An unattached kinetochore screams “Wait!”

I n August of 1993, Conly Rieder and Green fi eld " Kip " Sluder skipped a few sessions of the annual Microscopy Society of America meeting in Cincinnati, Ohio, to visit the Air Force museum in Dayton and see the bombers on display. During the drive, they discussed a topic that had been in the back of both of their minds. Rieder's story began with a 1988 call from Leland Hartwell, shortly before Hartwell had put forward the idea of cell cycle checkpoints (Hartwell and Weinert, 1989). Hartwell asked whether anyone had defi nitively shown that cells delayed anaphase until all chromosomes were hooked up to the spindle. Rieder noted that there was one obscure abstract concluding that newt cells never started anaphase in the presence of a monooriented chromosome (Zirkle, 1970). And Sluder, as a graduate student, had found that inhibiting spindle assembly delayed anaphase onset in sea urchin eggs (Sluder, 1979). But although there were some anecdotal reports of a chromosome attachment or spindle assembly checkpoint in other cell types, a proper study had not been done. Now, the two road-trippers thought, was the perfect time to test the idea in mammalian somatic cells before someone else did. Rieder already had an undergraduate , Adriene Schultz, collecting video data on 126 individual PtK1 cells, which remain fl at through mitosis and have 12 easy-to-follow chromosomes. She was measuring the time from nuclear envelope breakdown to anaphase onset and how long the cell had unattached kinetochores. The data lined up on a near-perfect linear regression (Rieder et al., 1994). In addition, the group determined that a single unattached ki-netochore was enough to delay anaphase. But once the last kineto-chore attached to the spindle, anaphase would always proceed ‫02ف‬ min later. Sluder suggested using the drug Taxol to artifi cially delay microtubule attachment for up to three hours. These cells also entered anaphase when the last kinetochore joined up. " The unattached kinetochore was doing something that shuts down the whole spindle system, " says Sluder. According to Rieder: " The next question was, what does the cell moni-tor? Either it was monitoring bipolar attachment [of each chro-mosome] or the unattached kinetochore is screaming 'Wait!' through negative feedback. " To test what the checkpoint monitored, the group relied on a handy laser setup. Rieder had installed it to do chromosome microsurgery for aster ejection force studies. But he now …