Spatiotemporal Domains in Active Actin Gels

Abstract

Using a standard actin filament gliding assay, rhodamine-phalloidin labeled actin filaments were visualized moving over a myosin-II coated surface by fluorescence microscopy and filament tracks were analyzed by computer. When micromolar amounts of non-fluorescent phalloidin-stabilised actin were added, filament motion became aligned into a common direction (Butt et al., 2010, J Biol Chem285:4964-4974). Experiments conducted at low filament densities show that, on average, there is a small angular deflection that tends to align the paths of individual colliding filaments. Path alignment is independent of relative track directionality and increases with collision incident angle. Filament elastic deformation during collision events indicates the collision bending energy is ∼20kBT. As increasing amounts of bulk actin are added, individual filament paths become straighter and start to move in a common direction. Spatial correlation analysis shows that increased alignment is due to incremental recruitment of filament tracks into one orientation rather than fusion of seed domains to larger ones. Experiments performed over a wide range of actin filament concentrations and HMM surface densities, show that ordering of filament motion peaks over a range of conditions and decreases at extreme values. This finding differs from theoretical prediction of a critical phase transition (Kraikivski et al., 2006, Phys Rev. Lett.96:258103). Loss of order at high bulk actin concentration might be explained if there is increased filament exchange between bulk, disordered, actin and surface bound material. Loss of order at high myosin surface densities might arise from increased probability of motor attachment that would reduce ability of actin to change its path following a collision. Although our results are broadly compatible with generation of long-range order from mechanics of individual actin filament interactions; other phenomena become important at high protein concentrations.