Ciliary Proteins: Filling the Gaps. Recent Advances in Deciphering the Protein Composition of Motile Ciliary Complexes

Abstract

Cilia are highly evolutionarily conserved, microtubule-based cell protrusions present in eukaryotic organisms from protists to humans, with the exception of fungi and higher plants. Cilia can be broadly divided into non-motile sensory cilia, called primary cilia, and motile cilia, which are locomotory organelles. The skeleton (axoneme) of primary cilia is formed by nine outer doublet microtubules distributed on the cilium circumference. In contrast, the skeleton of motile cilia is more complex: in addition to outer doublets, it is composed of two central microtubules and several diverse multi-protein complexes that are distributed periodically along both types of microtubules. For many years, researchers have endeavored to fully characterize the protein composition of ciliary macro-complexes and the molecular basis of signal transduction between these complexes. Genetic and biochemical analyses have suggested that several hundreds of proteins could be involved in the assembly and function of motile cilia. Within the last several years, the combined efforts of researchers using cryo-electron tomography, genetic and biochemical approaches, and diverse model organisms have significantly advanced our knowledge of the ciliary structure and protein composition. Here, we summarize the recent progress in the identification of the subunits of ciliary complexes, their precise intraciliary localization determined by cryo-electron tomography data, and the role of newly identified proteins in cilia.

Keywords: MIPs; N-DRC; axoneme; dynein arms; motile cilia; radial spokes.

Figure 1

A schematic representation of the regions of the basal body and cilium. The basal body (microtubules in blue) is docked to the cell membrane through distal appendages called transition fibers (in red). The axonemal microtubules in the region of the transition zone (the most proximal part of the cilium) are connected to the ciliary membrane through Y-links (in light green). The most distal part of the cilium, called the ciliary tip, can structurally vary in cilia assembled by different cell types. Here, partial 10-protofilament microtubules (called B-tubules) of the outer doublets terminate earlier than 13-protofilament A-tubules. The plus ends of the ciliary microtubules are capped with the structure called the ciliary cap (in violet). The transmission electron microscopy (TEM) sections of a ciliate Tetrahymena thermophila. On the left are longitudinal TEM sections of (a) the ciliary tip, (b) the basal body, transition zone and part of the cilium. On the right are cross TEM sections of (c) the ciliary tip with central microtubules and peripheral singlets, (d) the main ciliary shaft, (e) region in which one of the central microtubules originates, (f) the transition zone, (g) the distal part of the basal body, (h) the middle part of the basal body, and (i) the proximal part of the basal body.

Figure 2

A schematic representation of the organization of macro-complexes within the 96 nm axonemal unit: ODAs (outer dynein arms, in green), IDAs (inner dynein arms, in violet), RSs (radial spokes, in grey), and the N-DRC (nexin–dynein regulatory complex, in blue) [4].

Figure 3

Organization of motile cilia ultrastructure: (a) transmission electron microscopy (TEM) cross-section of a Tetrahymena cilium, scale = 200 nm; (b) a schematic representation of the organization of axonemal macro-complexes: ODAs (outer dynein arms, in green), IDAs (inner dynein arms, in violet), RSs (radial spokes, in grey), the N-DRC (nexin–dynein regulatory complex, in blue), and the central apparatus (two microtubules, with their projections in yellow and dark yellow, and the bridge connecting the central microtubules in red); (c) schematic representation of the outer doublet showing protofilament numbering (A1–A13: protofilaments of the A-tubule, B1–B10: protofilaments of the B-tubule), position of the globular MIPs (microtubule inner proteins, 1–7), and the position of the outer doublet macro-complexes (colored as in (b)); the positions of the filamentous MIPs are marked by red stars. (c) Modified from [5,42,43,44,51].

Figure 4

A schematic representation of the 96 nm axonemal unit with the position of the modifier of inner arms (MIA) complex (in yellow) and tether/tether head (T/TH) complex (in red) indicated. ODAs (outer dynein arms, in green), IDAs (inner dynein arms, in violet), RSs (radial spokes, in grey), N-DRC (nexin–dynein regulatory complex, in blue) [4,118,151,152,153].