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Adseverin is a member of the calcium-regulated gelsolin superfamily of actin severing and capping proteins. Adseverin comprises six homologous domains (A1-A6) which share 60% homology with the six domains from gelsolin (G1-G6). Adseverin is truncated in comparison to gelsolin, lacking the C-terminal extension which masks the F-actin binding site in calcium-free gelsolin. Biochemical assays have indicated differences in the interaction of the C-terminus halves of adseverin and gelsolin with actin. Gelsolin contacts actin through a major site on G4 and a minor site on G6, while adseverin uses a site on A5. Here we present the X-ray structure of the activated C-terminal half of adseverin (A4-A6). This structure is highly similar to that of the activated form of the C-terminal half of gelsolin (G4-G6), both in arrangement of domains and in the three bound calcium ions. Comparative analysis of the actin-binding surfaces observed in the G4-G6/actin structure suggests that adseverin in this conformation will also be able to interact with actin through A4 and A6, while the A5 surface is obscured. A model of calcium-free adseverin constructed from the structure of gelsolin predicts that the interaction between A2 and A6 provides sterric inhibition to prevent interaction with F-actin in the absence of calcium. Actin-binding assays reveal that the minimal stoichiometry of adseverin to calcium needed to disassemble actin filaments is 1:1 as compared to the 1:2 that was previously observed for gelsolin. We propose that the absence of a gelsolin-like C-terminal extension in adseverin reduces the calcium requirement for activation.

Villin and gelsolin consist of six homologous domains of the gelsolin/cofilin fold (V1-V6 and G1-G6, respectively). Villin differs   from gelsolin in possessing at its C terminus an unrelated seventh   domain, the villin headpiece. Here, we present the crystal structure of   villin domain V6 in an environment in which intact villin would be  inactive, in the absence of bound Ca2+ or phosphorylation. The structure of V6 more closely resembles that of the activated form of  G6, which contains one bound Ca2+, rather than that of the calcium   ion-free form of G6 within intact inactive gelsolin. Strikingly   apparent is that the long helix in V6 is straight, as found in the   activated form of G6, as opposed to the kinked version in inactive   gelsolin. Molecular dynamics calculations suggest that the preferable   conformation for this helix in the isolated G6 domain is also straight   in the absence of Ca2+ and other gelsolin domains. However, the G6   helix bends in intact calcium ion-free gelsolin to allow interaction   with G2 and G4. We suggest that a similar situation exists in villin.   Within the intact protein, a bent V6 helix, when triggered by Ca2+,   straightens and helps push apart adjacent domains to expose actin-binding sites within the protein. The sixth domain in this   superfamily of proteins serves as a keystone that locks together a compact ensemble of domains in an inactive state. Perturbing the   keystone initiates reorganization of the structure to reveal previously buried actin-binding sites.