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.
Highlights
Motile cilia and homologous structures, flagella, are microtubule-based external cell protrusions assembled in eukaryotic organisms from evolutionarily distant lineages
The transition zone containing Y-links and the distal part of the basal body containing transition fibers form a ciliary gate that separates the cilium from the rest of the cell [1,2]
Such localization corresponds to the position and Rib72, are strongly attached to the protofilaments that are shared between the A- and B-tubule of some MIP densities
Summary
Motile cilia and homologous structures, flagella, are microtubule-based external cell protrusions assembled in eukaryotic organisms from evolutionarily distant lineages. Ciliary outer doublet microtubules are a continuation of two out of three microtubules of the basal body, a structure that anchors cilium to the cell body by so-called rootlets and docks to the cell membrane through transition fibers (Figure 1). The most proximal and distal regions of the cilium are called the transition zone and the ciliary tip, respectively, and they have unique ultrastructural organizations. A schematic schematic representation representation of 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 grey), and and the the N-DRC. Progress is the result the joint efforts of in the understanding of motile ultrastructure andThis protein composition This of progress is the result researchers using genetic, biochemical, and cryo-ET approaches. Of the joint efforts of researchers using genetic, biochemical, and cryo-ET approaches
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