Molecular Mechanism for the Actin-Binding Domain of α-Actinin Ain1 Elucidated by Molecular Dynamics Simulations and Mutagenesis Experiments

In the fission yeast Schizosaccharomyces pombe, alpha-actinin Ain1 bundles F-actin into the contractile ring (CR) in the middle of the cell. Previous studies have proposed that a conformational change of the actin-binding domain (ABD) of Ain1 enhances the actin-binding activity. However, the molecular mechanism of the conformational change remains to be unveiled at an atomic resolution due to the difficulties of experimental techniques to observe them. In the present study, we performed a set of microsecond-order molecular dynamics (MD) simulations for ABD of Ain1. Our MD simulations for a pathogenic point mutation (R216E) in ABD did not result in large domain motions as previously expected. However, local motions of the loop regions were detected. Besides the three conventional actin-binding sites, we found characteristic electrostatic interactions with the N-terminal of actin. The mutagenesis experiment in fission yeast showed that collapses of the electrostatic interactions at the binding site abolished the proper localization of Ain1 to the CR. Furthermore, the MD simulation of F-actin with the Ain1 ABD R216E indicated that the stronger affinity is caused by a direct interaction of the point mutation. Our findings might be applicable to other highly conserved ABP family proteins to explain their binding affinities.