Abstract
Chromosome segregation in eukaryotes is driven by the kinetochore, a macromolecular complex that connects centromeric DNA to microtubules of the spindle apparatus. Kinetochores in well-studied model eukaryotes consist of a core set of proteins that are broadly conserved among distant eukaryotic phyla. By contrast, unicellular flagellates of the class Kinetoplastida have a unique set of 36 kinetochore components. The evolutionary origin and history of these kinetochores remain unknown. Here, we report evidence of homology between axial element components of the synaptonemal complex and three kinetoplastid kinetochore proteins KKT16-18. The synaptonemal complex is a zipper-like structure that assembles between homologous chromosomes during meiosis to promote recombination. By using sensitive homology detection protocols, we identify divergent orthologues of KKT16-18 in most eukaryotic supergroups, including experimentally established chromosomal axis components, such as Red1 and Rec10 in budding and fission yeast, ASY3-4 in plants and SYCP2-3 in vertebrates. Furthermore, we found 12 recurrent duplications within this ancient eukaryotic SYCP2-3 gene family, providing opportunities for new functional complexes to arise, including KKT16-18 in the kinetoplastid parasite Trypanosoma brucei. We propose the kinetoplastid kinetochore system evolved by repurposing meiotic components of the chromosome synapsis and homologous recombination machinery that were already present in early eukaryotes.
Highlights
Chromosome segregation in eukaryotes is driven by spindle microtubules and kinetochores
Chromosome segregation depends on spindle microtubules in T. brucei [15], we previously identified 24 unique kinetoplastid kinetochore proteins (KKT1–20 and KKT22–25) that localize at centromeres in this species, which together constitute a functionally analogous kinetochore structure [16,17,18]
KKT17 and KKT18 have a moderate degree of sequence identity and similarity to one another (23% shared identity and 38% similarity for T. brucei sequences), suggesting that these two proteins are the product of a duplication event [16]
Summary
Chromosome segregation in eukaryotes is driven by spindle microtubules and kinetochores. Microtubules are dynamic polymers that consist of α-/β-tubulin subunits, while the kinetochore is the macromolecular protein complex that assembles onto the centromeric DNA and interacts with spindle microtubules during mitosis and meiosis [1]. All studied eukaryotes use spindle microtubules to drive the chromosome movement, and α-/β-tubulins are among the most conserved proteins in eukaryotes [2]. CENP-A is a centromere-specific histone H3 variant that specifies kinetochore assembly sites, while the NDC80 complex (NDC80, NUF2, SPC24 and SPC25) constitutes the primary microtubule-binding activity of kinetochores [4]. Functional studies have established that CENP-A and the NDC80 complex are essential for the kinetochore function in several model eukaryotes (e.g. yeasts, worms, flies and humans) [3]. The most extreme case known to date is found in Kinetoplastida, for which no apparent direct homologues of the canonical kinetochore proteins were detected [11]
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