Myosin-driven fragmentation of actin filaments triggers contraction of a disordered actin network

Abstract

The dynamic cytoskeletal network is responsible for cell shape changes and cell division. The actinbased motor protein myosin II drives the remodeling of a highly disordered actin-based network and enables the network to perform mechanical work such as contraction, migration and adhesion. Myosin II forms bipolar filaments that self-associate via their tail domains. Such myosin minifilaments generate both extensile and compressive forces that pull and push actin filaments, depending on the relative position of myosin and actin filaments in the network. However, it remains unclear how the mechanical properties of myosin II that rely on the energy of ATP hydrolysis spontaneously contract the disordered actin network. Here, we used a minimal in vitro reconstituted experimental system consisting of actin, myosin, and a cross-linking protein, to gain insights into the molecular mechanism by which myosin minifilaments organize disordered actin networks into contractile states. We found that contracted cluster size and time required for the onset of network contraction decreased as ATP concentration decreased. Contraction velocity was negatively correlated with ATP concentrations.Reduction of ATP concentration caused fragmentation of actin filaments by myosin minifilament. We also found that gelsolin, a Ca 2+ -regulated actin filament-severing protein, induced contraction of a mechanically stable network, implying that fragmentations of actin filaments in the network weaken the intra-network connectivity and trigger contraction. Our findings reveal that the disordered actin network contraction can be controlled by fragmentation of actin filaments, highlighting the molecular mechanism underlying the myosin motor-severing activities, other than the sliding tensile and compressive stress in the disordered actin network.

The biochemical composition of the actomyosin network sets the magnitude of cellular traction forces

“…However, their individual contents only partially predicted traction-force magnitude and the production of high traction forces appeared to require both phospho-myosin and F-actin in an optimum ratio (Figure 4C). This is consistent with the complex functional roles of myosin pulling on actin filaments in generating intracellular force (Thoresen et al, 2011;Reymann et al, 2012;Stachowiak et al, 2012) and in crosslinking and disassembling actin filaments (Haviv et al, 2008;Wilson et al, 2010;Reymann et al, 2012;Stachowiak et al, 2014;Matsuda et al, 2018).…”

Myosin and gelsolin cooperate in actin filament severing and actomyosin motor activity

“…The results reflect a complex interplay between actin, HMM, [ATP], [Ca 2+ ], and gelsolin. Myosin generates contractile, extensile, bending, and torsional forces ( 65 , 66 , 67 ) with shearing, buckling, and eventually severing of the actin filaments ( 25 ) even in the absence of actin-binding proteins such as cofilin and gelsolin. Myosin-driven actin filament fragmentation in the in vitro motility assay experiments occurs in the presence of nearly physiological (millimolar) concentrations of ATP ( 25 ).…”

Myosin-gelsolin cooperativity in actin filament severing and actomyosin motor activity

“…3). These effects can be accommodated by local effects of gelsolin at its binding site to the actin filament and does not necessarily rely on gelsolin induced long-range allosteric changes in filament structure as suggested previously [74,77]. However, the effects of gelsolin binding on the sliding velocity at different filament lengths are difficult to explain without such long-range changes along the actin filament, previously suggested based on spectroscopic [29,47] and electron microscopic [46,78] evidence.…”

“…The results reflect a complex interplay between actin, HMM, [ATP], [Ca 2+ ] and gelsolin. Myosin generates contractile, extensile, bending and torsional forces [74][75][76] with shearing, buckling and eventually severing of the actin filaments [26] even in the absence of actin-binding proteins such as cofilin and gelsolin. Myosin-driven actin filament fragmentation in the in vitro motility assay experiments occurs in the presence of nearly physiological millimolar concentrations of ATP [26].…”