F-actin subunit with the respective atomic model. b: Zoom on α-helices of the nucleotide binding cleft with ADP and the coordinated cation, probably Mg2+. c: Zoom on the β sheet of SD3. d: Atomic model of an F-actin subunit rainbow coloured from the N-terminus (blue) to the C-terminus (red). (MOV 26400 kb)
a-c: Morph between G-actin (yellow, PDB: 1J6Z) to F-actin (cyan) shown in side, top and front view, respectively. d: Zoom on the nucleotide binding cleft highlighting Q137, which has been shown to play a central role in ATP hydrolysis. For better illustration, the F-actin model has been hidden. After the transition to F-actin, ATP and the coordinated cation are cross-faded to ADP and the coordinated cation. (MOV 27281 kb)
a: Cryo-EM structure of the F-actin-tropomyosin complex with tropomyosin in the A-state (yellow) and M-state (PDB: 4A7H) (blue), F-actin (green). b-d: Side and bottom views on myosin (magenta, PDB: 1LKX) binding to the F-actin-tropomyosin filament in the absence of troponin. Actin-induced closure of the 50-kDa cleft of myosin results in a strong binding of myosin and tropomyosin moves to its M-state position (blue). There are two possible ways for the transition of tropomyosin between the two states. The video shows only the right-handed rotation of ∼ 110°. (MOV 26987 kb)
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von der Ecken, J., Müller, M., Lehman, W. et al. Structure of the F-actin–tropomyosin complex. Nature 519, 114–117 (2015). https://doi.org/10.1038/nature14033
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