Structural basis of actin sequestration by thymosin-β4: Implications for WH2 proteins

The WH2 (Wiscott-Aldridge syndrome protein homology domain 2) repeat is an actin interacting motif found in monomer sequestering and filament assembly proteins. We have stabilized the prototypical WH2 family member, thympsin-β4 (Tβ4), with respect to actin, by creating a hybrid between gelsolin domain 1 and the C-terminal half of Tβ4 (G1-Tβ4). This hybrid protein sequesters actin monomers, severs actin filaments and acts as a leaky barbed end cap. Here, we present the structure of the G1-Tβ4:actin complex at 2 Å resolution. The structure reveals that Tβ4 sequesters by capping both ends of the actin monomer, and that exchange of actin between Tβ4 and profilin is mediated by a minor overlap in binding sites. The structure implies that multiple WH2 motif-containing proteins will associate longitudinally with actin filaments. Finally, we discuss the role of the WH2 motif in arp2/3 activation.