The spindle checkpoint: two transitions, two pathways

The spindle checkpoint is an evolutionarily conserved mitotic regulatory me chanism that ensures that anaphase is not attempted until chromosomes are p roperly aligned on the spindle. Two different cell-cycle transitions must b e inhibited by the spindle checkpoint to arrest cells at metaphase and prev ent mitotic exit The checkpoint proteins interact in ways that are more com plex than was originally envisioned. This review summarizes the evidence fo r two pathways of spindle-checkpoint regulation in budding yeast. We descri be how the proteins are involved in these pathways and discuss the ways in which the spindle checkpoint inhibits the cell-cycle machinery.