How a single residue in individual β-thymosin/WH2 domains controls their functions in actin assembly

beta-Thymosin (beta T) and WH2 domains are widespread, intrinsically disordered actin-binding peptides that display significant sequence variability and different regulations of actin self-assembly in motile and morphogenetic processes. Here, we reveal the structural mechanisms by which, in their 1:1 stoichiometric complexes with actin, they either inhibit assembly by sequestering actin monomers like Thymosin-beta 4, or enhance motility by directing polarized filament assembly like Ciboulot beta T. We combined mutational, functional or structural analysis by X-ray crystallography, SAXS (small angle X-ray scattering) and NMR on Thymosin-beta 4, Ciboulot, TetraThymosin beta and the long WH2 domain of WASP-interacting protein. The latter sequesters G-actin with the same molecular mechanisms as Thymosin-beta 4. Functionally different beta T/WH2 domains differ by distinct dynamics of their C-terminal half interactions with G-actin pointed face. These C-terminal interaction dynamics are controlled by the strength of electrostatic interactions with G-actin. At physiological ionic strength, a single salt bridge with actin located next to their central LKKT/V motif induces G-actin sequestration in both isolated long beta T and WH2 domains. The results open perspectives for elucidating the functions of beta T/WH2 domains in other modular proteins. The EMBO Journal (2012) 31, 1000-1013. doi: 10.1038/emboj.2011.461; Published online 23 December 2011