title: Tropomyosin position on F-actin revealed by EM reconstruction and computational chemistry.
abstract: Electron microscopy and fiber diffraction studies of reconstituted F-actin-tropomyosin filaments reveal the azimuthal position of end-to-end linked tropomyosin molecules on the surface of actin. However, the longitudinal z-position of tropomyosin along F-actin is still uncertain. Without this information, atomic models of F-actin-tropomyosin filaments, free of constraints imposed by troponin or other actin-binding proteins, cannot be formulated, and thus optimal interfacial contacts between actin and tropomyosin remain unknown. Here, a computational search assessing electrostatic interactions for multiple azimuthal locations, z-positions, and pseudo-rotations of tropomyosin on F-actin was performed. The information gleaned was used to localize tropomyosin on F-actin, yielding an atomic model characterized by protein-protein contacts that primarily involve clusters of basic amino acids on actin subdomains 1 and 3 juxtaposed against acidic residues on the successive quasi-repeating units of tropomyosin. A virtually identical model generated by docking F-actin and tropomyosin atomic structures into electron microscopy reconstructions of F-actin-tropomyosin validated the above solution. Here, the z-position of tropomyosin alongside F-actin was defined by matching the seven broad and narrow motifs that typify tropomyosin's twisting superhelical coiled-coil to the wide and tapering tropomyosin densities seen in surface views of F-actin-tropomyosin reconstructions. The functional implications of the F-actin-tropomyosin models determined in this work are discussed.
authors.affiliation: Department of Physiology and Biophysics, Boston University School of Medicine, Boston, Massachusetts, USA.
authors.forename: Xiaochuan Edward
authors.initials: XE
authors.lastname: Li
authors.forename: Larry S
authors.initials: LS
authors.lastname: Tobacman
authors.forename: Ji Young
authors.initials: JY
authors.lastname: Mun
authors.forename: Roger
authors.initials: R
authors.lastname: Craig
authors.forename: Stefan
authors.initials: S
authors.lastname: Fischer
authors.forename: William
authors.initials: W
authors.lastname: Lehman
mesh_terms: D000199:Actins / Q000737:chemistry* / Q000378:metabolism / Q000648:ultrastructure*; D000596:Amino Acids / Q000378:metabolism; D000818:Animals; D003198:Computer Simulation; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D021621:Imaging, Three-Dimensional; D008854:Microscopy, Electron / Q000379:methods*; D008958:Models, Molecular; D011485:Protein Binding; D011817:Rabbits; D015203:Reproducibility of Results; D055672:Static Electricity; D014335:Tropomyosin / Q000737:chemistry* / Q000378:metabolism / Q000648:ultrastructure*
chemical_list: D000199:Actins; D000596:Amino Acids; D014335:Tropomyosin