Liquid-like VASP condensates drive actin polymerization and dynamic bundling

Abstract

The organization of actin filaments into bundles is required for cellular processes such as motility, morphogenesis, and cell division. Filament bundling is controlled by a network of actin binding proteins. Recently, several proteins that comprise this network have been found to undergo liquid-liquid phase separation. How might liquid-like condensates contribute to filament bundling? Here we show that the processive actin polymerase, VASP, forms liquid-like droplets under physiological conditions. As actin polymerizes within VASP droplets, elongating filaments partition to the edges of the droplet to minimize filament curvature, forming an actin-rich ring within the droplet. The rigidity of this ring is balanced by the droplet’s surface tension, as predicted by a continuum-scale computational model. However, as the ring grows thicker, its rigidity increases and eventually overcomes the surface tension of the droplet, deforming into a linear bundle. The resulting bundles contain parallel actin filaments that grow from their tips. Growing bundles zipper together upon contact with one another, an effect which is mediated by the surface tension of the liquid-like VASP droplets that encapsulate them. Once the parallel arrangement of filaments is created within a VASP droplet, it propagates through the addition of new actin monomers to achieve a length that is many times greater than the initial droplet. This droplet-based mechanism of bundling may be relevant to the assembly of cellular architectures rich in parallel actin filaments, such as filopodia, stress fibers, and focal adhesions.

Spatially controlled microtubule nucleation and organization from crosslinker MAP65 condensates

“…Such spindle-like microtubule assemblies are like liquid crystal tactoids which were previously observed for microtubules (34,35) and actin (56,57). A recent report demonstrated that condensates made of actin-bundling protein VASP can nucleate actin to make tactoid-shaped bundles (58). The use of cytoskeletal-associated proteins to form condensates that can direct the organization of microtubules and actin is likely a general organizational principle in cells.…”

“…The use of cytoskeletal-associated proteins to form condensates that can direct the organization of microtubules and actin is likely a general organizational principle in cells. Indeed, cytoskeletal fiber formation has recently been reported for numerous cellular and in vitro studies for microtubules (21,22,25) and actin (58)(59)(60)(61). This may suggest that cells use this universal strategy to control cytoskeletal filament organization ( 62), but it is especially important for cell-types or regions of the cell that do not have centrioles to nucleate and grow microtubules in the traditional manner.…”

“…Such spindle-like microtubule assemblies are like liquid crystal tactoids which were previously observed for microtubules (34, 35) and actin (56, 57). A recent report demonstrated that condensates made of actin-bundling protein VASP can nucleate actin to make tactoid-shaped bundles (58).…”

A phase-separated biomolecular condensate nucleates polymerization of the tubulin homolog FtsZ to spatiotemporally regulate bacterial cell division

“…The nucleation of FtsZ filament bundles by PomY condensates is phenomenonologically similar to synthetic coacervates that artificially enrich FtsZ (te Brinke et al, 2018). More importantly, it is strikingly reminiscent of eukaryotic condensates that stimulate polymerization of cytoskeletal proteins like tubulin (Hernandez-Vega et al, 2017; King and Petry, 2020; Woodruff et al, 2017) or actin (Banjade and Rosen, 2014; Graham et al, 2022; Li et al, 2012; Su et al, 2016; Yang et al, 2022), suggesting a broadly conserved role of condensates in the spatiotemporal regulation of protein filament formation and, in particular, that condensate stimulation of tubulin polymerization is an ancient mechanism. In vitro PomY condensates deformed during FtsZ filament formation and covered the FtsZ filament bundles leading to the alignment and bundling of growing filaments into larger networks.…”

“…For all condensates, the interface between the two phases forms a boundary that serves as a selective barrier for some molecules but not for others, resulting in selective enrichment of so-called client proteins and/or RNA molecules (Alberti and Hyman, 2021; Banani et al, 2017; Lyon et al, 2021; Shin and Brangwynne, 2017). In this way, biomolecular condensates can enhance chemical reactions, sequester molecules, or act as hubs to nucleate microtubule or actin polymerization (Banjade and Rosen, 2014; Graham et al, 2022; Hernandez-Vega et al, 2017; King and Petry, 2020; Li et al, 2012; Woodruff et al, 2017; Yang et al, 2022).…”