Distinct roles of spindle checkpoint proteins in meiosis.

Gametes are produced via meiosis, a specialized cell division associated with frequent errors that cause birth defects and infertility. Uniquely in meiosis I, homologous chromosomes segregate to opposite poles, usually requiring their linkage by chiasmata, the products of crossover recombination.1 The spindle checkpoint delays cell-cycle progression until all chromosomes are properly attached to microtubules,2 but the steps leading to the capture and alignment of chromosomes on the meiosis I spindle remain poorly understood. In budding yeast meiosis I, Mad2 and Mad3BUBR1 are equally important for spindle checkpoint delay, but biorientation of homologs on the meiosis I spindle requires Mad2, but not Mad3BUBR1.3,4 Here we reveal the distinct functions of Mad2 and Mad3BUBR1 in meiosis I chromosome segregation. Mad2 promotes the prophase to metaphase I transition, while Mad3BUBR1 associates with the TOGL1 domain of Stu1CLASP, a conserved plus-end microtubule protein that is important for chromosome capture onto the spindle. Homologous chromosome pairs that are proficient in crossover formation but fail to biorient rely on Mad3BUBR1-Stu1CLASP to ensure their efficient attachment to microtubules and segregation during meiosis I. Furthermore, we show that Mad3BUBR1-Stu1CLASP are essential to rescue the segregation of mini-chromosomes lacking crossovers. Our findings define a new pathway ensuring microtubule-dependent chromosome capture and demonstrate that spindle checkpoint proteins safeguard the fidelity of chromosome segregation both by actively promoting chromosome alignment and by delaying cell-cycle progression until this has occurred.

Spindle assembly checkpoint-dependent mitotic delay is required for cell division in absence of centrosomes.

The spindle assembly checkpoint (SAC) temporally regulates mitosis by preventing progression from metaphase to anaphase until all chromosomes are correctly attached to the mitotic spindle. Centrosomes refine the spatial organization of the mitotic spindle at the spindle poles. However, centrosome loss leads to elongated mitosis, suggesting that centrosomes also inform the temporal organization of mitosis in mammalian cells. Here, we find that the mitotic delay in acentrosomal cells is enforced by the SAC in a MPS1-dependent manner, and that a SAC-dependent mitotic delay is required for bipolar cell division to occur in acentrosomal cells. Although acentrosomal cells become polyploid, polyploidy is not sufficient to cause dependency on a SAC-mediated delay to complete cell division. Rather, the division failure in absence of MPS1 activity results from mitotic exit occurring before acentrosomal spindles can become bipolar. Furthermore, prevention of centrosome separation suffices to make cell division reliant on a SAC-dependent mitotic delay. Thus, centrosomes and their definition of two spindle poles early in mitosis provide a 'timely two-ness' that allows cell division to occur in absence of a SAC-dependent mitotic delay.