Bazooka Forms a Platform that Integrates Stem Cell Polarity and Cell Cycle Progression. Mayu Inaba^1,2, Yukiko Yamashita¹. 1) Center for stem cell biology, Life Sciences Institute, University of Michigan, Ann Arbor, MI; 2) Department of Molecular Biology The University of Texas Southwestern Medical Center at Dallas, Dallas, Texas.

   Many stem cells are known to divide asymmetrically to balance self-renewal and differentiation. The essence of asymmetric cell division is polarization of the cell and subsequent cell division that unequally compartmentalizes cellular components that confer distinct cell fates to daughter cells. Precocious cell division before establishment of cell polarity would lead to failure in asymmetric cell division. However, it is poorly understood how cell polarity is coordinated with the cell cycle progression. In Drosophila male germline stem cells (GSCs), asymmetric division is achieved by stereotypical positioning of the centrosomes. The centrosome orientation checkpoint (COC) serves as a mechanism to coordinate correct centrosome orientation and mitotic entry. Here, we show that Bazooka (Baz)/Par-3 is an integral component of the COC. Baz forms small foci between GSC and its niche component, hub cells. We provide evidence that Baz-centrosome interaction at hub-GSC interface triggers mitotic entry, whereas lack of Baz-centrosome interaction leads to association of Baz with the spectrosome, a germline-specific organelle, leading to cell cycle delay. We propose that Baz forms a signaling platform to monitor centrosome orientation and regulate cell cycle progression.