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.
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.).
Extended data figures
Extended data tables
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.
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).
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.
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°.
About this article
Mechanism of actin polymerization revealed by cryo-EM structures of actin filaments with three different bound nucleotides
Proceedings of the National Academy of Sciences (2019)