The architecture of the centriole cartwheel-containing region revealed by cryo-electron tomography

Abstract

Centrioles are evolutionarily conserved barrels of microtubule triplets that form the core of the centrosome and the base of the cilium. In the proximal region of the centriole, nine microtubule triplets attach to each other via A-C linkers and encircle a central cartwheel structure, which directs the early events of centriole assembly. While the crucial role of the proximal region in centriole biogenesis has been well documented in many species, its native architecture and evolutionary conservation remain relatively unexplored. Here, using cryo-electron tomography of centrioles from four evolutionarily distant species, including humans, we report on the architectural diversity of the centriolar proximal cartwheel-bearing region. Our work reveals that the cartwheel central hub, previously reported to have an 8.5 nm periodicity in Trichonympha, is constructed from a stack of paired rings with an average periodicity of ∼4 nm. In all four examined species, cartwheel inner densities are found inside the hub’s ring-pairs. In both Paramecium and Chlamydomonas, the repeating structural unit of the cartwheel has a periodicity of 25 nm and consists of three ring-pairs with 6 radial spokes emanating and merging into a single bundle that connects to the triplet microtubule via the pinhead. Finally, we identified that the cartwheel is indirectly connected to the A-C linker through a flexible triplet-base structure extending from the pinhead. Together, our work provides unprecedented evolutionary insights into the architecture of the centriole proximal region, which underlies centriole biogenesis.

The centriolar cartwheel structure: symmetric, stacked, and polarized

“…1A). This top wheel-like view of a hub decorated with spokes inspired the cartwheel name and is consistent across all species studied thus far, from humans [8] to green algae [1], with the exception of nematodes, where a central tube scaffold was observed instead [3].…”

“…In most cartwheels studied, pairs of SAS-6 rings were stacked ~5 nm apart, thereby creating an average vertical periodicity at the cartwheel hub of ~4.2 nm [8,21]. It is worth noting that this matches the spacings of cartwheels reconstituted in vitro by Chlamydomonas SAS-6 [21] and suggests that stacking is an inherent property of SAS-6 rings.…”

“…centrioles at ~40 Å resolution originally suggested that stacking periodicity at the cartwheel hub was 8.5 nm; thus, that SAS-6 rings do not directly contact each other in the stack [19,20]. However, more recent electron tomography data of up to ~24 Å resolution from purified Trichonympha spp., T. agilis, Teranympha mirabilis, Paramecium tetraurelia, Naegleria gruberi and human centrioles [8,21], as well as in situ tomography of Chlamydomonas reinhardtii cell sections [8], altered our view of cartwheel stacking.…”

“…2A-C). CIDs were first observed in Trichonympha centrioles and thought to be specific to the exceptionally long organelles of this genus that feature cartwheels up to ~4 m in height [19]; however, CIDs have now also been observed in human, Chlamydomonas, Paramecium and Naegleria centrioles that feature short cartwheels of ~150 nm height on average [8], suggesting that CIDs perform a conserved but unknown function. Cartwheels from Teranympha, another termite gut symbiont with long centrioles, lacked CIDs but featured a filamentous density at their centre; it remains unclear whether and how CIDs and these filaments are related.…”

A first-takes-all model of centriole copy number control

“…It is well accepted that average cartwheel length is rather conserved within but may differ between distinct cell types and organisms [32]. Whereas recent studies found cartwheel length variation [26,33], it is unclear if this represents changes in the number of stacked rings. Notwithstanding, cartwheels undergo length fluctuations during the cell cycle in Chlamydomonas and Spermatozopsis [34,35] and they disappear during mitosis in human cells (reviewed in [24,25]).…”