Presence of a novel inhibitor of capping protein in neutrophil extract.

Capping of actin filament barbed ends regulates the duration of filament elongation and the steady-state level of actin polymerization. We find that the specific capping activity (capping activity per milligram protein) increased when a high speed supernatant of lysed neutrophils was diluted with buffer. The specific capping activity also increased when the concentration of barbed ends increased.

Mammalian CARMIL inhibits actin filament capping by capping protein.

Actin polymerization in cells occurs via filament elongation at the barbed end. Proteins that cap the barbed end terminate this elongation. Heterodimeric capping protein (CP) is an abundant and ubiquitous protein that caps the barbed end.

Beginning and ending an actin filament: control at the barbed end.

Dynamic actin filaments contribute to cell migration, organelle movements, memory, and gene regulation. These dynamic processes are often regulated by extracellular and?or cell cycle signals. Regulation targets, not actin itself, but the factors that determine it's dynamic properties.

Formin-induced nucleation of actin filaments.

Formins are proteins best defined by the presence of the unique, highly conserved formin homology domain 2 (FH2). FH2 is necessary and sufficient to nucleate an actin filament in vitro. The FH2 domain also binds to the filament's barbed end, modulating its elongation and protecting it from capping proteins.

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%.

Mechanism of formin-induced nucleation of actin filaments.

A fragment of the yeast formin Bni1 containing the FH1FH2 domains increases the rate of filament nucleation from pure G-actin [Pruyne et al. (2002) Science 297, 612-615]. To determine the mechanism of nucleation, we compared the G-actin dependence of Bni1FH1FH2-induced polymerization with theoretical models.

An unexplained sequestration of latrunculin A is required in neutrophils for inhibition of actin polymerization.

Latrunculin A (LatA) is a toxic natural product that causes disruption of the actin cytoskeleton in many eukaryotic cells at submicromolar concentrations. LatA has been found to bind G-actin with a dissociation constant of 0.2 microM, and more recently to bind profilin-G-actin and, weakly, thymosin beta4-G-actin.