Résumé
Thymosin-beta 4 (T beta 4) and profilin are the two major sequestering proteins that maintain the pool of monomeric actin (G-actin) within cells of higher eukaryotes. T beta 4 prevents G-actin from joining a filament, whereas profilin: actin only supports barbed-end elongation. Here, we report two T beta 4: actin structures. The first structure shows that T beta 4 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 T beta 4 C-terminal helix interaction. These structures, combined with biochemical assays and molecular dynamics simulations, show that the exchange of bound actin between T beta 4 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.