Functional characterization of the Csm1-like protein TITAN 9 in Arabidopsis thaliana

19 September 2025, by Website Team Biologie

Accurate segregation of chromosomes is essential for cell division. However, different patterns of chromosome segregation are found in mitosis versus meiosis: In mitosis and meiosis II, sister chromatids are pulled to opposite spindle poles while in meiosis I homologous chromosomes (homologs) are pulled apart with their sister chromatids remaining together. These distinct segregation patterns are reflected by a different organization of the kinetochores.  In mitosis and meiosis II, the kinetochores of sister chromatids are bi-oriented, i.e., they face different spindle poles while in meiosis I, the sister kinetochores are mono-oriented so that microtubules from the same pole attach to both sister chromatids.

 In budding yeast mono-orientation in meiosis I has been shown to depend on the monopolin-complex, including the component Csm1. However, Cms1’s function appears to be diversified as the fission yeast homolog Pcs1 prevents merotelic spindle microtubule attachments during mitosis and meiosis II and has no prominent role in meiosis I. A recent in-depth phylogenetic analysis predicted distantly related Csm1-like proteins in plants and other eukaryotes except most animals, raising the question how its function in plants would relate to the role of Csm1 in budding yeast versus fission yeast.

Here, we have characterized TITAN9 (TTN9), a distantly related Csm1-like protein in the flowering plant Arabidopsis. We show that TTN9 accumulates in mitotic and meiotic tissue and localizes to centromeres throughout the cell cycle. By analyzing proteome-wide TTN9 associated proteins, we identified a substantial subset of the Arabidopsis kinetochore proteome, including DSN1, mirroring known Csm1 interactions in yeast. While homozygous ttn9 mutants are not viable, a meiosis-specific knock-down of TTN9 causes chromosome segregation defects and split centromeres during meiosis I. These findings are consistent with vital kinetochore associated roles of TTN9 in mitosis and meiosis, including mono-orientation in meiosis I and suggest that Csm1-like proteins contribute to conserved kinetochore functions across eukaryotes.

 These results have been published in iScience: 

Zhang & Böwer et al. (2025) Functional characterization of the Csm1-like protein TITAN 9 in Arabidopsis thaliana. iScience, Volume 28, Issue 9, 113251