monopolin

Monopolin is a meiosis-specific protein complex in the budding yeast Saccharomyces cerevisiae that promotes the co-orientation of sister kinetochores during meiosis I. By enforcing that sister kinetochores attach to microtubules coming from the same spindle pole, Monopolin ensures that homologous chromosomes segregate to opposite poles rather than sister chromatids.

The complex is built around a core comprising Mam1 and Csm1, together with additional meiosis-specific components Lrs4 and Spo13. Mam1 is a meiosis-specific subunit essential for Monopolin assembly and function, while Csm1 and Lrs4 form a subcomplex that associates with centromeric chromatin. Spo13 acts as a regulatory partner that facilitates Monopolin targeting to kinetochores during meiosis I. The four subunits together mediate the physical linkage or coordination necessary for monopolar kinetochore attachments.

Function and mechanism: During meiosis I, Monopolin localizes to centromeres and promotes the monopolar arrangement of kinetochore–microtubule attachments. Loss of Monopolin components leads to randomized attachments and increased nondisjunction, undermining the reductional division that characterizes meiosis I. The complex operates in concert with other meiotic regulators to ensure that sister chromatids do not segregate prematurely.

Evolution and context: Monopolin is a yeast-specific adaptation linked to the unique requirements of meiosis I in budding yeast. While direct orthologs are not evident in higher eukaryotes, the principle of directing sister kinetochores to a single pole is a widespread feature of meiosis, achieved by different molecular strategies across species.