Architecture of the centriole cartwheel-containing region revealed by cryo-electron tomography.

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.

Overview of the centriole architecture.

The centriole is a magnificent molecular assembly of several giga-daltons, one of the largest of the eukaryotic cell, and whose atomic structure remains unsolved to date. However, numerous electron microscopy, cryo-tomography, and super-resolution studies now make it possible to establish a global architectural view of it with its different sub-regions. These analyses broaden our understanding by providing additional informations to cell biology and structural biology approaches. In this review, we describe current knowledge on the overall organization of the centriole. We will highlight each sub-structural element, their differences between species and their putative protein composition. We will conclude on the current limitations that still take us away from a complete atomic view of the centriole architecture.