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Thymosin-β4 (Tβ4) and profilin are the two major sequestering proteins that maintain the pool of monomeric actin (G-actin) within cells of higher eukaryotes. Tβ4 prevents G-actin from joining a filament, whereas profilin:actin only supports barbed-end elongation. Here, we report two Tβ4:actin structures. The first structure shows that Tβ4 has two helices that bind at the barbed and pointed faces of G-actin, preventing the incorporation of the bound G-actin into a filament. The second structure displays a more open nucleotide binding cleft on G-actin, which is typical of profilin:actin structures, with a concomitant disruption of the Tβ4 C-terminal helix interaction. These structures, combined with biochemical assays and molecular dynamics simulations, show that the exchange of bound actin between Tβ4 and profilin involves both steric and allosteric components. The sensitivity of profilin to the conformational state of actin indicates a similar allosteric mechanism for the dissociation of profilin during filament elongation.

Original publication

DOI

10.1073/pnas.1412271111

Type

Journal article

Journal

Proc Natl Acad Sci U S A

Publication Date

28/10/2014

Volume

111

Pages

E4596 - E4605

Keywords

crystallography, fluorescence, molecular dynamics, protein complex, thermodynamics, Actin Cytoskeleton, Actins, Allosteric Regulation, Amino Acid Sequence, Animals, Binding, Competitive, Crystallography, X-Ray, Fluorescence Polarization, Humans, Kinetics, Molecular Dynamics Simulation, Molecular Sequence Data, Polymerization, Principal Component Analysis, Profilins, Rabbits, Thymosin