Ena/VASP Enabled is a highly processive actin polymerase tailored to self-assemble parallel-bundled F-actin networks with Fascin

Abstract

Filopodia are exploratory finger-like projections composed of multiple long, straight, parallel-bundled actin filaments that protrude from the leading edge of migrating cells. Drosophila melanogaster Enabled (Ena) is a member of the Ena/vasodilatorstimulated phosphoprotein protein family, which facilitates the assembly of filopodial actin filaments that are bundled by Fascin. However, the mechanism by which Ena and Fascin promote the assembly of uniformly thick F-actin bundles that are capable of producing coordinated protrusive forces without buckling is not well understood. We used multicolor evanescent wave fluorescence microscopy imaging to follow individual Ena molecules on both single and Fascinbundled F-actin in vitro. Individual Ena tetramers increase the elongation rate approximately two-to threefold and inhibit capping protein by remaining processively associated with the barbed end for an average of ∼10 s in solution, for ∼60 s when immobilized on a surface, and for ∼110 s when multiple Ena tetramers are clustered on a surface. Ena also can gather and simultaneously elongate multiple barbed ends. Collectively, these properties could facilitate the recruitment of Fascin and initiate filopodia formation. Remarkably, we found that Ena's actin-assembly properties are tunable on Fascinbundled filaments, facilitating the formation of filopodia-like F-actin networks without tapered barbed ends. Ena-associated trailing barbed ends in Fascin-bundled actin filaments have approximately twofold more frequent and approximately fivefold longer processive runs, allowing them to catch up with leading barbed ends efficiently. Therefore, Fascin and Ena cooperate to extend and maintain robust filopodia of uniform thickness with aligned barbed ends by a unique mechanistic cycle.profilin | formin | TIRF microscopy | self organization | single molecule T he actin cytoskeleton facilitates fundamental cellular processes including division, polarization, and motility. The organization and dynamics of particular F-actin networks are determined by the coordinated action of specific subsets of actinbinding proteins with complementary biochemical properties such as sequestering, nucleating, elongating, bundling/crosslinking, and severing (1-3).Cell motility is driven primarily by lamellipodia, protrusive structures at the cell's leading edge composed of a dendritic network of short-branched filaments produced by the rapid capping of filaments nucleated by the actin-related proteins 2 and 3 (Arp2/3) complex (4). Filopodia are exploratory finger-like projections composed of uniformly long, straight, parallel-bundled filaments that extend from lamellipodia. One current model for filopodia assembly is convergent elongation (5). Formin and/or Enabled/vasodilator-stimulated phosphoprotein (Ena/VASP) proteins gather and rapidly elongate subpopulations of lamellipodial actin-barbed ends, antagonizing inhibition by capping protein (CP), and the parallel actin-crosslinking protein Fascin aligns and bundles elongating filopodial filaments (...

Distinct VASP tetramers synergize in the processive elongation of individual actin filaments from clustered arrays

“…The requirement of PFN for VASP-mediated actin filament assembly is still controversial. Although some laboratories found that PFN does not accelerate VASP-mediated filament elongation in vitro (6,20), others reported on increased filament elongation rates in the presence of PFN (19,21). To conclusively clarify this issue, we analyzed filament elongation of VASP-4M by TIRF imaging in the absence or presence of different human PFN isoforms and compared it side-by-side to filament elongation driven by a C-terminal fragment of the formin mDia1 (mDia1-C) in solution and after clustering on beads.…”

“…Comparable to formins, Ena/VASP proteins associate with growing barbed ends and accelerate filament elongation twofold to sevenfold (6,9,19,20). However, contrary to formins, they deliver actin monomers directly to filament barbed ends by virtue of their GAB domains (6,9,19,20).…”

“…However, contrary to formins, they deliver actin monomers directly to filament barbed ends by virtue of their GAB domains (6,9,19,20). Moreover, PFN appears dispensable for VASP-mediated filament elongation in vitro (6,20), albeit some studies have reported increased filament elongation rates in the presence of PFN (19,21). Finally, because single Ena/VASP tetramers are not as processive as formins, they consequently only poorly protect barbed end growth from capping protein (CP) in solution (6,19,20).…”

“…Moreover, PFN appears dispensable for VASP-mediated filament elongation in vitro (6,20), albeit some studies have reported increased filament elongation rates in the presence of PFN (19,21). Finally, because single Ena/VASP tetramers are not as processive as formins, they consequently only poorly protect barbed end growth from capping protein (CP) in solution (6,19,20). By contrast, when clustered in high density on surfaces, Ena/VASP proteins become highly processive and are virtually resistant to CP (6,9).…”

“…Indeed, processive filament elongation has been reported with single vertebrate and Drosophila Ena/VASP tetramers (19,20). However, the marked differences in the context-specific processivity of VASP and its resistance to CP, indicated major differences in the underlying mechanisms.…”