Different Drosophila cell types exhibit differences in mitotic centrosome assembly dynamics

Centrosomes are major microtubule organising centres comprising a pair of centrioles surrounded by pericentriolar material (PCM). The PCM expands dramatically as cells enter mitosis, and we previously showed that two key PCM components, Centrosomin (Cnn) and Spd-2, cooperate to form a scaffold structure around the centrioles that recruits the mitotic PCM in Drosophila; the SPD-5 and SPD-2 proteins appear to play a similar function in C. elegans [1–3]. In fl y syncytial embryos, Cnn and Spd-2 are initially recruited into a central region of the PCM and then fl ux outwards [4–6]. This centrosomal fl ux is potentially important, but it has so far not been reported in any other cell type. Here we examine the dynamic behaviour of Cnn and Spd-2 in Drosophila larval brain cells. Spd-2 fl uxes outwards from the centrioles in both brains and embryos in a microtubule-independent manner. In contrast, although Cnn is initially incorporated into the region of the PCM occupied by Spd-2 in both brains and embryos, Cnn fl uxes outwards along microtubules in embryos, but not in brain cells, where it remains concentrated around the centrosomal Spd-2. Thus, the microtubule-independent centrosomalfl ux of Spd-2 occurs in multiple fl y cell types, while the microtubule-dependent outward fl ux of Cnn appears to be restricted to the syncytial embryo. We analysed the dynamic behaviour of Spd-2–GFP or GFP–Cnn at centrosomes in mitotic Drosophila larval brain cells using fl uorescence recovery after photobleaching (FRAP). Both Spd-2–GFP and GFP–Cnn fl uorescence recovered at centrosomes after photobleaching the centrosomal GFP signal, although Spd-2–GFP recovered faster than GFP–Cnn and both recovered more slowly than at embryonic centrosomes (Figure 1A–F; Correspondence