Pruyne and colleagues report that formins might directly nucleate unbranched actin filaments.
When it comes to actin-filament assembly, the Arp2/3 complex seems to have received all the attention recently. In budding yeast, though, the formins Bni1 and Bnr1 promote the assembly of actin cables (bundles of actin filaments) when Arp2/3 is absent, and in Science Express, Pruyne and colleagues report that formins might directly nucleate unbranched actin filaments.
Bni1 is made up of a Rho-binding domain, a formin homology-1 (FH1) domain, an FH2 domain and a carboxy-terminal extension. Expression of Bni1 that lacks the first domain (Bni1FH1FH2COOH) can induce filament assembly in vivo, so the authors attempted to reconstitute this in vitro. They succeeded — Bni1FH1FH2COOH nucleated actin filaments in actin-polymerization assays. Nucleating activity was dose-dependent on Bni1FH1FH2 (deletion of the carboxy-terminal extension had no effect) but, in contrast to Arp2/3-mediated nucleation, was not enhanced by pre-formed actin filaments. Further analysis showed that FH2 alone could nucleate actin, but that FH1, which binds profilin (a nucleation enhancer) in vivo, is probably important too.
Actin filaments have their own polarity in the form of opposing barbed and pointed ends, and Pruyne and colleagues wanted to know at which end Bni1FH1FH2 was mediating filament growth. Bni1FH1FH2-stimulated filament growth was sensitive to cytochalasin B — an inhibitor of barbed-end filament growth — which indicates that Bni1FH1FH2-nucleated filaments grow from the barbed end. Furthermore, the authors found that Bni1FH1FH2 localized with assembling filaments at the barbed end. Notably, electron-microscopy examination showed all the filaments to be long and unbranched.
As Bni1FH1FH2 nucleates unbranched actin filaments and remains associated with the growing end of barbed filaments, the authors propose that these unique properties help Bni1 to establish the polarity of growing actin filaments, a feature that could apply to actin dynamics in other eukaryotes.
Katrin Bussell
References
Pruyne, D. et al. Role of formins in actin assembly: nucleation and barbed end association. Science Express 2002 June 6 (DOI 10.1126/science.1072309)
