Formins are thought both to extend existing actin filaments and to nucleate new ones. Work by Quinlan et al., however, reveals that one particular formin may not initiate new filaments but instead assist in a nucleation complex.
Figure 1.
Spire (green) and Cappuccino (red) come together to nucleate actin.
Actin nucleation and filament assembly are required for, among other things, cell polarity. This polarity fails in fly eggs when either Cappuccino (a type of formin) or Spire (a distinct actin nucleator) are missing. Both can nucleate filaments in vitro, and genetic and biochemical evidence suggest that the two work together to create polarity-inducing actin networks in vivo.
Quinlan et al. now show that Cappuccino and Spire proteins interact in vivo. But within this complex, only Spire seems to be capable of nucleation. When brought together with Spire, Cappuccino's nucleation ability was inhibited. Spire's nucleation activity, by contrast, was boosted.
Loss of Spire might be thought to leave Cappuccino able to nucleate actin, yet the Spire mutant flies still have polarity defects. Without Spire, Cappuccino is diffusely expressed in fibroblasts; but with Spire and its membrane-targeting domain, Cappuccino localizes to membrane-associated puncta. Perhaps, despite having its own nucleating capability, Cappuccino is simply not in the right position to use it without Spire. 
Reference:
Quinlan, M.E., et al. 2007. J. Cell Biol. 179:117–128.