F. Uecker, S. Wargenau, M. Boiero Sanders, L. Prange, R. Jekabson, A. Janning, V. Krausel, M. Gass, H. Pavenstaedt, D. Braun, M. P. Krahn, C. Schuberth, S. Raunser, P. Bieling, R. Wedlich-Soldner
The actin cytoskeleton rapidly reorganizes in response to intracellular calcium signals, driving cellular morphogenesis and wound healing. Among actin regulators, the formin INF2 uniquely mediates the "Calcium-mediated Actin Reset" (CaAR) reaction, orchestrating transient and global actin remodeling upon calcium influx. Excessive INF2 activity is linked to kidney and neuronal diseases, underscoring the need for its tight control. Combining live cell imaging with single molecule tracking, biochemistry and structural analysis we discover that INF2 activity is tightly controlled by two interlinked mechanisms: canonical intramolecular autoinhibition and binding of the INF2 N-terminus to the side of actin filaments. Side-binding limits actin elongation and supports re-establishment of autoinhibition. Disruption of this negative feedback prolongs INF2 activity, affecting plasma membrane organization and repair as well as transcriptional control. Our findings uncover a novel product-inhibition mechanism that limits INF2 function and offer important insight into disease mechanisms linked to actin dysregulation.