The Molecular Basis of Monopolin Recruitment to the Kinetochore

In budding yeast meiosis I, the kinetochores of each sister chromatid pair are fused by the monopolin complex to mediate their monoorientation on the meiosis I spindle, enabling the biorientation and segregation of homologs. Monopolin forms a V-shaped complex with binding sites for the kinetochore protein Dsn1 at the apices of the V, suggesting that monopolin forms a physical bridge between the two sister kinetochores. Here, we reveal the molecular basis of the monopolin-kinetochore interaction and identify the key interfaces required for monopolin function at the kinetochore. The disordered N-terminus of budding-yeast Dsn1 unexpectedly possesses two binding motifs for the monopolin subunit Csm1, encompassing the previously-identified “Box 1” and “Box 2-3” regions of Dsn1. Strikingly, Dsn1 Box 1 and Box 2-3 bind the same conserved hydrophobic cavity on the monopolin complex subunit Csm1, suggesting that they are mutually exclusive for Csm1 binding, yet both regions are critical for monopolin function in Saccharomyces cerevisiae meiosis I. We find that Dsn1 Box 1 is an ancestral monopolin-binding motif that is conserved throughout fungi, including in the fission yeast Schizosaccharomyces pombe . In contrast, Box 2-3 is found only in species with sequence-defined point centromeres ( S. cerevisiae and its close relatives), suggesting that this region contributes specifically to sister kinetochore crosslinking in meiosis I. Finally, we propose that phosphorylation of two conserved serine residues in Box 3 may stabilize monopolin at the kinetochore, providing a potential mechanism for enforcing specific sister kinetochore crosslinking in meiosis I.