A Quantitative Exploration of Tension Sensing at Metaphase in Budding Yeast

Abstract

During mitosis, motors associate with microtubules to exert forces that push spindle poles apart, thus establishing a mitotic spindle. These pushing forces in turn cause tension in the chromatin that connects oppositely attached sister chromosomes. This tension has been hypothesized to act as a mechanical signal that allows the cell to detect chromosome attachment errors during mitosis. However, the magnitude of changes in tension that could be detected by the cell to initiate an error correction response during metaphase has not been measured, and the underlying mechanics of tension based error detection and error correction remains unknown. In this study, we generated a gradient in tension over multiple isogenic budding yeast cell lines by genetically altering the magnitude of motor-based spindle forces. This allowed us, for the first time, to quantitatively elucidate the mechanics of tension based error detection pathway in mitosis. We found that a decreasing gradient in tension led to an increasing gradient in rates of kinetochore detachment and anaphase chromosome mis-segregration, with a corresponding gradient in metaphase times. Further, these tension-based cellular response gradients were abrogated in the absence of key error-correction pathway proteins. The underlying mechanism involves as increasing gradient in the degree of phosphorylation of proteins, comprising the load-bearing component of the kinetochore-microtubule interface, in response to a decreasing gradient in the magnitude of tension. We conclude that the cell is exquisitely tuned to the magnitude of tension as a signal to detect potential chromosome segregation errors during mitosis.