Meiotic budding yeast assemble bundled triple helices but not ladders

Abstract

The synaptonemal complex (SC) is the large, conserved, proteinaceous scaffold that assembles between and holds together homologous chromosomes in meiotic prophase. Knowledge of the native structure of this complex is needed to evaluate how the SC carries out its functions. Traditional electron microscopy and superresolution light microscopy have revealed that in many organisms, the SC has a ladder-like structure: two rail-like lateral elements are bridged by a set of rung-like transverse filaments. The transverse filaments are connected along their centers by a central element. To determine the 3-D architecture of the SC in situ, we studied frozen-hydrated meiotic yeast cell cryosections by Volta phase-contrast electron cryotomography and subtomogram analysis. We find the SC is built from triplehelical filaments that pack into dense polycrystalline bundles. These structures are also abundant in the polycomplexes of pachytene-arrested cells. Dissolution by 1,6hexanediol treatment suggests that these triple-helical filaments belong to the central region of the SCs. Subtomogram averaging revealed that the SC's triple-helical filaments are up to 12-nm thick and have a 5-nm rise and 130-nm pitch. Single triplehelices and polymers thinner than the triple helix, such as single or double strands, were not detected, consistent with the strong self-oligomerization properties of SC proteins. The dense packing of SC subunits supports the notion that the SC's mechanical properties help coordinate the rapid end-to-end communication across synapsed chromosomes.

Ultrastructural analysis in yeast reveals a meiosis-specific actin-containing nuclear bundle

“…3, 4). We also noted that cryo-electron microscope tomography showed a nuclear bundle in meiotic yeast nuclei referred to as the "meiotic triple helix" 39 , which is similar to the nuclear bundle described in this paper.…”

“…However, the angle of putative branching in nuclear bundles is quite different from the angle of branches of actin filaments mediated by the Arp2/3 complex, which has a value of 70°4 2 . Previous staining analyses with phalloidin or imaging with actin probes, Abp140-GFP 43 or Life-Act 39 , has not shown the presence of any actin polymers inside yeast meiotic nuclei. These results suggest that the nuclear bundle is a novel structure containing actin.…”

“…8). The bundle is also sensitive to the other actin-depolymerising drug, latrunculin A 39 . Are the filaments in the nuclear and cytoplasmic bundles described here considered so-called typical actin filaments with a doublehelix structure?…”

Sharing Marks: H3K4 Methylation and H2B Ubiquitination as Features of Meiotic Recombination and Transcription

“…In spite of the mechanisms not being completely understood, the SC plays an important role for crossover recombination, and several links between the SC and recombination have been discussed [39]. Among its described subunits in yeast, we can find, on the one hand, Rec8, Red1, and Hop1, which conform the two parallel lateral elements (LEs) [40,41]. On the other hand, the central element (CE) proteins Ecm11 and Gmc2, and the traverse filament component Zip1, localize to centromeres in early prophase [42], mediating the coupling of homologous centromeres, and afterwards, the correct pairing of whole homologous chromosomes [43].…”

Serial Cryomicrotomy of Saccharomyces cerevisiae for Serial Electron Cryotomography

“…We also explore the cryomicrotomy parameters that minimize artefacts in yeast cryosections. This approach has also been used to study mitotic fission yeast and meiotic budding yeast (Cai et al, 2018;Ma et al, 2019). 4.…”