α-Catenin Structure and Nanoscale Dynamics in Solution and in Complex with F-Actin

Abstract As a core component of the adherens junction, α -catenin stabilizes the cadherin/catenin complexes to the actin cytoskeleton for the mechanical coupling of cell-cell adhesion. α -catenin also modulates actin dynamics, cell polarity, and cell-migration functions that are independent of the adherens junction. We have determined the solution structures of the α -catenin monomer and dimer using in-line size-exclusion chromatography small-angle X-ray scattering, as well as the structure of α -catenin dimer in complex to F-actin filament using selective deuteration and contrast-matching small angle neutron scattering. We further present the first observation, to our knowledge, of the nanoscale dynamics of α -catenin by neutron spin-echo spectroscopy, which explicitly reveals the mobile regions of α -catenin that are crucial for binding to F-actin. In solution, the α -catenin monomer is more expanded than either protomer shown in the crystal structure dimer, with the vinculin-binding M fragment and the actin-binding domain being able to adopt different configurations. The α -catenin dimer in solution is also significantly more expanded than the dimer crystal structure, with fewer interdomain and intersubunit contacts than the crystal structure. When in complex to F-actin, the α -catenin dimer has an even more open and extended conformation than in solution, with the actin-binding domain further separated from the main body of the dimer. The α -catenin-assembled F-actin bundle develops into an ordered filament packing arrangement at increasing α -catenin/F-actin molar ratios. Together, the structural and dynamic studies reveal that α -catenin possesses dynamic molecular conformations that prime this protein to function as a mechanosensor protein.