During all eukaryotic cell divisions, the mitotic spindle must be positioned properly to ensure faithful segregation of cellular determinants into the progeny cells. My lab aims at understanding how dividing cells know where to position the mitotic spindle, and what mechanism they use to move the mitotic spindle to the proper location. We use the budding yeast Saccharomyces cerevisiae as our experimental system. Yeast supports a unique combination of classical and molecular genetic studies, high resolution in vivo imaging, and biochemical approaches. Current studies aim at understanding how cytoplasmic microtubules, the microtubule motor dynein and its regulator dynactin, and the cortical attachment protein Num1 in mediating movement of the mitotic spindle into the mother-bud neck. This network of cytoskeletal and regulatory proteins, termed the dynein pathway components, function in the bud to pull the mitotic spindle into the neck. Positioning of the spindle across the neck guarantees that the daughter and mother cells receive an exact copy of the duplicated chromosomes prior to cytokinesis. Our results show that, during the dynein pathway, dynamic astral/cytoplasmic microtubules probe the bud cortex for attachment sites containing the cortical protein Num1 and other associated proteins. Dynein is targeted to the distal plus ends of these cytoplasmic microtubules, and its localization is dependent on the its regulator Pac1, the yeast homologue of human LIS1. Our results suggest that upon productive interactions between the distal ends of microtubules and cortical attachment sites, dynein is offloaded, anchored and subsequently activated to be utilized in force production for pulling the mitotic spindle into the neck.

Mechanism of Dynein-Mediated Nuclear Migration and Spindle Positioning