Asymmetric cell division is the main mode of division of stem cells and plays a central role in generating cell diversity during development. Indeed, during asymmetric cell division of polarized cells, biomolecules are asymmetrically segregated between daughter cells. The position of the mitotic spindle within the cell determines whether the cell will divide symmetrically or asymmetrically. Spindle positioning, in turn, depends on the interaction of astral microtubules (cMTs) with cortical factors. A complex network of proteins, such as non-motor microtubule-associated proteins (plus-end tracking proteins, +TIPs), kinesins, dynein and actin-interacting proteins, are involved in these interactions.
Budding yeast also divides asymmetrically and is used as a model organism to study asymmetric cell division. Spindle positioning in yeast depends on two genetically identified pathways: i) the dynein pathway that includes complexes of the microtubule-dependent motor dynein, and ii) the Kar9 pathway. Kar9 is the yeast functional equivalent of the Adenomatous Polyposis Coli (APC) tumour suppressor, a protein with a central role in spindle positioning from Drosophila to mammals. Dynein, Kar9 and APC homologues are universal mediators of spindle positioning during eukaryotic asymmetric division, but their regulation is poorly understood.
In the laboratory, we explore how cell cycle regulators modulate the temporal activation and dynamics of dynein and Kar9 complexes during spindle positioning in budding yeast. During their transport along microtubules and interactions with cortical proteins, these factors form highly dynamic protein complexes. Cyclin-dependent kinases control both the formation of these complexes and the coordination of their activity with other cytoskeletal events, such as chromosome segregation and cytokinesis. To understand the regulation of Kar9 and dynein complexes we use genetics to study their interactions. Using protein biochemistry, we analyse the Kar9/dynein complex composition and phosphorylation. Finally, we investigate complex dynamics in living yeast cells by time-lapse fluorescence imaging.