26.3
La formation de microtubules
Les microtubules sont des structures dynamiques qui subissent un assemblage et un désassemblage continus. Ils proviennent de complexes multiprotéiques…
La formation des microtubules commence dans des zones spécialisées des cellules, les centres d’organisation des microtubules ou MTOC.
Les cellules eucaryotes ont plusieurs types de MTOC, y compris les corps basaux et les centrosomes, qui recrutent des protéines qui composent les complexes cycliques gamma-tubuline.
Tout d’abord, deux sous-unités de la gamma-tubuline se couplent avec deux protéines différentes du complexe de la gamma-tubuline ou GCP formant un noyau hétérotétramérique - le petit complexe gamma-tubuline. Ensuite, sept copies de ce complexe central s’organisent en une structure hélicoïdale et s’associent à d’autres GCP accessoires pour former le complexe complet de l’anneau gamma-tubuline.
Le complexe cyclique résultant comporte treize sous-unités gamma-tubulines exposées qui servent de modèle pour la formation des microtubules.
La nucléation des microtubules commence lorsque l’alpha-tubuline se lie aux sous-unités gamma-tubuline exposées formant l’extrémité négative du complexe cyclique. Cela laisse la bêta-tubuline, ou l’extrémité positive de l’hétérodimère, libre pour un assemblage ultérieur.
Le complexe cyclique gamma-tubuline à l’extrémité négative du filament agit comme une coiffe et permet à l’allongement de se poursuivre uniquement à l’extrémité positive.
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Q1: What are microtubule organizing centers and what role do they play in microtubule formation?
Microtubule organizing centers (MTOCs) are specialized cellular structures where microtubule formation begins. In eukaryotic cells, MTOCs include centrosomes and basal bodies that recruit gamma-tubulin ring complexes. These complexes serve as templates for microtubule nucleation, with the minus end anchored at the MTOC and the plus end extending outward for continued assembly.
Q2: How is the gamma-tubulin ring complex assembled?
The gamma-tubulin ring complex forms through a multi-step process. First, two gamma-tubulin subunits couple with two Gamma-tubulin-Complex-Proteins (GCPs) to create a heterotetrameric core. Seven copies of this core arrange helically and associate with additional accessory GCPs, resulting in a complete ring complex with thirteen exposed gamma-tubulin subunits that template microtubule formation.
Q3: What happens when alpha-tubulin binds to the gamma-tubulin ring complex?
When alpha-tubulin binds to exposed gamma-tubulin subunits, it initiates microtubule nucleation and forms the minus end of the microtubule. This binding leaves the beta-tubulin (plus end) of the heterodimer free for further assembly. The gamma-tubulin ring complex acts as a cap at the minus end, restricting elongation to occur only at the plus end.
Q4: What is the difference between the minus end and plus end of a microtubule?
The minus end originates at the MTOC where alpha-tubulin binds to the gamma-tubulin ring complex, serving as the nucleation site. The plus end extends outward from the MTOC and is the primary site for microtubule elongation. This polarity allows directional microtubule growth and is essential for organizing spindle complexes during cell division.
Q5: How do intrinsic and extrinsic factors influence microtubule nucleation?
Intrinsic factors affecting nucleation include alpha- and beta-tubulin isotypes, free heterodimer concentration, post-translational modifications, and microtubule-associated proteins (MAPs). Extrinsic factors such as temperature, pH, and microtubule interfering drugs regulate polymerization and depolymerization rates. Together, these factors control the dynamics of microtubule assembly within the cell.
Q6: What are the primary functions of microtubules in eukaryotic cells?
Microtubules serve two main functions: organizing spindle complexes to separate sister chromatids during mitotic or meiotic cell division, and forming locomotory appendages like cilia and flagella. Their dynamic nature allows continuous assembly and disassembly, enabling cells to rapidly reorganize these structures in response to cellular needs.
Q7: How do MTOCs vary across different eukaryotic cell types?
The structure and location of MTOCs vary within different eukaryotic cell types depending on microtubule function. Animal cells contain organized centrosomes with centrioles and pericentriolar material, while some lower eukaryotes like most fungi lack organized MTOCs entirely. This variation reflects the specialized roles microtubules play in different cell types.