Structure, Function, and Evolution of the β-Thymosin/WH2 (WASP-Homology2) Actin-Binding Module

β-thymosins are acknowledged G-actin sequesterers. However, in the recent years, the conserved β-thymosins/WH2 actin-binding module, has been identified in a large number of proteins that all interact with actin and play diverse functions in cell motility. The functional evolution of the WH2 domain has been approached by a combination of structural and biochemical methods, using thymosin β4 (Tβ4) and Ciboulot, a 3 β-thymosin repeat protein from Drosophila as models. Ciboulot binds actin like Tβ4 but promotes actin assembly like profilin. The first repeat of Ciboulot (D1) has the profilin function of the whole protein. The crystal structure of Ciboulot-actin shows that the major interaction with G-actin lies in the N-terminal amphipathic helix of D1. By point mutagenesis the sequestering activity of Tβ4 can be changed into a profilin activity. (1H, 15N)-NMR studies show that the functional switch from inhibition to promotion of actin assembly is linked to a change in the dynamics of interaction of the central and C-terminal regions of the WH2 domain with subdomains 1 and 2 of G-actin. Further systematic mutagenesis studies have been performed by engineering a series of chimeras of Ciboulot and Tβ4. Proteins displaying either profilin function or enhanced sequestering activity compared to Tβ4 have been characterized. The results provide insight into the structural basis for the regulation of the multiple functions of the WH2 domain.