Formins Are a Family of Actin Nucleating Proteins

The nucleation of actin filaments is an important rate-limiting step in de novo actin filament polymerization. Currently there are three known classes of proteins with the ability to nucleate actin filaments: the Arp2/3 complex, Spire, and the formin family of proteins (Machesky et al. 1994; Quinlan et al. 2005). The best-characterized actin nucleator is the Arp2/3 complex, which binds to the side of existing actin filaments to nucleate new filaments that grow at the barbed end, resulting in the formation of a branched actin network. Much less is known about Spire, which nucleates linear actin filaments and binds to the pointed end of the new filament (Quinlan et al. 2005). The formins comprise seven mammalian subfamilies of proteins characterized by conserved Formin Homology 1 (FH1) and FH2 domains, which convey the ability to nucleate actin filaments (Higgs 2005; Higgs and Peterson 2005). Unlike the Arp2/3 complex and Spire, formins nucleate actin filaments from the barbed end and remain bound to the elongating barbed end (Pruyne et al. 2002). The actin-nucleating activity of the formins is linked to such cellular processes as cytokinesis, cell migration, endocytosis, and the establishment of cell polarity (for a review of formin structure and function, see Faix and Grosse 2006).

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