Role of delta-tubulin and the C-tubule in assembly of Paramecium basal bodies

BACKGROUND: A breakthrough in the understanding of centriole assembly was provided by the characterization of the UNI3 gene in Chlamydomonas. Deletion of this gene, found to encode a novel member of the tubulin superfamily, delta-tubulin, results in the loss of the C-tubule, in the nine microtubule triplets which are the hallmark of centrioles and basal bodies. Delta-tubulin homologs have been identified in the genomes of mammals and protozoa, but their phylogenetic relationships are unclear and their function is not yet known. RESULTS: Using the method of gene-specific silencing, we have inactivated the Paramecium delta-tubulin gene, which was recently identified. This inactivation leads to loss of the C-tubule in all basal bodies, without any effect on ciliogenesis. This deficiency does not directly affect basal body duplication, but perturbs the cortical cytoskeleton, progressively leading to mislocalization and loss of basal bodies and to altered cell size and shape. Furthermore, additional loss of B- and even A-tubules at one or more triplet sites are observed: around these incomplete cylinders, the remaining doublets are nevertheless positioned according to the native ninefold symmetry. CONCLUSIONS: The fact that in two distinct phyla, delta-tubulin plays a similar role provides a new basis for interpreting phylogenetic relationships among delta-tubulins. The role of delta-tubulin in C-tubule assembly reveals that tubulins contribute subtle specificities at microtubule nucleation sites. Our observations also demonstrate the existence of a prepattern for the ninefold symmetry of the organelle which is maintained even if less than 9 triplets develop

Épigénétique : la paramécie comme modèle d’étude

Si la paramécie apparaît comme un modèle de choix pour analyser les composantes épigénétiques de l’hérédité, cela tient sans doute à sa complexité structurale et fonctionnelle qui en fait une sorte de métazoaire non cellularisé. Ses deux caractéristiques les plus atypiques, en effet, sont la complexité de son organisation corticale - offrant une énorme amplification des structures centriolaires qui n’existent ailleurs qu’en deux exemplaires par cellule - et son dualisme nucléaire, avec un micronoyau diploïde à fonction germinale et un macronoyau très polyploïde, dérivé du micronoyau, mais contenant un génome « simplifié » dédié à la transcription. Cet article tente de décrire comment l’analyse génétique de caractères touchant justement à ces particularités - l’organisation du cortex et l’expression de fonctions macronucléaires - a conduit à mettre en évidence le rôle, dans l’hérédité cellulaire chez la paramécie, de trois composantes : génome, transcriptome et « structurome », trilogie qui a quelques chances d’avoir une signification biologique, voire évolutive, générale.Since the middle of the last century, Paramecium has appeared as an intriguing genetic model, displaying a variety of heritable characters which do not follow the Mendel laws but are cytoplasmically inherited. The analysis of the hereditary mechanisms at play in this eukaryotic unicellular organism has provided new insight into epigenetics mechanisms. Interestingly, the revealing phenomena concern two pecularities of Paramecium, its highly elaborate surface structure (with thousands of ciliary basal bodies as cytoskeleton organizers), and its nuclear dualism (coexistence of a diploid « germline » micronucleus and a highly polyploid somatic macronucleus devoted to transcription, which contains a rearranged version of the germline genome). Analysis of variant cortical organization has led to the concept of structural inheritance, implying that assembly of new organelles and supramolecular protein complexes is guided by pre-existing organization. Analysis of other cytoplasmically inherited characters revealed that the developing macronucleus is epigenetically programmed by the maternal macronucleus through RNA-mediated, homology-dependent effects, suggesting the transcriptome should be recognized as a third actor in cellular inheritance, along with the « structurome » and the genome

Épigénétique : la paramecia comme modèle d’étude

Si la paramécie apparaît comme un modèle dechoix pour analyser les composantes épigénétiquesde l’hérédité, cela tient sans doute à sa complexité structurale et fonctionnelle qui en fait une sorte de métazoaire non cellularisé. Ses deux caractéristiques les plus atypiques, en effet, sont la complexité de son organisation corticale - offrant une énorme amplification des structures centriolaires qui n’existent ailleurs qu’en deux exemplaires par cellule - et son dualisme nucléaire, avec un micronoyau diploïde à fonction germinale et un macronoyau très polyploïde, dérivé du micronoyau, mais contenant un génome «simplifié» dédié à la transcription. Cet article tente de décrire comment l’analyse génétique de caractères touchant justement à ces particularités - l’organisation du cortex et l’expression de fonctions macronucléaires - a conduit à mettre en évidence le rôle, dans l’hérédité cellulaire chez la paramécie, de trois composantes: génome, transcriptome et « structurome », trilogie qui a quelques chances d’avoir une signification biologique, voire évolutive, générale

The SM19 gene, required for duplication of basal bodies in Paramecium, encodes a novel tubulin, η-tubulin

AbstractThe discovery of δ-tubulin, the fourth member of the tubulin superfamily, in Chlamydomonas[1] has led to the identification in the genomes of vertebrates and protozoa of putative δ homologues and of additional tubulins, ε and ζ[2–4]. These discoveries raise questions concerning the functions of these novel tubulins, their interactions with microtubule arrays and microtubule-organising centres, and their evolutionary status. The sm19-1 mutation of Paramecium specifically inhibits basal body duplication [5] and causes delocalisation of γ-tubulin, which is also required for basal body duplication [6]. We have cloned the SM19 gene by functional complementation and found that it encodes another new member of the tubulin superfamily. SM19p, provisionally called eta-tubulin (η-tubulin), shows low sequence identity with the tubulins previously identified in Paramecium, namely, α[7], β[8], γ[6], δ (this work) and ε (P. Dupuis-Williams, personal communication). Phylogenetic analysis indicated that SM19p is not consistently grouped with any phylogenetic entity