The proteomic landscape of centromeric chromatin reveals an essential role for the Ctf19^CCANcomplex in meiotic kinetochore assembly

Abstract

Kinetochores direct chromosome segregation in mitosis and meiosis. Faithful gamete formation through meiosis requires that kinetochores take on new functions that impact homolog pairing, recombination and the orientation of kinetochore attachment to microtubules in meiosis I. Using an unbiased proteomics pipeline, we determined the composition of centromeric chromatin and kinetochores at distinct cell-cycle stages, revealing extensive reorganisation of kinetochores during meiosis.The data uncover a network of meiotic chromosome axis and recombination proteins that replace the microtubule-binding outer kinetochore sub-complexes during meiotic prophase. We show that this kinetochore remodelling in meiosis requires the Ctf19c CCAN inner kinetochore complex. Through functional analyses, we identify a Ctf19c CCAN -dependent kinetochore assembly pathway that is dispensable for mitotic growth, but becomes critical upon meiotic entry. Therefore, extensive kinetochore remodelling and a distinct assembly pathway direct the specialization of meiotic kinetochores for successful gametogenesis.

A Grave Affair

“…This phenomenon, sometimes referred to as "asymmetric tetrads", has been observed in certain genetic backgrounds, such as ctf19∆ and mcm21∆ (kinetochore mutants)as well as sgs1 9A mms4 mn (double mutant eliminating "back up" processing in aberrant joint molecules) [59,60,93].…”

“…COMA consists of two heterodimers: the first, consisting of Okp1 and Ame1 and the second consisting of, Ctf19 and Mcm21 [54,[56][57][58]. Ctf19 and Mcm21 are two proteins of interest in this study because of their documented effect on Zip1 localization and defect in chromosome segregation when either protein is absent [59,60]. Moreover, the Ctf19 complex is important for the recruitment of Zip1 to centromeres to prevent deleterious crossovers in the centromere-proximal region [61].…”

“…Centromeres and kinetochores together are central players in chromosome segregation. Failure to propagate the centromere and kinetochore factors (such as Ctf19 and Mcm21) results in the inability of a chromosome to attach to the spindle, leading to the loss of or mis-segregation of the chromosomes [59,60,62].…”

“…proteins make up a heterodimer in vitro, they have been studied independently of one another meiotically thus it is currently unclear if these proteins perform some functions independent of one another [106]. Ctf19 has been shown to have a role in minimizing potentially deleterious centromere-proximal crossovers, regulating microtubule dynamics, recruiting Zip1 localization to the centromere, and overall promoting proper segregation of chromosomes [59][60][61]. Mcm21's role has not been explored as in-depth, however the protein has been implicated in the recruitment of several important kinetochore factors and its similar overall meiotic phenotype to ctf19∆ mutants is consistent with the protein carrying out Ctf19-associated roles [60].…”

“…ctf19∆ and mcm21∆ mutants have been reported to have extremely low spore viability, resembling the fpr3∆ zip3∆ double mutant strain [59,60]. We first built strains to evaluate the spore viability phenotypes of ctf19∆ and mcm21∆ mutants in our lab strain's genetic background ("BR"), as prior reports of the ctf19∆ and mcm21∆ phenotypes were regarding strains of the SK1 background; the BR and SK1 strains sometimes exhibit different sensitivities to the absence of a meiotic protein.…”