FHOD1 is a combined actin filament capping and bundling factor that selectively associates with actin arcs and stress fibers

Abstract

SummaryFormins are actin polymerization factors that are known to nucleate and elongate actin filaments at the barbed end. In the present study we show that human FHOD1 lacks actin nucleation and elongation capacity, but acts as an actin bundling factor with capping activity toward the filament barbed end. Constitutively active FHOD1 associates with actin filaments in filopodia and lamellipodia at the leading edge, where it moves with the actin retrograde flow. At the base of lamellipodia, FHOD1 is enriched in nascent, bundled actin arcs as well as in more mature stress fibers. This function requires actin-binding domains located N-terminally to the canonical FH1-FH2 element. The bundling phenotype is maintained in the presence of tropomyosin, confirmed by electron microscopy showing assembly of 5 to 10 actin filaments into parallel, closely spaced filament bundles. Taken together, our data suggest a model in which FHOD1 stabilizes actin filaments by protecting barbed ends from depolymerization with its dimeric FH2 domain, whereas the region N-terminal to the FH1 domain mediates F-actin bundling by simultaneously binding to the sides of adjacent F-actin filaments.

The formins FHOD1 and INF2 regulate inter- and intra-structural contractility of podosomes

“…In fact, both scenarios could be envisioned, considering the currently known biochemical activities of FHOD1. In vitro, FHOD1 shows no actin polymerization or depolymerization activities, but acts as capping and bundling protein for actin filaments (Schonichen et al, 2013). The disordered connecting cables in FHOD1-knockdown cells could thus result from a lack of bundling.…”

“…Notably, FHOD1 has recently been shown to regulate maturation of integrin-based adhesion sites (Iskratsch et al, 2013). In vitro, FHOD1 acts as capping and bundling protein for actin filaments (Schonichen et al, 2013), although the exact biochemical activity of FHOD proteins is currently unclear . In contrast to these more accessory activities of FHOD1, INF2 has been shown to be involved in polymerization and severing of actin filaments (Gurel et al, 2014).…”

The Drosophila FHOD1-like formin Knittrig acts through Rok to promote stress fiber formation and directed macrophage migration during the cellular immune response

“…The FH2 dimer nucleates filament assembly by stabilizing actin intermediates, whereas an additional processive capping activity of formins is thought to promote actin filament elongation by protecting growing barbed ends from capping proteins. Nucleation and processive elongation seem to be the major conserved activities of formins, although a number of studies also revealed additional effects on filament bundling, severing or even depolymerization (Harris et al, 2004;Harris et al, 2006;Schönichen et al, 2013). The broad range of biochemical activities suggest that formins regulate different aspects of the actin cytoskeleton architecture and actin dynamics during morphogenesis.…”

“…Indeed, constitutively active forms of FHOD1 inhibit actin assembly by capping growing actin filaments (Schönichen et al, 2013). Thus, the strong induction of stress fibers observed in cell transfection experiments might be the result of bundling pre-existing actin filaments.…”

“…The Drosophila FHOD1-like formin Knittrig acts through Rok to promote stress fiber formation and directed macrophage migration during the cellular immune response nucleator promoting actin polymerization (Schönichen et al, 2013). Indeed, constitutively active forms of FHOD1 inhibit actin assembly by capping growing actin filaments (Schönichen et al, 2013).…”