The bundling of actin filaments underlies essential cellular processes such as motility, morphogenesis, and cell division. Organization of actin filaments into bundles is regulated by actin-binding proteins, some of which undergo liquid-liquid phase separation. For example, VASP (vasodilator-stimulated phosphoprotein), a processive actin polymerase and bundling protein, forms liquid-like condensates under physiological conditions. Within these condensates, actin polymerization drives filament partitioning to the droplet edge, where filaments align into an actin-rich ring. As the ring thickens, its increased rigidity overcomes the surface tension of the condensate, deforming it into a rod-like structure filled with a bundle of parallel actin filaments. Myosin motor proteins are responsible for transporting cargo by moving unidirectionally along actin filaments. For example, myosin V moves toward the barbed (plus) end of an actin filament, while myosin VI moves toward the pointed (minus) end. Here, we asked how the ability of VASP condensates to bundle actin filaments might impact the motility of myosin. We hypothesized that the directional motility of myosin motors could be used to determine the orientation of actin filaments within bundles, and that condensates might alter myosin processivity relative to bare actin filaments. Our approach includes: (1) using myosin V and VI to confirm bundle polarity in linear VASP-actin droplets, (2) quantifying changes in myosin processivity in condensate versus non-condensate environments, and (3) assessing how condensates of different proteins affect myosin activity. This work will advance our understanding of how condensates regulate actin-myosin interactions, with implications for intracellular transport, a process involved in many essential cellular functions.
Jordan et al. (Sun,) studied this question.