Figure 1. Overview of evolutionarily conserved components and compartments of cilia, including the transition zone “ciliary gate”.
(A) All cilia possess a microtubule‐based axoneme that stems from a basal body foundation anchored to the plasma membrane via transition fibers. To build the cilium and make it dynamic, an intraflagellar transport (IFT) machinery powered by molecular motors (IFT‐Kinesin and IFT‐Dynein) docks at transition fibers and uses three functional modules (IFT‐A, IFT‐B and BBS protein complex or BBSome) to traffic ciliary proteins (cargoes) into and out of the organelle. Immediately distal to the basal body is a transition zone (TZ) region, widely believed to form a ciliary gate or diffusion barrier for membrane and soluble proteins. The TZ harbors typically Y‐shaped structures that span from the axoneme to the overlying membrane. In metazoans, the proximal segment includes a so‐called Inversin compartment that functionally interacts with the TZ. Most cilia also possess a distal segment with singlet microtubules. (i) TZ‐associated Y‐links are shown with arrowheads in cross‐section electron microscope images of motile cilia TZs from Chlamydomonas (modified from Craige et al, 2010) and rabbit oviduct (Anderson, 1974), as well as nonmotile cilia TZs from C. elegans (Jensen et al, 2018) and rat photoreceptor (Besharse et al, 1985; “Copyright 1985 Society for Neuroscience”). (ii) The ends of the TZ Y‐links, which connect to the membrane, likely form ciliary necklace “beads.” Shown is an electron micrograph of freeze‐fractured hamster respiratory airway cilia showing the ciliary necklace region, which contains 7–8 strands of membrane particles (arrowheads) and may form a spiral (arrows show start/end; modified from Heller & Gordon, 1986). Scale bars, 100 nm. (B) Rhodopsin (arrowheads) is highly concentrated in the distal ciliary region of the cilium (outer segment), and virtually absent from the connecting cilium (TZ) and near the basal body. Modified from ref. (Liu et al, 1999; “Copyright 1999 Society for Neuroscience”). Scale bar, 0.5 μm.