Cilium Assembly (Homo sapiens)
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Description
This pathway describes cilia formation, with an emphasis on the primary cilium. The primary cilium is a sensory organelle that is required for the transduction of numerous external signals such as growth factors, hormones and morphogens, and an intact primary cilium is needed for signaling pathways mediated by Hh, WNT, calcium, G-protein coupled receptors and receptor tyrosine kinases, among others (reviewed in Goetz and Anderson, 2010; Berbari et al, 2009; Nachury, 2014). Unlike the motile cilia, which are generally present in large numbers on epithelial cells and are responsible for sensory function as well as wave-like beating motions, the primary cilium is a non-motile sensory organelle that is present in a single copy at the apical surface of most quiescent cells (reviewed in Hsiao et al, 2012).
Cilium biogenesis involves the anchoring of the basal body, a centriole-derived organelle, near the plasma membrane and the subsequent polymerization of the microtubule-based axoneme and extension of the plasma membrane (reviewed in Ishikawa and Marshall, 2011; Reiter et al, 2012). Although the ciliary membrane is continuous with the plasma membrane, the protein and lipid content of the cilium and the ciliary membrane are distinct from those of the bulk cytoplasm and plasma membrane (reviewed in Emmer et al, 2010; Rohatgi and Snell, 2010). This specialized compartment is established and maintained during cilium biogenesis by the formation of a ciliary transition zone, a proteinaceous structure that, with the transition fibres, anchors the basal body to the plasma membrane and acts as a ciliary pore to limit free diffusion from the cytosol to the cilium (reviewed in Nachury et al, 2010; Reiter et al, 2012). Ciliary components are targeted from the secretory system to the ciliary base and subsequently transported to the ciliary tip, where extension of the axoneme occurs, by a motor-driven process called intraflagellar transport (IFT). Anterograde transport of cargo from the ciliary base to the tip of the cilium requires kinesin-2 type motors, while the dynein-2 motor is required for retrograde transport back to the ciliary base. In addition, both anterograde and retrograde transport depend on the IFT complex, a multiprotein assembly consisting of two subcomplexes, IFT A and IFT B. The primary cilium is a dynamic structure that undergoes continuous steady-state turnover of tubulin at the tip; as a consequence, the IFT machinery is required for cilium maintenance as well as biogenesis (reviewed in Bhogaraju et al, 2013; Hsiao et al, 2012; Li et al, 2012; Taschner et al, 2012; Sung and Leroux, 2013).
The importance of the cilium in signaling and cell biology is highlighted by the wide range of defects and disorders, collectively known as ciliopathies, that arise as the result of mutations in genes encoding components of the ciliary machinery (reviewed in Goetz and Anderson, 2010; Madhivanan and Aguilar, 2014).
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DataNodes
dimer:
ARF4:GTP:VxPx-containing ciliary membrane proteinsdimer:ASAP1
dimer:ARF4:GTP:VxPx-containing ciliary membrane proteinsdimer:ASAP1
dimer:VxPx-containing ciliary membrane proteinsciliary membrane
proteinsciliary membrane
proteinsbody:transition zone
proteins:RAB3IP:RAB11A:GTP:Golgi-derived vesiclebody:transition
zone proteinsAnnotated Interactions
dimer:
ARF4:GTP:VxPx-containing ciliary membrane proteinsdimer:
ARF4:GTP:VxPx-containing ciliary membrane proteinsThis reaction shows putative human IFT B proteins assembling in a single step; details of how and when this assembly occurs are not shown, nor are the specific protein-protein interactions within the complex or details of how IFT B is regulated. Moreover, this reaction shows the formation of a presumptive IFT B* complex, lacking IFT20, to allow the recruitment of IFT20 from the Golgi compartment to be depicted.
Details of protein-protein interactions within the IFT A complex are not known, nor are the details of how and where the complex assembles in a human cell.
In mammalian cells, formation of the BBSome depends on a BBS/CCT complex that consists of MKKS/BBS6, BBS10, BBS12 and 6 members of the CCT/TRiC family of chaperonins. The BBS/CCT complex interacts with a subset of the BBSome protein and plays a role in the BBS7 stability, promoting the formation of an intermediate "BBSome core complex" (Seo et al, 2010; Jin et al, 2010; Zhang el al, 2012).
dimer:ASAP1
dimer:ARF4:GTP:VxPx-containing ciliary membrane proteinsdimer:ASAP1
dimer:ARF4:GTP:VxPx-containing ciliary membrane proteinsdimer:ASAP1
dimer:ARF4:GTP:VxPx-containing ciliary membrane proteinsdimer:ASAP1
dimer:VxPx-containing ciliary membrane proteinsdimer:ASAP1
dimer:VxPx-containing ciliary membrane proteinsciliary membrane
proteinsciliary membrane
proteinsbody:transition zone
proteins:RAB3IP:RAB11A:GTP:Golgi-derived vesiclebody:transition zone
proteins:RAB3IP:RAB11A:GTP:Golgi-derived vesiclebody:transition
zone proteinsbody:transition
zone proteins