MICROTUBULES ORIENT THE MITOTIC SPINDLE IN YEAST THROUGH DYNEIN-DEPENDENT INTERACTIONS WITH THE CELL CORTEX

Proper orientation of the mitotic spindle is critical for successful c ell division in budding yeast, To investigate the mechanism of spindle orientation, we used a green fluorescent protein (GFP)-tubulin fusion protein to observe microtubules in living yeast cells, GFP-tubulin is incorporated into microtubules, allowing visualization of both cytopl asmic and spindle microtubules, and does not interfere with normal mic rotubule function, Microtubules in yeast cells exhibit dynamic instabi lity, although they grow and shrink more slowly than microtubules in a nimal cells, The dynamic properties of yeast microtubules are modulate d during the cell cycle. The behavior of cytoplasmic microtubules reve aled distinct interactions with the cell cortex that result in associa ted spindle movement and orientation. Dynein-mutant cells had defects in these cortical interactions, resulting in misoriented spindles. In addition, microtubule dynamics were altered in the absence of dynein. These results indicate that microtubules and dynein interact to produc e dynamic cortical interactions, and that these interactions result in the force driving spindle orientation.