Direct Evidence for Sister Kinetochore Fusion in Meiosis I

Abstract

For decades, biologists have wondered how replicated sister chromatids, which always separate from one another during normal mitotic cell division, instead co-migrate together during phase I of meiosis, the specialized division that produces eggs and sperm. Genetic, cytological, and biochemical studies using budding yeast have suggested that sister kinetochores, which link the sister chromatids to microtubules and drive their movement, may be mechanically fused by a meiosis I-specific kinetochore-binding factor called monopolin. But thus far the evidence for monopolin-dependent sister kinetochore fusion has been indirect, and the possibility that monopolin could instead promote co-migration by binding to and inhibiting one of the two sister kinetochores has not been excluded. To test these ideas directly, we are isolating native meiotic and mitotic kinetochore particles, reconstituting their function in vitro, and applying advanced tools for manipulating and tracking individual molecules. Using laser trapping, we find that kinetochore particles from meiosis I form substantially stronger attachments to dynamic microtubule tips than those from mitosis or from meiosis II. The high strength of meiosis I particles is lost if DNA replication is blocked during pre-meiotic S-phase, demonstrating that sisters are required. High strength is also lost if monopolin is disrupted prior to kinetochore particle isolation and, conversely, the strength of mitotic kinetochores can be increased by artificially inducing monopolin expression. Quantitative fluorescence microscopy also confirms that meiosis I particles have more copies of the core microtubule-binding component, Nuf2. Together these data suggest that sister kinetochores are mechanically fused by the monopolin complex during meiosis I, in such a way that the microtubule-binding elements from the two sister kinetochores can cooperate together to form a single attachment site.