The Central Apparatus of Cilia and Eukaryotic Flagella

Abstract

The motile cilium is a complex organelle that is typically comprised of a 9+2 microtubule skeleton; nine doublet microtubules surrounding a pair of central singlet microtubules. Like the doublet microtubules, the central microtubules form a scaffold for the assembly of protein complexes forming an intricate network of interconnected projections. The central microtubules and associated structures are collectively referred to as the central apparatus (CA). Studies using a variety of experimental approaches and model organisms have led to the discovery of a number of highly conserved protein complexes, unprecedented high-resolution views of projection structure, and new insights into regulation of dynein-driven microtubule sliding. Here, we review recent progress in defining mechanisms for the assembly and function of the CA and include possible implications for the importance of the CA in human health.

Figure 1.

Central apparatus (CA) structure. (A) Transmission electron microscopy (TEM) image of an axoneme transverse section from Chlamydomonas reinhardtii. The central microtubules and their associated projections are surrounded by the outer doublet microtubules. (B) Diagram of axoneme transverse section seen in A. (C) Diagram of CA with projections labeled. DMT, Doublet microtubule; IDA, inner dynein arm; ODA, outer dynein arm; RS, radial spoke.

Figure 2.

Detailed structural elements of the central apparatus (CA). Isosurface renderings derived from cryo-electron tomography (cryo-ET) of the Chlamydomonas CA viewed in transverse section. The projections form an interconnected network surrounding the two singlet microtubules with connections between C1a and C2a and C1b and C2b. The bridge structure connecting the two microtubules is in gray. MIP-C2a is highlighted within the lumen of the C2 microtubule. The projections labeled C1e, C1f, C2d, and C2e likely correspond to the material previously described as a sheath. (From Carbajal-Gonzalez et al. 2013; reprinted, with permission, from Wiley.)