Structural and Biochemical Basis for the Inhibitory Effect of Liprin-α3 on Mouse Diaphanous 1 (mDia1) Function

Diaphanous-related formins are eukaryotic actin nucleation factors regulated by an autoinhibitory interaction between the N-terminal RhoGTPase-binding domain (mDia(N)) and the C-terminal Diaphanous-autoregulatory domain (DAD). Although the activation of formins by Rho proteins is well characterized, its inactivation is only marginally understood. Recently, liprin-alpha 3 was shown to interact with mDia1. Overexpression of liprin-alpha 3 resulted in a reduction of the cellular actin filament content. The molecular mechanisms of how liprin-alpha 3 exerts this effect and counteracts mDia1 activation by RhoA are unknown. Here, we functionally and structurally define a minimal liprin-alpha 3 core region, sufficient to recapitulate the liprin-alpha 3 determined mDia1-respective cellular functions. We show that liprin-alpha 3 alters the interaction kinetics and thermodynamics of mDia(N) with RhoA center dot GTP and DAD. RhoA displaces liprin-alpha 3 allosterically, whereas DAD competes with liprin-alpha 3 for a highly overlapping binding site on mDia(N). Liprin-alpha 3 regulates actin polymerization by lowering the regulatory potency of RhoA and DAD on mDia(N). We present a model of a mechanistically unexplored and new aspect of mDia(N) regulation by liprin-alpha 3.