Aip1 Destabilizes Cofilin-Saturated Actin Filaments by Severing and Accelerating Monomer Dissociation from Ends

Abstract

SUMMARY Background Depolymerization of actin filaments is vital for the morphogenesis of dynamic cytoskeletal arrays and actin-dependent cell-motility. Cofilin is necessary for actin disassembly in cells, and it severs filaments most efficiently at low cofilin to actin ratios, whereas higher concentrations of cofilin suppress severing. However, the cofilin concentration in thymocytes is too high to allow the severing of single actin filaments. Results We observed that filaments sever efficiently in thymus cytosol. We identified Aip1 as a critical factor responsible for the severing and destabilization of actin filaments even in the presence of high amounts of cofilin. By FRET-based spectroscopy and single-filament imaging of actin, we show that besides driving the rapid severing of cofilin-actin filaments, Aip1 also augments the monomer dissociation rate at both the barbed and pointed ends of actin. Our results also demonstrate that Aip1 does not cap the barbed ends of actin filaments, as was previously thought. Significance Our results indicate that Aip1 is a cofilin-dependent actin depolymerization factor and not a barbed-end capping factor as was previously thought. Aip1 inverts the rules of cofilin-mediated actin disassembly such that increasing ratios of cofilin to actin now result in filament destabilization through faster severing and accelerated monomer loss from barbed and pointed ends. Aip1 therefore offers a potential control point for disassembly mechanisms in cells to switch from a regime of cofilin-saturation and stabilization to one that favors fast disassembly and destabilization.

Actin Mechanics and Fragmentation

“…One model of enhanced severing (61) implicates further structural changes in cofilin-actin induced by Aip1, possibly a wedge-like mechanism where cofilin is inserted between adjacent long-axis neighbors. Consistent with a functional ternary Aip1-cofilin-actin complex, Aip1 does not dissociate cofilin and enhances severing at all cofilin occupancies (62). However, a different study (63) favors an alternate mechanism (64) in which Aip1 simply competes with cofilin, dissociating it from actin and introducing additional boundaries where severing can occur.…”

Regulators of actin filament barbed ends at a glance

“…The latter effect is responsible for enhanced growth of individual filament barbed ends, which supports motility. Recently, other factors such as Aip1 (also known as DAB2IP), coronin proteins, twinfilin proteins and cyclase-associated proteins (CAP, Srv2 in yeast) have been reported to further enhance filament disassembly in synergy with ADF and cofilin proteins (Gressin et al, 2015;Johnston et al, 2015;Kueh et al, 2008;Mikati et al, 2015;Nadkarni and Brieher, 2014). These effects, however, do not appear to involve barbed-end capping by these proteins, in contrast with what has been previously suggested (Okada et al, 2002).…”

Cellular functions of the ADF/cofilin family at a glance

“…Other factors have recently been identified to cooperate with cofilin and to enhance the depolymerization of actin filaments. These factors include actininteracting protein 1 (Aip1), twinfilin proteins, adenylyl cyclaseassociated protein 1 (CAP1) and the coronins (Chen et al, 2015;Gressin et al, 2015;Johnston et al, 2015;Kueh et al, 2008;Mikati et al, 2015;Nadkarni and Brieher, 2014).…”