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Review
. 2017 Apr;40(4):243-253.
doi: 10.14348/molcells.2017.0054. Epub 2017 Apr 12.

The Ciliary Transition Zone: Finding the Pieces and Assembling the Gate

João Gonçalves  1 , Laurence Pelletier  1   2
Affiliations
Review

The Ciliary Transition Zone: Finding the Pieces and Assembling the Gate

João Gonçalves et al. Mol Cells. 2017 Apr.

Abstract

Eukaryotic cilia are organelles that project from the surface of cells to fulfill motility and sensory functions. In vertebrates, the functions of both motile and immotile cilia are critical for embryonic development and adult tissue homeostasis. Importantly, a multitude of human diseases is caused by abnormal cilia biogenesis and functions which rely on the compartmentalization of the cilium and the maintenance of its protein composition. The transition zone (TZ) is a specialized ciliary domain present at the base of the cilium and is part of a gate that controls protein entry and exit from this organelle. The relevance of the TZ is highlighted by the fact that several of its components are coded by ciliopathy genes. Here we review recent developments in the study of TZ proteomes, the mapping of individual components to the TZ structure and the establishment of the TZ as a lipid gate.

Keywords: centriole; centrosome; cilia; transition zone.

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Figures

Fig. 1
Fig. 1
Cilia structure. The scheme depicts the structure of a primary cilium. At their base eukaryotic cilia present a centriole/basal body, from which the axoneme microtubules elongate, and accessory structures such as the basal foot and the transition fibers. The ciliary membrane is a specialized membrane domain enriched in specific proteins (e.g. ARL13B) and lipid species (e.g. PI(4)P). At the proximal region of the axoneme is the transition zone characterized by microtubule-membrane connectors. Distal to the transition zone is localized the inversin domain which lacks y-links and has a distinct protein composition from the transition zone. The figure shows the protein modules present at the transition zone largely as they were described by Chih et al. (2011), Garcia-Gonzalo et al. (2011), and Sang et al. (2011). Indicated are also genetic interactions between components of the transition zone modules and components of the BBSome and IFT.
Fig. 2
Fig. 2
Localization of transition zone proteins. The schemes represent the localization of transition zone and basal body components in human, Drosophila and Trypanosome cilia and flagella, and were adapted from Dean et al. (2016), Pratt et al. (2016), and Yang et al. (2015). MT – microtubules; CM – ciliary membrane; CP – ciliary pocket; TF – transition fibers; BB – basal body; TZ – transition zone; BP – basal plate; TP – terminal plate.
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