Biology subjects

Bournonville, L.

Publications and source records attributed to Bournonville, L..

5 recordsLinked to original sources

bioRxiv2025

The Luminal Ring Protein C2CD3 Acts as a Radial In-to-Out Organizer of the Distal Centriole and Appendages

Centrioles are polarized microtubule-based structures with appendages at their distal end that are essential for cilia formation and function. The protein C2CD3 is critical for distal appendage assembly, with mutations linked to orofaciodigital syndrome and other ciliopathies. However, its precise molecular role in appendage recruitment remains unclear. Using Ultrastructure Expansion Microscopy (U-ExM), iterative U-ExM, and in situ cryo-electron tomography (cryo-ET), we reveal that C2CD3 adopts a radially symmetric 9-fold organization within the centrioles distal lumen. We show that the C-terminal region of C2CD3 localizes close to a [~]100 nm luminal ring structure consisting of [~]27 nodes, while its N-terminal region localizes close to a hook-like structure that attaches to the A-microtubule as it extends from the centriole interior to exterior. This hook structure is adjacent to the DISCO complex (MNR/CEP90/OFD1), which marks future appendage sites. C2CD3 depletion disrupts not only the recruitment of the DISCO complex via direct interaction with MNR but also destabilizes the luminal ring network composed of C2CD3/SFI1/centrin-2/CEP135/NA14, as well as the distal microtubule tip protein CEP162. This reveals an intricate "in-to-out" molecular hub connecting the centriolar lumen, distal microtubule cap, and appendages. Although C2CD3 loss results in shorter centrioles and appendage defects, key structural elements remain intact, permitting continued centriole duplication. We propose that C2CD3 forms the luminal ring structure and extends radially to the space between triplet microtubules, functioning as an architectural hub that scaffolds the distal end of the centriole, orchestrating its assembly and directing appendage formation.

cell biology↗

bioRxiv2024

The C. elegans homolog of Sjögrens Syndrome Nuclear Antigen 1 is required for the structural integrity of the centriole and bipolar mitotic spindle assembly

Centrioles play central roles in ciliogenesis and mitotic spindle assembly. Once assembled, centrioles exhibit long-term stability, a property essential for maintaining numerical control. How centriole stability is achieved and how it is lost in certain biological contexts are still not completely understood. In this study we show that SSNA-1, the Caenorhabditis elegans ortholog of Sjogrens Syndrome Nuclear Antigen 1, is a centriole constituent that localizes close to the microtubule outer wall, while also exhibiting a developmentally regulated association with centriole satellite-like structures. A complete deletion of the ssna-1 gene results in an embryonic lethal phenotype marked by the appearance of extra centrioles and spindle poles. We show that SSNA-1 genetically interacts with the centriole stability factor SAS-1 and is required post assembly for centriole structural integrity. In SSNA-1s absence, centrioles assemble but fracture leading to extra spindle poles. However, if the efficiency of cartwheel assembly is reduced, the absence of SSNA-1 results in daughter centriole loss and monopolar spindle formation, indicating that the cartwheel and SSNA-1 cooperate to stabilize the centriole during assembly. Our work thus shows that SSNA-1 contributes to centriole stability during and after assembly, thereby ensuring proper centriole number.

cell biology↗

bioRxiv2024

The A-C Linker controls centriole cohesion and duplication

Centrioles are evolutionarily conserved barrel-shaped organelles playing crucial roles in cell division and ciliogenesis. These functions are underpinned by specific structural sub-elements whose functions have been under investigation since many years. The A- C linker structure, connecting adjacent microtubule triplets in the proximal region, has remained unexplored due to its unknown composition. Here, using ultrastructure expansion microscopy, we characterized two recently identified A-C linker proteins, CCDC77 and WDR67, along with a newly discovered protein, MIIP. Our findings reveal that these proteins localize between microtubule triplets at the A-C linker, forming a complex. Depletion of A-C linker components disrupt microtubule triplet cohesion, leading to breakage at the proximal end. Co-removal of the A-C linker and the inner scaffold demonstrates their joint role in maintaining centriole architecture. Moreover, we uncover an unexpected function of the A-C linker in centriole duplication through torus regulation, underscoring the interplay between these protein modules.

cell biology↗

bioRxiv2024

Functional monocentricity with holocentric characteristics andchromosome-specific centromeres in a stick insect

Centromeres are specialized genomic regions that are critical for chromosome segregation in eukaryotes. However, how the diversity of centromeres evolved, ranging from localized centromeres in monocentric species to complex chromosome-wide distributed centromere regions in holocentric species, remains elusive. Our cellular and genomic analyses in Timema stick insects reveal that within cell variation of the major centromere protein CenH3 recapitulates the variation typically observed across species. While CenH3-containing nucleosomes are distributed in a monocentric fashion on autosomes and bind tandem repeat sequences specific to individual or small groups of chromosomes, they exhibit a holocentric-like distribution on the sex chromosome and bind to more complex intergenic regions. Despite this contrasting distribution, all chromosomes, including the sex chromosome, attach to spindle microtubules at a single location, marking the first instance of a functionally monocentric species with holocentric-like attributes. Together, our findings highlight the potential for gradual transitions towards holocentricity or CenH3-independent centromere determination, and help to understand the rapid centromere sequence divergence between species.

evolutionary biology↗

bioRxiv2020

Two alveolin network proteins are essential for the subpellicular microtubules assembly and conoid anchoring to the apical pole of mature Toxoplasma gondii

Toxoplasma gondii belongs to the coccidian sub-group of Apicomplexa that possess an apical complex harboring a conoid, made of unique tubulin polymer fibers. This enigmatic and dynamic organelle extrudes in extracellular invasive parasites and is associated to the apical polar ring (APR), a microtubule-organizing center for the 22 subpellicular microtubules (SPMTs). The SPMTs are linked to the Inner Membrane Complex (IMC), a patchwork of flattened vesicles, via an intricate network of small filaments composed of alveolins proteins. Here, we capitalize on super-resolution techniques including stimulated emission depletion (STED) microscopy and ultrastructure expansion microscopy (U-ExM) to localize the Apical Cap protein 9 (AC9) and its close partner AC10, identified by BioID, to the alveolin network and intercalated between the SPMTs. Conditional depletion of AC9 or AC10 using the Auxin-induced Degron (AiD) system uncovered a severe loss of fitness. Parasites lacking AC9 or AC10 replicate normally but are defective in microneme secretion and hence fail to invade and egress from infected cells. Remarkably, a series of crucial apical complex proteins (MyoH, AKMT, FRM1, CPH1, ICMAP1 and RNG2) are lost in the mature parasites although they are still present in the forming daughter cells. Electron microscopy on intracellular or deoxycholate-extracted parasites revealed that the mature parasite mutants are conoidless. Closer examination of the SPMTs by U-ExM highlighted the disassembly of the SPMTs in the apical cap region that is presumably at the origin of the catastrophic loss of APR and conoid. AC9 and AC10 are two critical components of the alveolin network that ensure the integrity of the whole apical complex in T. gondii and likely other coccidians.

microbiology↗