Sizes of actin networks sharing a common environment are determined by the relative rates of assembly

Abstract

Within the cytoplasm of a single cell, several actin networks can coexist with distinct sizes, geometries, and protein compositions. These actin networks assemble in competition for a limited pool of proteins present in a common cellular environment. To predict how two distinct networks of actin filaments control this balance, the simultaneous assembly of actin-related protein 2/3 (Arp2/3)-branched networks and formin-linear networks of actin filaments around polystyrene microbeads was investigated with a range of actin accessory proteins (profilin, capping protein, actin-depolymerizing factor [ADF]/cofilin, and tropomyosin). Accessory proteins generally affected actin assembly rates for the distinct networks differently. These effects at the scale of individual actin networks were surprisingly not always correlated with corresponding loss-of-function phenotypes in cells. However, our observations agreed with a global interpretation, which compared relative actin assembly rates of individual actin networks. This work supports a general model in which the size of distinct actin networks is determined by their relative capacity to assemble in a common and competing environment.

The dynamics of actin network turnover is self-organized by a growth-depletion feedback

“…www.nature.com/scientificreports www.nature.com/scientificreports/ pool 24,44 to also include the localized depletion effects of the actin binding partners. Thus it is not only the actin monomer availability which regulates the homeostasis of actin cytoskeletal networks, but the local availability of depolymerization factors and accessory proteins as well 25,45 .…”

“…While these processes have been identified by the in vitro reconstitution of single filament dynamics, how these effects can be scaled up to the mechanisms governing the dynamics of actin networks is so far unknown 22 . Recent publications emphasize the importance of such systems, as certain effects can only be achieved as a consquence of global actin turnover [23][24][25] . Here, the competition for a shared actin pool, as well as maintained gradients of the actin monomer concentration are of significance, which are not taken into account from a single filament perspective.…”

A functional family of fluorescent nucleotide analogues to investigate actin dynamics and energetics

“…The following proteins were overexpressed in Rosetta 2(DE3)pLysS cells and purified as described in the following references: S. cerevisiae profilin (Pfy1p) 34 , S. cerevisiae WASp (Gst-Las17(375-Cter)-6xHis) 35 , S. cerevisiae ADF/cofilin (Cof1p) 36 , and S. cerevisiae capping protein (Cap1p/Cap2p) 35 . Human tropomyosin-1.6 (Tpm1.6) N-terminally fused to sfGFP (sfGFP-Tpm1.6) was purified as described in Gateva et al 14 .…”

Diversity from similarity: cellular strategies for assigning particular identities to actin filaments and networks

“…Tropomyosins are dimers of α-helices forming parallel coiled-coils that span several actin subunits [ 123 ]. A biochemical link between formins and tropomyosins has been described in vitro , and cooperativity between these proteins is established [ 136 , 142 , 143 ]. In budding yeast, the presence of tropomyosin can specifically increase the nucleation rate of a formin.…”