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Curr Biol. 2003 Oct 14;13(20):1820-3.

Formin leaky cap allows elongation in the presence of tight capping proteins.

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Department of Biology, University of Pennsylvania, Philadelphia, PA 19104, USA.


Formins, characterized by formin homology domains FH1 and FH2, are required to assemble certain F-actin structures including actin cables, stress fibers, and the contractile ring. FH1FH2 in a recombinant fragment from a yeast formin (Bni1p) nucleates actin filaments in vitro. It also binds to the filament barbed end where it appears to act as a "leaky" capper, slowing both polymerization and depolymerization by approximately 50%. We now find that FH1FH2 competes with tight capping proteins (including gelsolin and heterodimeric capping protein) for the barbed end. We also find that FH1FH2 forms a tetramer. The observation that this formin protects an end from capping but still allows elongation confirms that it is a leaky capper. This is significant because a nucleator that protects a new barbed end from tight cappers will increase the duration of elongation and thus the total amount of F-actin. The ability of FH1FH2 to dimerize probably allows the formin to walk processively with the barbed end as the filament elongates.

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