Proper connections between centromeres and spindle microtubules are of critical importance in ensuring accurate segregation of the genome during cell division. Using an in vitro approach based on the sequence-specific budding yeast centromere, we identified a complex of the chromosomal passenger proteins Bir1 and Sli15 (Survivin and INCENP) that links centromeres to microtubules. This linkage does not require Ipl1/Aurora B kinase, whose targeting and activation are controlled by Bir1 and Sli15. Ipl1 is the tension-dependent regulator of centromere-microtubule interactions that ensures chromosome biorientation on the spindle. Elimination of the linkage between centromeres and microtubules mediated by Bir1-Sli15 phenocopies mutations that selectively cripple Ipl1 kinase activation. These findings lead us to propose that the Bir1-Sli15-mediated linkage, which bridges centromeres and microtubules and includes the Aurora kinase-activating domain of INCENP family proteins, is the tension sensor that relays the mechanical state of centromere-microtubule attachments into local control of Ipl1 kinase activity.

The kinetochore is a proteinaceous structure that assembles onto centromeric DNA and mediates chromosome attachment to microtubules during mitosis. This description is deceivingly simple: recent proteomic studies suggest that the diminutive kinetochores of Saccharomyces cerevisiae are comprised of at least 60 proteins organized into as many as 14 different subcomplexes. Many of these proteins, such as the centromeric histone variant CENP-A, and entire subcomplexes, such as the Ndc80(Hec1) complex, are conserved from yeast to humans despite the diverse nature of the DNA sequences on which they assemble. There have recently been advances in our understanding of the molecular basis of how kinetochores establish dynamic attachments to spindle microtubules, and how these attachments are correctly oriented to ensure segregation of sister chromatids to daughter cells.

Tissue stem cells divide to self-renew and generate differentiated cells to maintain homeostasis. Although influenced by both intrinsic and extrinsic factors, the genetic mechanisms coordinating the decision between self-renewal and initiation of differentiation remain poorly understood. The escargot (esg) gene encodes a transcription factor that is expressed in stem cells in multiple tissues in Drosophila melanogaster, including intestinal stem cells (ISCs). Here, we demonstrate that Esg plays a pivotal role in intestinal homeostasis, maintaining the stem cell pool while influencing fate decisions through modulation of Notch activity. Loss of esg induced ISC differentiation, a decline in Notch activity in daughter enteroblasts (EB), and an increase in differentiated enteroendocrine (EE) cells. Amun, an inhibitor of Notch in other systems, was identified as a target of Esg in the intestine. Decreased expression of esg resulted in upregulation of Amun, while downregulation of Amun rescued the ectopic EE cell phenotype resulting from loss of esg. Thus, our findings provide a framework for further comparative studies addressing the conserved roles of Snail factors in coordinating self-renewal and differentiation of stem cells across tissues and species.