The Difficulty in Separating Sisters

The separation of the tightly associated sister chromatids during the anaphase step of cell division is an intricately orchestrated event. Orr-Weaver, in her Perspective, discusses new findings by Zou et al . in this issue that reveal the underlying molecular machinery involved. A protein called a separin, Esp1, is held in check by a securin, which in humans turns out to be the protein encoded by PTTG, an oncogene. Once released from its complex with securin, Esp1 cleaves off a subunit from the cohesin complex that is responsible for keeping the sister chromatids together. Orr-Weaver points out the intriguing possibility that in some tumors, mutation of PTTG may result in abnormal separation of sister chromatids and chromosome instability.