The relationship of HsEg5 and the actin cytoskeleton to centrosome separation.

Although centrosome separation is essential to the formation of a bipolar spindle, it can proceed along several different pathways. This raises questions as to the similarity between the mechanism(s) underlying these various forms of separation. To address this question we reinvestigated centrosome separation in HeLa cells using a variety of techniques. We present a refined description of the two major pathways of centrosome separation found in HeLa cells and demonstrate that each of these pathways has its own timing, protein requirements, morphological characteristics, and relationship to spindle assembly. The first pathway, which occurs in prophase cells, is dependent on an intact actin cytoskeleton, and when this pathway is completed prior to nuclear envelope breakdown, the microtubules associated with this process do not become part of the spindle. Thus, centrosome separation and spindle pole organization can occur as two separate events. The second centrosome separation pathway is found in cells in which separation occurs concurrent with prometaphase. In this case, centrosome separation and the formation of the mitotic spindle are integrated together and an intact actin cytoskeleton is not required. The relationship between these multiple pathways of centrosome separation and the distribution of the human kinesin-like protein HsEg5 was also investigated. This protein was found associated with all centrosomal microtubules present during both prophase and prometaphase centrosome separation, as well as with prophase centrosomes displaying independent movement in Cytochalasin-D treated cells. In addition, we demonstrate that this protein is associated with post-mitotic centrosome movement which involves a single centrosome. Thus, HsEg5 is a feature of individual centrosome function and does not require anti-parallel microtubule arrays.