PLP helps the mother centrosome stay mum

PLP helps the mother centrosome stay mum D rosophila neuroblasts give rise to the fl y's central nervous system by dividing asymmetrically to produce a daughter neuroblast and a gan-glion mother cell (GMC). The neuroblast's centrosomes are themselves asymmetric in terms of their activity, localization, and pattern of inheritance. Lerit and Rusan describe how the centrosomal protein pericentrin controls this asymmetry to ensure that neuro-blast centrosomes segregate correctly (1). After a neuroblast's centrosome duplicates in S phase, the younger, " daughter " centrosome remains active throughout inter-phase, nucleating and organizing micro-tubules from the apical side of the cell. The older, " mother " centrosome, on the other hand, is inactivated and translocated to the basal side of the cell, only gaining the proteins it needs to nucleate microtubules as the neuroblast enters mitosis (2, 3). The positions of the two centrosomes help orient the mitotic spindle so that the mother centrosome segregates into the GMC while the apical daughter centrosome partitions into the new neuroblast. Nasser Rusan, from the National Heart, Lung, and Blood Institute in Bethesda, Maryland, described the asymmetric activity of neu-roblast centrosomes as a postdoc at the University of North Carolina (2). But the functional signifi cance of this behavior remained unclear. " We thought this might be important to the function of neuroblasts, " Rusan explains, " but we had no way to test this hypothesis until we understood how the centrosome's activity is regulated so that we could turn the inactive centrosome on. " Rusan and his postdoc, Dorothy Lerit, therefore set out to identify differences between the mother and daughter centro somes that might explain their asymmetric activity levels. The researchers stained neuroblasts for a variety of centrosomal proteins and identifi ed one, known as pericentrin-like protein (PLP), that was enriched on the inactive , basal centrosome during interphase. When neuroblasts entered mitosis, however, PLP localized to both centrosomes evenly, and, as the amount of PLP on the basal cen-trosome decreased, levels of the microtu-bule-nucleating protein ␥-tubulin increased, indicating that the centrosome was active. PLP is the Drosophila orthologue of pericentrin, a protein that organizes the peri-centriolar material of centrosomes during mitosis to promote the organelle's microtu-bule-organizing activity. But PLP's local-ization pattern suggested that, in interphase cells, it might inhibit the activity of the neu-roblast's basal centrosome. To investigate this possibility , Lerit and Rusan examined the neuroblasts of fl ies lacking PLP. " …