Figure 6.

Model of Centriole Symmetry Breaking and Spindle Orientation in Drosophila NBs

At the end of mitosis, the mother-daughter centrioles of the NB disengage. The mother (or basal) centriole retains Plk4 activity, which phosphorylates Spd2, causing (1) PCM shedding and, thus, loss of MT nucleation and (2) Fzr displacement, which inhibits apical anchoring. An as yet undiscovered mechanism, Plk4 is inactive on the daughter (or apical) centriole, resulting in the stable maintenance of a centriole-bound population of non-phosphorylated Spd2, which promotes both MT nucleation and Fzr-dependent cortical anchoring.