Mechanical strain in actin networks regulates FilGAP and integrin binding to filamin A

Abstract

Living cells need to respond to mechanical forces for many essential biological functions. This mechanosensing activity is thought to be a property of the actin cytoskeleton, but no specific mechanisms have yet been identified. In this study, Ehrlicher et al. identify the actin-binding protein filamin A (FLNa) as a central mechanotransduction element. In a minimal reconstituted system, ligand binding to filamin is affected by mechanical forces, causing certain binding partners to dissociate and others to adhere more strongly. This selectivity may provide a direct molecular link between physical forces and biological activity. Mechanical stresses elicit cellular reactions mediated by chemical signals. Defective responses to forces underlie human medical disorders1,2,3,4 such as cardiac failure5 and pulmonary injury6. The actin cytoskeleton’s connectivity enables it to transmit forces rapidly over large distances7, implicating it in these physiological and pathological responses. Despite detailed knowledge of the cytoskeletal structure, the specific molecular switches that convert mechanical stimuli into chemical signals have remained elusive. Here we identify the actin-binding protein filamin A (FLNA)8,9 as a central mechanotransduction element of the cytoskeleton. We reconstituted a minimal system consisting of actin filaments, FLNA and two FLNA-binding partners: the cytoplasmic tail of β-integrin, and FilGAP. Integrins form an essential mechanical linkage between extracellular and intracellular environments, with β-integrin tails connecting to the actin cytoskeleton by binding directly to filamin4. FilGAP is an FLNA-binding GTPase-activating protein specific for RAC, which in vivo regulates cell spreading and bleb formation10. Using fluorescence loss after photoconversion, a novel, high-speed alternative to fluorescence recovery after photobleaching11, we demonstrate that both externally imposed bulk shear and myosin-II-driven forces differentially regulate the binding of these partners to FLNA. Consistent with structural predictions, strain increases β-integrin binding to FLNA, whereas it causes FilGAP to dissociate from FLNA, providing a direct and specific molecular basis for cellular mechanotransduction. These results identify a molecular mechanotransduction element within the actin cytoskeleton, revealing that mechanical strain of key proteins regulates the binding of signalling molecules.