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A basal body is the centriole-derived structure at the base of a cilium or a eukaryotic flagellum. It anchors and organizes the microtubules of the projecting structure. A typical basal body contains nine peripheral microtubule triplets; farther along the cilium, the corresponding outer elements usually continue as doublets. The central pair found in many motile cilia is not a pair of central microtubules running through the basal body.^[ref]

In a cell bearing a primary cilium, a mature centriole can dock at the cell membrane and act as its basal body. The same underlying organelle therefore has different roles depending on its position and associated structures: a centriole within a centrosome helps organize an intracellular microtubule system, whereas a basal body organizes a projection from the surface. The ninefold arrangement is connected to assembly of SAS-6 into a central cartwheel scaffold.^[ref]

A centriole is a cylindrical organelle with a characteristic ninefold organization of microtubules and associated proteins. In many animal cells, two centrioles lie within the surrounding material of a centrosome. Much of the microtubule nucleation occurs in that surrounding material: a centriole and a centrosome are therefore different structural levels. A centriole can also become the basal body that organizes a cilium. ^[ref]

The geometry has a molecular basis. Structural and reconstitution experiments showed that SAS-6 family proteins form elongated dimers whose head-to-head interactions can assemble a cartwheel-like hub with radiating spokes. Mutations interfering with oligomerization impaired centriole formation. This connects a repeated protein interaction to the symmetry of a larger cellular structure. ^[ref]

A microtubule-organizing structure in relevant animal cells. Positioning centrosomes relative to a niche can prepare the orientation of a later mitotic spindle. Budding yeast instead has a spindle pole body.

Its microtubule-nucleating material helps organize arrays without requiring every filament to remain permanently attached. A centrosome and a centriole are different structural levels: centrioles can be components of a centrosome, while the surrounding material carries much of the organizing activity. ^[ref]

Daiju Kitagawa led work explaining the ninefold organization of centrioles through interactions between SAS-6 molecules. The 2011 study combined structural analysis, biochemical assembly and cellular experiments: rod-shaped dimers associated through their ends, and disrupting the relevant association impaired centriole formation.^[ref]

A model built from the related Chlamydomonas reinhardtii protein placed nine dimers in a ring with spokes extending outward. Recombinant protein also formed cartwheel-like assemblies. The work connects a repeated protein structure to the larger geometry of an organelle, rather than treating its symmetry as an unexplained cellular pattern.^[ref]

SAS-6 is a family of scaffold proteins required for normal centriole assembly. Elongated SAS-6 dimers can associate through their head domains to form the central hub and radiating spokes of the centriole cartwheel. This supplies a molecular route from interactions between proteins to the ninefold organization of a much larger organelle.

Daiju Kitagawa and colleagues combined protein structure determination, assembly experiments and cellular perturbations. Their model arranged nine dimers into a ring; electron microscopy of recombinant algal SAS-6 showed cartwheel-like assemblies, while mutations that impaired head-domain association also impaired centriole formation. The isolated structure alone would show a possible geometry; its agreement with assembly and perturbation experiments connects that geometry to function.^[ref]