TENSION-SENSITIVE KINETOCHORE PHOSPHORYLATION AND THE CHROMOSOME DISTRIBUTION CHECKPOINT IN PRAYING MANTID SPERMATOCYTES

Improper chromosome attachment to the spindle can lead to daughter cel ls with missing or extra chromosomes. Such mishaps are avoided in many cells by a checkpoint that detects even a single improperly attached chromosome. What is detected? A misattached chromosome is not under te nsion from opposed mitotic forces, and in praying mantid spermatocytes , direct experiments show that the absence of tension is what the chec kpoint detects. How is the absence of tension detected? Tension-sensit ive kinetochore protein phosphorylation is the most likely possibility . We combined micromanipulation with immunostaining for phosphoprotein s in order to study the effect of tension on kinetochore phosphorylati on in mantid spermatocytes. We confirm earlier observations on mammali an cells and grasshopper spermatocytes that misattached chromosomes ha ve phosphorylated kinetochore proteins. We also confirm experiments in grasshopper spermatocytes showing that tension alters kinetochore che mistry: tension from a micromanipulation needle causes kinetochore pro tein dephosphorylation, and relaxation of tension causes kinetochore p rotein rephosphorylation. Beyond confirmation, our results provide fre sh evidence for phosphorylation as the signal to the checkpoint. First , mantid cells are the only ones in which an effect of tension on the checkpoint has been directly demonstrated; by equally direct experimen ts, we now show that tension affects kinetochore phosphorylation in th ese same cells. Second, sex chromosome behavior in mantids provides a natural experiment to test the relationship between phosphorylation an d the checkpoint. In grasshoppers, an unpaired sex chromosome is norma l, its kinetochore is under-phosphorylated, and the checkpoint is not activated. In mantids, exactly the opposite is true: an unpaired sex c hromosome is abnormal, its kinetochore is phosphorylated and, as predi cted, the checkpoint is activated. We conclude that tension-sensitive kinetochore protein phosphorylation very likely is the essential link between proper chromosome attachment and the checkpoint, the link that permits potential errors in chromosome distribution to be detected an d avoided.