Figure 2.

The mechanism of spindle orientation.

A) In budding yeast, spindle formation starts in the mother cell body with the older centrosome/SPB (red) oriented towards the bud by the action of Kar9 (yellow; Apc2 homolog) and associated proteins (not depicted). The Kar9 pathway positions the nucleus at the bud neck region, which is maintained by the septin scaffold (orange). The dynein-dependent pathway (purple) works at later stages of the cell cycle and provides the forces to move the spindle into the daughter cell, allowing proper spindle orientation. If either the Kar9 or Dyn1 pathways fail, mitotic spindle elongation occurs inside the mother cells and causes the activation of SPOC components (not depicted). Chromosomes, blue; cell polarity cap, magenta; spindle, green.

In Drosophila male GSCs (light blue), the mother centrosome (red circle) is associated with the cell-cell junction between GSCs and hub cells (pink), the major component of the stem cell niche, leading to perpendicular orientation of spindles. As a result, GSCs divide asymmetrically. Apc2 (yellow), a putative Kar9 homolog, concentrates at the cell-cell junction to anchor the mother centrosome, while the daughter centrosome is on the opposite side (green circle). In CySCs (lime) of the Drosophila testis, which associate with GSCs and hub cells, spindle repositioning during anaphase results in asymmetric stem cell division. Spindles are formed with random orientation but rotate during mitosis to acquire the desired orientation of the spindle. Dynein might function to pull the spindle pole (green) to the adherens junction (orange).