Overexpressed CNN (yellow) nucleates microtubules (green).

In studies of Drosophila cells, aurora-A kinase and centrosomin (CNN) are sometimes used as markers for centrosomes, but, other than their localization, relatively little is known about these proteins. On page 757, Terada et al. show that aurora-A and CNN must physically interact to be translocated to the centrosome, and, once there, CNN acts as a powerful microtubule nucleator, placing these two convenient markers at the center of centrosomal function.

Using a two-hybrid screen, the authors discovered that aurora-A specifically binds to a COOH-terminal domain of CNN, and RNAi inhibition of either protein prevents the translocation of the other to the centrosome. The NH2-terminal half of CNN interacts with the γ-tubulin complex. When overexpressed in fly or mammalian cells, or in an in vitro system, CNN induces microtubule nucleation with striking efficiency.Terada et al. speculate that the two functionally distinct domains of CNN connect the γ-tubulin complex to aurora-A and the centrosome. Although the natural CNN analogues in mammalian cells have so far eluded detection, the ability of Drosophila CNN to induce microtubule nucleation in mammalian cells underscores the evolutionary conservation of this mechanism. The authors hope to use their new in vitro system to determine how the nucleating site is constructed. ▪