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Sally Zigmond
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Formins: signaling effectors for assembly and polarization of actin filaments.
Eukaryotic cells require filamentous actin to maintain their shape and for movement, growth and replication. New actin filaments are formed by the cutting of existing filaments or de novo through the action of specialized nucleators. The most highly characterized nucleator is the Arp2/3 complex, which nucleates the branched actin networks in the lamellae of migrating cells. Recently, Bni1p, which is a member of the formin family of proteins, has been shown to nucleate actin filaments in vitro. Formins are implicated in the formation of actin cables in yeast, stress fibers in tissue culture cells and cytokinesis in many cell types. Formins contain two highly conserved formin-homology domains, FH1 and FH2. The Bni1p FH2 domain is sufficient to mediate nucleation. The Bni1p FH1 domain binds profilin, an actin-monomer-binding protein that delivers actin to the growing barbed end of filaments. The Bni1p FH1-profilin interaction enhances nucleation. Formins participate in a number of signaling pathways that control the assembly of specific actin structures and bind the barbed end of actin filaments, thereby providing a cytoskeletal basis for the establishment of cell polarity.
Actin Cytoskeleton↗
Role of formins in actin assembly: nucleation and barbed-end association.
Nucleation of branched actin filaments by the Arp2/3 complex is a conserved process in eukaryotic cells, yet the source of unbranched actin filaments has remained obscure. In yeast, formins stimulate assembly of actin cables independently of Arp2/3. Here, the conserved core of formin homology domains 1 and 2 of Bni1p (Bni1pFH1FH2) was found to nucleate unbranched actin filaments in vitro. Bni1pFH2 provided the minimal region sufficient for nucleation. Unique among actin nucleators, Bni1pFH1FH2 remained associated with the growing barbed ends of filaments. This combination of properties suggests a direct role for formins in regulating nucleation and polarization of unbranched filamentous actin structures.
Actin Cytoskeleton↗