Formin leaky cap allows elongation in the presence of tight capping proteins.
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.
Zigmond
Sally H
SH
Department of Biology, University of Pennsylvania, Philadelphia, PA 19104, USA. szigmond@sas.upenn.edu
Evangelista
Marie
M
Boone
Charles
C
Yang
Changsong
C
Dar
Arvin C
AC
Sicheri
Frank
F
Forkey
Joe
J
Pring
Martin
M
D008841:Actin Cytoskeleton / Q000378:metabolism*; D002845:Chromatography; D002850:Chromatography, Gel; D019281:Dimerization; D008840:Microfilament Proteins / Q000235:genetics* / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics* / Q000378:metabolism
C105191:Bni1 protein, S cerevisiae; D008840:Microfilament Proteins; D011993:Recombinant Fusion Proteins; D029701:Saccharomyces cerevisiae Proteins