Live imaging of Centrosomin reveals novel centrosome dynamics. T.L. Megraw , S. Kilaru , T.C. Kaufman. HHMI and Dept Biol, Indiana Univ, Bloomington, IN.

   The centrosome is the major microtubule organizing center in animal cells. The centrosomin (cnn) gene was originally identified as a target of Hox regulation in the visceral mesoderm of the midgut. Cnn is a core component of centrosomes that appears to have homologs in other species. We have expressed a Cnn-GFP fusion construct in syncytial embryos, and this fusion protein localizes to centrosomes and is capable of rescuing cnn maternal effect null embryos. Live imaging of centrosomes with Cnn-GFP revealed the surprisingly dynamic nature of the centrosome. Extracentrosomal particles of Cnn move from the centrosome outward and back again. We have named these outbursts "flares". Flares are dependent on microtubules, since disruption of the microtubule array severs the particles from their transits to and from centrosomes. The free particles then surround the centrosomes, which oscillate and appear to attempt to bleb off more flares. By injecting rhodamine-actin we observed that flares extend no farther than the actin cage. However, disruption of the microfilament array does not affect the activity or extent of flares. We also examined live chromosome behavior in cnn mutant embryos using a GFP-histone. The production of aneuploid nuclei, which had previously been observed in fixed whole-mount specimens, was observed live. The mechanism for producing giant nuclei in cnn mutant embryos is due to the fusion of nuclei at joined spindle poles. Thus, in syncytial embryos the centrosomes are important for blocking the fusion of spindle poles at multiple mitotic spindles, thereby preventing the fusion of nuclei at division.