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. While the crucial role of the proximal region in centriole biogenesis has been well documented, its native architecture and evolutionary conservation remain relatively unexplored. Here, using cryo-electron tomography of centrioles from four evolutionarily distant species, we report on the architectural diversity of the centriole's proximal cartwheel-bearing region. Our work reveals that the cartwheel central hub is constructed from a stack of paired rings with cartwheel inner densities inside. 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 microtubule triplet via the D2-rod and the pinhead. Finally, we identified that the cartwheel is indirectly connected to the A-C linker through the 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.

Novel features of centriole polarity and cartwheel stacking revealed by cryo‐tomography

“…The first corresponds to the offset double ring configuration, with a tight packing of SAS‐6 rings, which are ~3.2 nm apart for instance in the T. mirabilis 36% class. In line with our findings, directly superimposed hub densities with remarkably similar spacing are observed also in Paramecium and C. reinhardtii (Klena et al , 2020). In the second stacking mode, two such offset double rings stack in register, being ~5.2 nm apart from each other in the T. mirabilis 36% class.…”

“…Note that spoke angles are merely illustrative and limited by the resolution of the available STA maps. Note also that alternative modes of spoke merging may be revealed by analyzing much larger sub‐volumes and may be present in other species (Klena et al , 2020).…”

“…E Working model of cartwheel stacking, with alternation of two modes of SAS-6 ring stacking: Pairs of rings are tightly superimposed with a rotational offset (dashed line), and such offset double rings are then stacked in register (solid line); the offset direction is consistent in T. spp., T. agilis, and T. mirabilis, yet may be different in other species, as shown in Paramecium (Klena et al, 2020). Surface representation of SAS-6 rings in different colors for clarity; homodimers highlighted by black contour.…”

“…and less so in T. agilis (see Fig 2D and I), although it is discontinuous and smaller in diameter than the fCID. Moreover, a density that may be related to either CID or fCID is apparent inside the hub in other species, including Chlamydomonas, Chytrid and several insects (Olson & Fuller, 1968;McNitt, 1974;Geimer & Melkonian, 2004;O'Toole & Dutcher, 2014;Gottardo et al, 2015;Guichard et al, 2017;Uzbekov et al, 2018;Klena et al, 2020) (Fig 6A). Determining the molecular nature of CID and fCID in diverse systems is expected to help assess whether they share an evolutionary history and may serve a similar function.…”

The 3D architecture and molecular foundations of de novo centriole assembly via bicentrioles

“…The A-tubule grows from its distal-end, while B and C-tubules elongate bidirectionally (Guichard et al ., 2010). In contrast, cartwheels show proximal-end directional elongation (Aydogan et al ., 2018) with a small (10-40 nm) portion overhanging from the centriole wall (Klena et al ., 2020). Indeed, cartwheel and centriolar microtubules appear to assemble interdependently in order to define and stabilize centriole architecture (Hilbert et al ., 2016).…”