Structure of the actin-depolymerizing factor homology domain in complex with actin.

Actin filament turnover is essential for cell motility and endocytosis. ADF/cofilin and twinfilin are key regulators that influence actin dynamics. ADF/cofilin promotes filament disassembly, while twinfilin sequesters monomers and caps filament ends. Both proteins use an ADF homology (ADF-H) domain to bind actin. Despite their roles, the exact mechanism of how ADF-H domains interact with actin remained unclear. No atomic structure of an ADF-H domain bound to actin had been determined. This gap motivated researchers to investigate the structural basis of these interactions. Understanding this mechanism could clarify how actin regulators function in vivo. Prior studies showed that ADF-H domains are conserved across various actin-binding proteins. However, the specific interface and functional implications were not resolved.

This study aimed to determine the structural details of the ADF-H domain in complex with actin. The researchers focused on twinfilin’s C-terminal ADF-H domain. They sought to identify how this domain interacts with actin monomers. The goal was to provide a structural framework for understanding how ADF-H domains regulate actin dynamics. The study also aimed to propose a mechanism for how these domains inhibit nucleotide exchange. Additionally, the researchers wanted to model how ADF/cofilin weakens actin filament interactions. This would clarify the molecular basis of actin regulation by ADF-H domain proteins. The findings could help explain how actin regulators influence cytoskeletal processes.

The researchers used X-ray crystallography to determine the structure of twinfilin’s C-terminal ADF-H domain bound to an actin monomer. They crystallized the complex and collected diffraction data. The structure was solved at atomic resolution to reveal the binding interface. The study analyzed the interaction between the ADF-H domain and actin subdomains 1 and 3. The team compared the interface to known ADF-H domain proteins to assess conservation. They used structural modeling to propose functional implications of the binding site. Computational tools helped simulate how the ADF-H domain might affect actin filament stability. The approach combined structural biology with biochemical analysis to validate the model.

The crystal structure revealed that the ADF-H domain binds between actin subdomains 1 and 3. This interface is conserved across ADF-H domain proteins. The binding site overlaps with regions involved in nucleotide exchange. The structure suggests a mechanism for inhibiting nucleotide exchange in actin monomers. The researchers propose that ADF-H domains block the binding of profilin or other nucleotide-exchange factors. The model indicates that ADF/cofilin weakens interactions between actin subunits in filaments. This weakening could promote filament disassembly and depolymerization. The findings provide a structural basis for how ADF-H domains regulate actin dynamics.

The study presents the first atomic structure of an ADF-H domain bound to actin. This structure identifies a conserved interface between the domain and actin subdomains 1 and 3. The findings suggest that ADF-H domains inhibit nucleotide exchange in actin monomers. The researchers propose a model for how ADF/cofilin weakens filament interactions to induce depolymerization. The study supports the idea that ADF-H domains use a shared mechanism across different proteins. The results clarify how twinfilin and ADF/cofilin regulate actin dynamics. The authors suggest that this structural framework could help explain the functional diversity of ADF-H domain proteins. The conclusions are based on the observed structure and proposed modeling of actin interactions.