Mechanics of blunting of actin-myosin interaction dynamics by the actinopathy-causing mutation E334Q in cytoskeletal γ-actin

Cellular processes such as cytokinesis, apoptosis and migration rely heavily on the myosin-based contractility of the γ-actin network in the submembrane cortex. Direct measurements of γ-actin–myosin interactions through morphological and depletion investigations remain elusive. Here, we use a synthetic nanomachine, consisting of an array of myosin motors carried on a nanopositioner and brought to interact with an actin filament attached to a bead trapped in the focus of dual laser optical tweezers. The nanomachine is able to mimic the loading conditions of γ-actin–myosin interactions in situ, allowing measurements of the maximum steady force (F0) and of the shortening velocity against loads < F0. Comparative measurements are conducted on wild-type γ-actin and γ-actin carrying the E334Q mutation, associated with non-muscle actinopathies. Our results show that the force of the single actin–myosin interaction is 2.5 pN for the wild-type actin and is halved by the mutation. The kinetics of motor attachment–detachment, underpinning the rate of isometric force rise and the force–velocity relation, are also reduced by a factor of two, resulting in a reduction of the maximum nanomachine power to one-fifth. The identification and quantitative definition of the loss of basic function caused by the E334Q γ-actin mutation serve as a starting point for understanding the chain of remodelling events leading to the pathological phenotype and demonstrate the potential of the nanomachine for targeted therapeutic interventions.