Depletion of F-actin Near Surfaces

Abstract

Proximity to membranes is required of actin networks for many key cell functions, including mechanics and motility. However, F-actin rigidity should hinder a filament's approach to surfaces. Using confocal microscopy, we monitor the distribution of fluorescent actin near non-adherent glass surfaces. Initially uniform, monomers polymerize to create a depletion zone where F-actin is absent at the surface but increases monotonically with distance from the surface. At its largest, depletion effects can extend >35 μm, comparable to mass-weighted filament lengths. Increasing the rigidity of actin filaments with phalloidin increases the extent of depletion, whereas shortening filaments using capping protein reduces it proportionally. In addition, depletion kinetics are faster with higher actin concentrations, consistent with faster polymerization and faster Brownian-ratchet-driven motion. Conversely, the extent of depletion decreases with actin concentration, suggesting that entropy is the thermodynamic driving force. Quantitatively, depletion kinetics and extent match existing actin kinetics, rigidity and lengths. However, explaining depletion profiles and concentration-dependence (power-law of -1) requires modifying the rigid rod model. Dynamically crosslinked and dendritic (ARP2/3) networks either slow or enhance the extent of depletion, respectively. In cells, surface depletion should slow membrane-associated F-actin reactions another ∼10-fold beyond hydrodynamic considerations, and to favor membrane invaginations by decreased surface tension. Similar depletion principles underlie the thermodynamics of all surface-associated reactions with mechanical structures, ranging from DNA to filaments to networks. For various functions, cells must actively resist the thermodynamics of depletion.