KINETOCHORE CHEMISTRY IS SENSITIVE TO TENSION AND MAY LINK MITOTIC FORCES TO A CELL-CYCLE CHECKPOINT

Some cells have a quality control checkpoint that can detect a single misattached chromosome and delay the onset of anaphase, thus allowing time for error correction. The mechanical error in attachment must som ehow be linked to the chemical regulation of cell cycle progression. T he 3F3 antibody detects phosphorylated kinetochore proteins that might serve as the required link (Gorbsky, G. J., and W. A. Ricketts. 1993. J. Cell Biol. 122:1311-1321). We show by direct micromanipulation exp eriments that tension alters the phosphorylation of kinetochore protei ns. Tension, whether from a micromanipulation needle or from normal mi totic forces, causes dephosphorylation of the kinetochore proteins rec ognized by 3F3. If tension is absent, either naturally or as a result of chromosome detachment by micromanipulation, the proteins are phosph orylated. Equally direct experiments identify tension as the checkpoin t signal: tension from a microneedle on a misattached chromosome leads to anaphase (Li, X., and R. B. Nicklas. 1995. Nature (Lend.). 373:630 -632), and we show here that the absence of tension caused by detachin g chromosomes from the spindle delays anaphase indefinitely. Thus, the absence of tension is linked to both kinetochore phosphorylation and delayed anaphase onset. We propose that the kinetochore protein dephos phorylation caused by tension is the all clear signal to the checkpoin t. The evidence is circumstantial but rich. In any event, tension alte rs kinetochore chemistry. Very likely, tension affects chemistry direc tly, by altering the conformation of a tension-sensitive protein, whic h leads directly to dephosphorylation.