Nucleation of microtubule assembly by a gamma-tubulin-containing ring complex

There are three types of cytoskeletal structures in eukaryotic cells—microfilaments, intermediate filaments, and microtubules. With a diameter of about 25 nm, microtubules are the thickest of these fibers. Microtubules carry out a variety of functions that include cell structure and support, transport of organelles, cell motility (movement), and the separation of chromosomes during cell division.Microtubules are hollow tubes whose walls are made up of globular tubulin proteins. Each tubulin...

Cytoskeletal filaments are polymeric forms of smaller protein subunits. However, individual cytoskeletal filaments may easily disassemble or associate with other similar filaments to form rigid structures. Microfilaments, made of actin monomers, rely on actin-binding proteins to form bundles and create networks of individual actin filaments. Microtubules rely on microtubule-associated proteins (MAPs) to form sturdy cylindrical structures. However, the proteins involved in forming complex...

Microtubules are the thickest cytoskeletal filaments with a diameter of 25 nm. In prokaryotic organisms, microtubules are commonly found in locomotory appendages like cilia and flagella. In eukaryotic cells, microtubules form specialized extensions for moving fluid over the surface, like those found in cells lining the intestine.
Microtubules have two structurally similar globular protein subunits: α and β tubulins. In the cytosol, the α and β tubulins form a heterodimer. These αβ-heterodimers...

Microtubules are dynamic structures that undergo continuous assembly and disassembly. They originate from specialized multi-protein complexes known as microtubule organizing centers or MTOCs. Within the MTOC, the point of origin of the microtubule is known as the minus end, while the end radiating outward is the plus end. Microtubules serve two primary functions — the organization of spindle complexes to separate sister chromatids during mitotic or meiotic cell division and the formation of...

Complex microtubule structures are present in resting cells and in dividing cells. In resting cells, they are responsible for maintaining the cellular architecture, tracks for intracellular transport, positioning of organelles, assembly of cilia and flagella. They mediate the bipolar spindle assembly for chromosomal segregation and positioning of the cell division plate in dividing cells. The formation of microtubule complex structures depends on the cell type, cell stage, and cell function.

Spindle assembly occurs through three, often coexisting, pathways – the centrosome-mediated pathway, the chromatin-mediated pathway, and the microtubule-mediated pathway – collectively contributing to form a robust spindle apparatus.
In most cells, centrosomes are the primary microtubule nucleation centers. In the centrosome-mediated pathway, the G2-prophase transition triggers centrosome maturation and increased microtubule nucleation. Progressive nucleation results in a microtubule array...