title: Cations Stiffen Actin Filaments by Adhering a Key Structural Element to Adjacent Subunits.
abstract: Ions regulate the assembly and mechanical properties of actin filaments. Recent work using structural bioinformatics and site-specific mutagenesis favors the existence of two discrete and specific divalent cation binding sites on actin filaments, positioned in the long axis between actin subunits. Cation binding at one site drives polymerization, while the other modulates filament stiffness and plays a role in filament severing by the regulatory protein, cofilin. Existing structural methods have not been able to resolve filament-associated cations, and so in this work we turn to molecular dynamics simulations to suggest a candidate binding pocket geometry for each site and to elucidate the mechanism by which occupancy of the "stiffness site" affects filament mechanical properties. Incorporating a magnesium ion in the "polymerization site" does not seem to require any large-scale change to an actin subunit's conformation. Binding of a magnesium ion in the "stiffness site" adheres the actin DNase-binding loop (D-loop) to its long-axis neighbor, which increases the filament torsional stiffness and bending persistence length. Our analysis shows that bound D-loops occupy a smaller region of accessible conformational space. Cation occupancy buries key conserved residues of the D-loop, restricting accessibility to regulatory proteins and enzymes that target these amino acids.
authors.affiliation: Department of Chemistry, James Franck Institute, Institute for Biophysical Dynamics, and Computation Institute, The University of Chicago , Chicago, Illinois 60637, United States.
authors.forename: Glen M
authors.initials: GM
authors.lastname: Hocky
authors.affiliation: Department of Chemistry, The College of New Jersey , Ewing Township, New Jersey 08628, United States.
authors.forename: Joseph L
authors.initials: JL
authors.lastname: Baker
authors.affiliation: Molecular Biophysics and Biochemistry, Yale University , New Haven, Connecticut 06520, United States.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Bradley
authors.affiliation: Department of Chemistry, James Franck Institute, Institute for Biophysical Dynamics, and Computation Institute, The University of Chicago , Chicago, Illinois 60637, United States.
authors.forename: Anton V
authors.initials: AV
authors.lastname: Sinitskiy
authors.affiliation: Molecular Biophysics and Biochemistry, Yale University , New Haven, Connecticut 06520, United States.
authors.forename: Enrique M
authors.initials: EM
authors.lastname: De La Cruz
authors.affiliation: Department of Chemistry, James Franck Institute, Institute for Biophysical Dynamics, and Computation Institute, The University of Chicago , Chicago, Illinois 60637, United States.
authors.forename: Gregory A
authors.initials: GA
authors.lastname: Voth
mesh_terms: D008841:Actin Cytoskeleton / Q000737:chemistry* / Q000378:metabolism; D000199:Actins / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D002412:Cations / Q000737:chemistry / Q000378:metabolism; D008274:Magnesium / Q000737:chemistry / Q000378:metabolism; D056004:Molecular Dynamics Simulation; D011485:Protein Binding; D017510:Protein Folding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary
chemical_list: D000199:Actins; D002412:Cations; D008274:Magnesium