DYNEIN AND DYNACTIN ARE LOCALIZED TO ASTRAL MICROTUBULES AND AT CORTICAL SITES IN MITOTIC EPITHELIAL-CELLS

The mitotic spindle is often positioned in a characteristic location d uring development, for example to enable the proper segregation of dev elopmental determinants [1,2], When epithelial cells divide, the mitot ic spindle is often positioned parallel to the plane of the epithelium , so that both daughter cells contribute to the epithelium [3], The me chanisms by which mitotic spindles are positioned have not been charac terized in great detail, but evidence is accumulating that in some sys tems the dynein-dynactin microtubule motor complex plays a role [4-6], Dynein has yet not been localized to cortical sites where it could bi nd to microtubules and exert a force that might orient the mitotic spi ndle, however [7,8], Here, we report that in mitotic polarized epithel ial cells, the dynein-dynactin complex accumulates, from prometaphase onwards, along astral microtubules and at cortical spots, into which m any of the astral microtubules dock. The spots are assembled at the la teral plasma membrane, in the region below the tight junctions. Their formation is inhibited by cytochalasin D, and under these conditions t he spindles do not orient properly. This novel localization of the dyn ein-dynactin complex is consistent with a role for the complex in the positioning of the mitotic spindle. We also show that, during prophase , the motor complex colocalizes with the nuclear envelope, consistent with it having a role in separating the centrosomes that are associate d with the nuclear envelope.