Introduction The centrosome is the major microtubule-organizing center (MTOC) in most animal cells, but its functions are still relatively obscure (Palazzo and Schatten, 2000). At mitosis, a pair of centrosomes nucleates and organizes microtubules into the bipolar spindle apparatus, the accurate assembly of which is necessary for the proper segregation of chromosomes into the daughter cells. If the spindle apparatus is not assembled into a bipolar array, the chromosomes may not be distributed equally at cell division, and the resulting aneuploidy can lead to apoptosis or oncogenesis (Brinkley, 2001; Lingle and Salisbury, 2000; Doxsey, 1998). In the absence of centrosomes, the bipolar spindle assembles by an alternate, anastral, pathway where the microtubules are nucleated on the chromosomes and subsequently organized into the bipolar spindle with the minus ends of the microtubules focused at the poles (Hyman, 2000; Compton, 2000). This process involves the coordinate activities of motor proteins (Wittmann et al., 2001; Walczak, 2000; Heald, 2000; Sharp et al., 2000; Compton, 2000). Since centrioles are degraded during oogenesis in many species, this alternate spindle assembly pathway appears to act exclusively during female meiosis (Theurkauf and Hawley, 1992; Rieder et al., 1993; McKim and Hawley, 1995; Megraw and Kaufman, 2000; Compton, 2000). Found throughout the animal kingdom, the centrosome consists of over 100 proteins (Kalt and Schliwa, 1993; Kellogg et al., 1994). At the heart of the centrosome lies a pair of centrioles. These are surrounded by the pericentriolar material (PCM), which has a filamentous structure (Dictenberg et al., 1998; Schnackenberg et al., 1998; Schnackenberg and Palazzo, 1999). The centromatrix, a substructure of the PCM, is a filamentous basal component of the centrosome that remains after many PCM proteins are removed by high salt (Schnackenberg et al., 1998; Schnackenberg and Palazzo, 1999; Moritz et al., 1998; Dictenberg et al., 1998; Palazzo et al., 2000). Additionally, γ-tubulin ring complexes (γTURCs) are bound to the PCM and are the sites of microtubule nucleation at the centrosome (Gunawardane et al., 2000). Cnn is a core centrosome constituent, and a major protein component of purified Drosophila centrosomes (Lange et al., 2000), but it is not part of the centromatrix (Moritz et al., 1998). In the hierarchical assembly of the centrosome (Palazzo et al., 2000), Cnn lies between the centromatrix and the incorporation of other PCM components including CP60, CP190 and γ-tubulin (Megraw et al., 1999; Megraw et al., 2001). Changes in the centrosome composition with the cell cycle are reflected in the discovery that Cnn is required for γ-tubulin accumulation at mitotic, but not interphase, centrosomes (Megraw et al., 2001). 4707 The Drosophila Centrosomin (Cnn) protein is an essential core component of centrosomes in the early embryo. We have expressed a Cnn-GFP fusion construct in cleavage stage embryos, which rescues the maternal effect lethality of cnn mutant animals. The localization patterns seen with GFP-Cnn are identical to the patterns we see by immunofluorescent staining with anti-Cnn antibodies. Live imaging of centrosomes with Cnn-GFP reveals surprisingly dynamic features of the centrosome. Extracentrosomal particles of Cnn move radially from the centrosome and frequently change their direction. D-TACC colocalized with Cnn at these particles. We have named these extrusions ‘flares’. Flares are dependent on microtubules, since disruption of the microtubule array severs the movement of these particles. Movement of flare particles is cleavage-cycle-dependent and appears to be attributed mostly to their association with dynamic astral microtubules. Flare activity decreases at metaphase, then increases at telophase and remains at this higher level of activity until the next metaphase. Flares appear to be similar to vertebrate PCM-1-containing ‘centriolar satellites’ in their behavior. By injecting rhodamine-actin, we observed that flares extend no farther than the actin cage. Additionally, disruption of the microfilament array increased the extent of flare movement. These observations indicate that centrosomes eject particles of Cnn-containing pericentriolar material that move on dynamic astral microtubules at a rate that varies with the cell cycle. We propose that flare particles play a role in organizing the actin cytoskeleton during syncytial cleavage. Movies available on-line Key words: Drosophila, Centrosomes, Centrosomin (Cnn), PCM, Flares Summary The centrosome is a dynamic structure that ejects PCM flares Timothy L. Megraw 1,2, *, Sandhya Kilaru 1,2 , F. Rudolf Turner 1 and Thomas C. Kaufman 1,2,‡ 1 Department of Biology and 2 Howard Hughes Medical Institute, Indiana University, Bloomington, IN 47405, USA *Present address: Cecil H. and Ida Green Center for Reproductive Biology Sciences and Dept of Pharmacology, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75390-9051, USA ‡ Author for correspondence (e-mail: kaufman@bio.indiana.edu) Accepted 27 August 2002 Journal of Cell Science 115, 4707-4718 © 2002 The Company of Biologists Ltd doi:10.1242/jcs.00134 Research Article