Role and regulation of BubR1 on acentric chromosome segregation. Nicolas Derive¹, Zohra Rahmani², Anne Royou¹. 1) Institut Européen de Chimie et Biologie, CNRS, Université Bordeaux Segalen, 2 rue Robert Escarpit, 33607 Pessac, France; 2) Institut Jacques Monod, CNRS, Université Paris Diderot Paris 7, 4 rue Marie-Andrée Lagroua Weill-Halle, 75205 Paris, France.

   Correct transmission of the genetic material during mitosis requires the proper chromosome attachment to the spindle microtubules. The centromere of the chromosome serves as an assembly site for a multiprotein structure called the kinetochore, the functional unit that binds the microtubules. The centromere is thus essential for proper chromosome segregation. However, we have recently identified a mechanism by which fragments of chromosomes lacking the centromere segregate properly. The observation of live Drosophila neuroblasts transiting mitosis with broken chromosomes revealed that acentric chromatids segregate faithfully. This is achieved through a DNA tether that attaches the acentric fragments to its centric partners. The integrity of the tether depends on BubR1 function, a protein that localizes on the kinetochore until anaphase, and accumulates on the tether throughout mitosis. To determined how BubR1 recruitment to the tether is regulated, we monitored dividing cells expressing fluorescent BubR1 constructs lacking specific domains. We found that the Glebs motif and more specifically the Glutamate 481 are required for BubR1 localization on the tether. Since E481 is essential for BubR1 interaction with Bub3 and its localization on the kinetochore, we investigated whether Bub3 localizes to the tether and more generally whether the tether is an assembly site for other kinetochore proteins. We found that five core kinetochore proteins did not localize on the tether ruling out the hypothesis that a neo-kinetochore forms on the tether. In contrast, Bub3 accumulated on the tether throughout mitosis. Moreover, BubR1 was not required for Bub3 localization to the tether suggesting that Bub3 acts upstream of BubR1 on the tether. All together these results suggest that the molecular pathway regulating BubR1 recruitment to the tether is similar but not identical to that of BubR1 localization to the kinetochore.