Cell division is a feature of all life by which each cell duplicates into two daughter cells. Eukaryotic cells (such as yeast and humans) divide by forming a mitotic spindle, which is a bipolar structure composed of microtubules. The dynamic elongation and antiparallel organization of microtubules segregates each duplicated chromosome towards opposite sides of the cell, eventually leading to the creation of new daughter cells. Across eukaryotic life, there are variations in how mitotic spindles are assembled. In yeast, the mitotic spindle assembles inside the nucleus, while in humans and fruit flies, the nucleus is disassembled. The assembly and organization processes are driven by motor molecules, which are essential for cell division, and are highly similar in eukaryotes. Among them, Kinesin-5 motors possess a unique dumbbell shape, with two sets of motile regions located at opposite ends, which simultaneously move along two filaments emanating from opposite poles of the mitotic spindle. Through this activity, the Kinesin-5 motors promote the sliding-apart of microtubules during cell division, leading to elongation of spindles and chromosome segregation. When Kinesin-5 function is impaired, dividing cells stall and eventually die, because cells cannot segregate duplicated chromosomes. We will investigate the unique mechanisms and adaptations that have evolved in different versions of Kinesin-5 found in eukaryotes and their role in organizing mitotic spindles. We hypothesize that such unique adaptations specify features of the observed differences in mitotic spindle assembly found among eukaryotes.
A successful collaboration between US and Israeli research groups revealed the remarkable evolution of Kinesin-5 molecules from organisms with unique forms of cell division. In lower eukaryotes, Kinesin-5 motors move towards the opposite end of mitotic spindles compared to those found in higher eukaryotes such as fruit fly kinesin-5 motors. Yet the lower eukaryotic kinesin-5 motors reverse direction as they cluster in a unique way near the ends of filaments, once capturing second filaments; while in animals the kinesin-5 molecules do not possess these features. Here, we apply an interdisciplinary approach including biochemical analyses, yeast genetics and single motor fluorescence imaging, to elucidate how these unique adaptations evolved in Kinesin-5 motors to influence the diverse assembly and organization states of mitotic spindles observed in different organisms. First, we will determine the origin of clustering among kinesin-5 molecules found in both yeast humans and fruit fly versions and how that leads molecules persist at different ends of spindle filaments. Second, we will determine the features that lead one motor to direction switch (in yeast, but not in the humans and fruit flies). Third, we will determine how the physical organization of the dumbbell-shaped kinesin-5 molecules specifies microtubule sliding. These studies will provide a deep understanding of kinesin-5 generated motility in relation to the evolution of eukaryotes.
This research was funded by a joint Israeli and United State program in which the NSF provides funding for the US laboratory, located at UC Davis, while the Israeli BSF provides funds for the Israeli lab located at Ben-Gurion University in Negev.
