title: Structure of the F-actin-tropomyosin complex.
abstract: 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 loss, familial thoracic aortic aneurysms and dissections, and multiple variations of myopathies. In striated muscle fibres, the binding of myosin motors to actin filaments is mainly regulated by tropomyosin and troponin. 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 troponin. Although crystal structures for monomeric actin (G-actin) are available, 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 Mg(2+) 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 structure 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.
authors.affiliation: Department of Structural Biochemistry, Max Planck Institute of Molecular Physiology, 44227 Dortmund, Germany.
authors.forename: Julian
authors.initials: J
authors.lastname: von der Ecken
authors.affiliation: Institute for Biophysical Chemistry, Hannover Medical School, 30625 Hannover, Germany.
authors.forename: Mirco
authors.initials: M
authors.lastname: Müller
authors.affiliation: Department of Physiology and Biophysics, Boston University School of Medicine, Boston, Massachusetts 02118, USA.
authors.forename: William
authors.initials: W
authors.lastname: Lehman
authors.affiliation: Institute for Biophysical Chemistry, Hannover Medical School, 30625 Hannover, Germany.
authors.forename: Dietmar J
authors.initials: DJ
authors.lastname: Manstein
authors.affiliation: Department of Biochemistry and Molecular Biology, The University of Texas, Houston Medical School, Houston, Texas 77030, USA.
authors.forename: Pawel A
authors.initials: PA
authors.lastname: Penczek
authors.affiliation: Department of Structural Biochemistry, Max Planck Institute of Molecular Physiology, 44227 Dortmund, Germany.
authors.forename: Stefan
authors.initials: S
authors.lastname: Raunser
mesh_terms: D000199:Actins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D000244:Adenosine Diphosphate / Q000378:metabolism; D000818:Animals; D020285:Cryoelectron Microscopy; D008274:Magnesium / Q000378:metabolism; D051379:Mice; D008958:Models, Molecular; D009154:Mutation / Q000235:genetics; D011487:Protein Conformation; D011817:Rabbits; D055672:Static Electricity; D014335:Tropomyosin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*
chemical_list: D000199:Actins; D014335:Tropomyosin; D000244:Adenosine Diphosphate; D008274:Magnesium