Add a new actin nucleator to the mix. Rashmi Ahuja, Michael Kessels, Britta Qualmann (Leibniz Institute for Neurobiology, Magdeburg, Germany), and colleagues identify Cordon-Bleu (Cobl) as a nucleator of actin filaments.

“Previously, there were really only two known vertebrate actin nucleators: formins and the Arp2/3 complex,” says Qualmann. “That's surprising if you consider the wealth of different actin structures that form.”

While studying proteins that interface with Arp2/3 and link actin polymerization to vesicle trafficking, the team noticed that new actin filaments still formed in extracts lacking Arp2/3. They decided it was time to fish for a new nucleator.

Using yeast two-hybrid analyses with those actin/vesicle linking proteins as bait, the group pulled out Cobl. Unlike Arp2/3 and its output of branched actin networks, Cobl created unbranched filaments similar to those made by formins. But formins are weak compared with Cobl, which polymerized filaments at 10-fold lower concentrations and with fewer available actin monomers.

Cobl's physiological duties were most apparent in neurons, where it increased both neurite numbers and branching. Cobl knockdown resulted in poor branching and impaired neuronal network formation. Cobl is also seen during early development in specialized patterning cells that undergo plenty of actin building and reorganization. The group is now excited to tease out where and when Cobl is needed, how it is regulated, and whether it cooperates with other nucleators.


Ahuja, R., et al.