Electron Cryo-Tomography Provides Insight into Procentriole Architecture and Assembly Mechanism

Abstract

Centriole is an essential structure with multiple functions in cellular processes. Centriole biogenesis and homeostasis is tightly regulated. Using electron cryo-tomography (cryoET) we present the structure of procentrioles from Chlamydomonas reinhardtii. We identified a set of non-tubulin components attached to the triplet microtubule (MT), many are at the junctions of tubules likely to reinforce the triplet. We describe structure of the A-C linker that bridges neighboring triplets. We find that POC1 is an integral component of the A-C linker. Its conserved WD40 β-propeller domain provides sites for attachment to other A-C linker components. The twist of A-C linker results in an iris diaphragm-like motion of the triplets in the longitudinal direction of procentriole. Finally, we identified two assembly intermediates at the growing ends of procentriole allowing us to propose a model for the procentriole assembly. Our results provide a comprehensive structural framework for understanding the molecular mechanisms underpinning procentriole biogenesis and assembly.

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

“…4Bb-d). As POC1 in Chlamydomonas was suggested to localize to the A-C linker and inner scaffold of the centriole wall (Le Guennec et al ., 2020; Li et al ., 2019), this partial elongation of POC1 signal may be associated with the longer microtubule triplets within a centriole, in agreement with our 3D model (Fig. 3).…”

Towards understanding centriole elimination

“…Whereas cytoplasmic microtubules are single tube-like polymers typically composed of 13 protofilaments, in most species centriolar microtubules form triplets and doublets with a unique geometry. Triplets consist of one complete microtubule (A-microtubule) containing 13 protofilaments, plus two incomplete ones (Band C-microtubules), each with 10 protofilaments proper, whereas doublets harbour A-and B-microtubules (figure 1b) [105,106,173]. Microtubule triplets are present in the proximal region of the centriole, microtubule doublets in the distal one.…”

“…Given their tiny dimensions and low copy number in most cells, the presence or absence of centrioles has been evaluated often through the monitoring of a focus bearing centriolar proteins rather than by lower throughput electron microscopy (EM) approaches. In some cases, including in human cells upon depletion of RBM14, Neurl4 or TRIM37, foci bearing centriolar proteins that act as MTOCs have been revealed by EM analysis not to harbour microtubules [104][105][106][107][108]. These examples illustrate that foci bearing centriolar proteins do not necessarily correspond to bona fide centrioles, despite retaining their function in recruiting PCM and acting as MTOCs.…”