Role of kinetochore stretching in controlling the spindle-assembly checkpoint

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Faithful segregation of chromosomes relies on the spindle-assembly checkpoint(SAC), which delays anaphase onset until all kinetochores are attached to microtubules(MT). The SAC monitors MT attachment at kinetochores, but whether it also senses the presence of 'tension' has long been a matter of debate. We found that 'tension' applied across sister kinetochores, which stretches centromeres, is dispensable, but stretching of kinetochores is required for silencing the SAC(Uchida et al., 2009). It is therefore crucial to investigate if 'tension' underlies kinetochore stretching. We addressed this question experimentally by engineering the conditions that microtubules are attached to kinetochors but free from the pulling force. Under these conditions, we found that kinetochores underwent stretching upon MT attachment. These results suggest that intact kinetochore-pulling force is not required for kinetochore stretching, thus challenging the notion that SAC directly monitors the presence of tension.