Abstract
Primary cilium is a rod-like plasma membrane protrusion that plays important roles in sensing the cellular environment and initiating corresponding signaling pathways. The sensory functions of the cilium critically depend on the unique enrichment of ciliary residents, which is maintained by the ciliary diffusion barrier. It is still unclear how ciliary cargoes specifically enter the diffusion barrier and accumulate within the cilium. In this review, the organization and trafficking mechanism of the cilium are compared to those of the nucleus, which are much better understood at the moment. Though the cilium differs significantly from the nucleus in terms of molecular and cellular functions, analogous themes and principles in the membrane organization and cargo trafficking are notable between them. Therefore, knowledge in the nuclear trafficking can likely shed light on our understanding of the ciliary trafficking. Here, with a focus on membrane cargoes in mammalian cells, we briefly review various ciliary trafficking pathways from the Golgi to the periciliary membrane. Models for the subsequent import translocation across the diffusion barrier and the enrichment of cargoes within the ciliary membrane are discussed in detail. Based on recent discoveries, we propose a Rab–importin-based model in an attempt to accommodate various observations on ciliary targeting.
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References
Ishikawa H, Marshall WF (2017) Intraflagellar transport and ciliary dynamics. Cold Spring Harb Perspect Biol 9(3):a021998
Nachury MV et al (2010) Trafficking to the ciliary membrane: how to get across the periciliary diffusion barrier? Annu Rev Cell Dev Biol 26:59–87
Basten SG, Giles RH (2013) Functional aspects of primary cilia in signaling, cell cycle and tumorigenesis. Cilia 2(1):6
Goetz SC, Anderson KV (2010) The primary cilium: a signalling centre during vertebrate development. Nat Rev Genet 11(5):331–344
Garcia-Gonzalo FR, Reiter JF (2012) Scoring a backstage pass: mechanisms of ciliogenesis and ciliary access. J Cell Biol 197(6):697–709
Vorobjev IA, Chentsov Yu S (1982) Centrioles in the cell cycle. I. Epithelial cells. J Cell Biol 93(3):938–949
Reiter JF et al (2012) The base of the cilium: roles for transition fibres and the transition zone in ciliary formation, maintenance and compartmentalization. EMBO Rep 13(7):608–618
Anderson RG (1972) The three-dimensional structure of the basal body from the rhesus monkey oviduct. J Cell Biol 54(2):246–265
Gilula NB, Satir P (1972) The ciliary necklace. A ciliary membrane specialization. J Cell Biol 53(2):494–509
Ounjai P et al (2013) Architectural insights into a ciliary partition. Curr Biol 23(4):339–344
Hu Q et al (2010) A septin diffusion barrier at the base of the primary cilium maintains ciliary membrane protein distribution. Science 329(5990):436–439
Chih B et al (2012) A ciliopathy complex at the transition zone protects the cilia as a privileged membrane domain. Nat Cell Biol 14(1):61–72
Leaf A, Von Zastrow M (2015) Dopamine receptors reveal an essential role of IFT-B, KIF17, and Rab23 in delivering specific receptors to primary cilia. Elife 4:e06996
Ye F et al (2013) Single molecule imaging reveals a major role for diffusion in the exploration of ciliary space by signaling receptors. Elife 2:e00654
Lin YC et al (2013) Chemically inducible diffusion trap at cilia reveals molecular sieve-like barrier. Nat Chem Biol 9(7):437–443
Breslow DK et al (2013) An in vitro assay for entry into cilia reveals unique properties of the soluble diffusion barrier. J Cell Biol 203(1):129–147
Williams CL et al (2011) MKS and NPHP modules cooperate to establish basal body/transition zone membrane associations and ciliary gate function during ciliogenesis. J Cell Biol 192(6):1023–1041
Lambacher NJ et al (2016) TMEM107 recruits ciliopathy proteins to subdomains of the ciliary transition zone and causes Joubert syndrome. Nat Cell Biol 18(1):122–131
Kee HL et al (2012) A size-exclusion permeability barrier and nucleoporins characterize a ciliary pore complex that regulates transport into cilia. Nat Cell Biol 14(4):431–437
Takao D et al (2014) An assay for clogging the ciliary pore complex distinguishes mechanisms of cytosolic and membrane protein entry. Curr Biol 24(19):2288–2294
Takao D, Verhey KJ (2016) Gated entry into the ciliary compartment. Cell Mol Life Sci 73(1):119–127
Vieira OV et al (2006) FAPP2, cilium formation, and compartmentalization of the apical membrane in polarized Madin–Darby canine kidney (MDCK) cells. Proc Natl Acad Sci USA 103(49):18556–18561
Del Viso F et al (2016) Congenital heart disease genetics uncovers context-dependent organization and function of nucleoporins at cilia. Dev Cell 38(5):478–492
Madugula V, Lu L (2016) A ternary complex comprising transportin1, Rab8 and the ciliary targeting signal directs proteins to ciliary membranes. J Cell Sci 129(20):3922–3934
Najafi M et al (2012) Steric volume exclusion sets soluble protein concentrations in photoreceptor sensory cilia. Proc Natl Acad Sci USA 109(1):203–208
Alberts B et al (eds) (2008) Molecular biology of the cell, 5th edn. Garland Science, New York
Hetzer MW (2010) The nuclear envelope. Cold Spring Harb Perspect Biol 2(3):a000539
Stewart M (2007) Molecular mechanism of the nuclear protein import cycle. Nat Rev Mol Cell Biol 8(3):195–208
Crisp M, Burke B (2008) The nuclear envelope as an integrator of nuclear and cytoplasmic architecture. FEBS Lett 582(14):2023–2032
Terry LJ et al (2007) Crossing the nuclear envelope: hierarchical regulation of nucleocytoplasmic transport. Science 318(5855):1412–1416
Burke B, Stewart CL (2013) The nuclear lamins: flexibility in function. Nat Rev Mol Cell Biol 14(1):13–24
Hetzer MW et al (2005) Pushing the envelope: structure, function, and dynamics of the nuclear periphery. Annu Rev Cell Dev Biol 21:347–380
Yang W et al (2004) Imaging of single-molecule translocation through nuclear pore complexes. Proc Natl Acad Sci USA 101(35):12887–12892
Antonin W et al (2011) Traversing the NPC along the pore membrane: targeting of membrane proteins to the INM. Nucleus 2(2):87–91
Katta SS et al (2014) Destination: inner nuclear membrane. Trends Cell Biol 24(4):221–229
Zuleger N et al (2011) System analysis shows distinct mechanisms and common principles of nuclear envelope protein dynamics. J Cell Biol 193(1):109–123
Soullam B, Worman HJ (1995) Signals and structural features involved in integral membrane protein targeting to the inner nuclear membrane. J Cell Biol 130(1):15–27
Boni A et al (2015) Live imaging and modeling of inner nuclear membrane targeting reveals its molecular requirements in mammalian cells. J Cell Biol 209(5):705–720
Ungricht R et al (2015) Diffusion and retention are major determinants of protein targeting to the inner nuclear membrane. J Cell Biol 209(5):687–703
Fan S et al (2007) A novel Crumbs3 isoform regulates cell division and ciliogenesis via importin beta interactions. J Cell Biol 178(3):387–398
Gruss OJ (2010) Nuclear transport receptor goes moonlighting. Nat Cell Biol 12(7):640–641
Jin H et al (2010) The conserved Bardet–Biedl syndrome proteins assemble a coat that traffics membrane proteins to cilia. Cell 141(7):1208–1219
Field MC et al (2011) Evolution: on a bender—BARs, ESCRTs, COPs, and finally getting your coat. J Cell Biol 193(6):963–972
Dishinger JF et al (2010) Ciliary entry of the kinesin-2 motor KIF17 is regulated by importin-beta2 and RanGTP. Nat Cell Biol 12(7):703–710
Hurd TW et al (2011) Localization of retinitis pigmentosa 2 to cilia is regulated by Importin beta2. J Cell Sci 124(Pt 5):718–726
Datta M et al (2011) Genome wide gene expression regulation by HIP1 Protein Interactor, HIPPI: prediction and validation. BMC Genom 12:463
Shi L et al. (2017) The ciliary protein IFT57 in the macronucleus of Paramecium. J Eukaryot Microbiol. doi:10.1111/jeu.12423
Madhivanan K, Aguilar RC (2014) Ciliopathies: the trafficking connection. Traffic 15(10):1031–1056
Hsiao YC et al (2012) Trafficking in and to the primary cilium. Cilia 1(1):4
Francis SS et al (2011) A hierarchy of signals regulates entry of membrane proteins into the ciliary membrane domain in epithelial cells. J Cell Biol 193(1):219–233
Molla-Herman A et al (2010) The ciliary pocket: an endocytic membrane domain at the base of primary and motile cilia. J Cell Sci 123(Pt 10):1785–1795
Stoops EH et al (2015) The periciliary ring in polarized epithelial cells is a hot spot for delivery of the apical protein gp135. J Cell Biol 211(2):287–294
Drubin DG, Nelson WJ (1996) Origins of cell polarity. Cell 84(3):335–344
Papermaster DS et al (1985) Vesicular transport of newly synthesized opsin from the Golgi apparatus toward the rod outer segment. Ultrastructural immunocytochemical and autoradiographic evidence in Xenopus retinas. Investig Ophthalmol Vis Sci 26(10):1386–1404
Moritz OL et al (2001) Mutant rab8 impairs docking and fusion of rhodopsin-bearing post-Golgi membranes and causes cell death of transgenic Xenopus rods. Mol Biol Cell 12(8):2341–2351
Wang J et al (2012) The Arf GAP ASAP1 provides a platform to regulate Arf4- and Rab11-Rab8-mediated ciliary receptor targeting. EMBO J 31(20):4057–4071
Boehlke C et al (2010) Differential role of Rab proteins in ciliary trafficking: Rab23 regulates smoothened levels. J Cell Sci 123(Pt 9):1460–1467
Das A, Guo W (2011) Rabs and the exocyst in ciliogenesis, tubulogenesis and beyond. Trends Cell Biol 21(7):383–386
Emmer BT et al (2010) Molecular mechanisms of protein and lipid targeting to ciliary membranes. J Cell Sci 123(Pt 4):529–536
Finetti F et al (2015) The small GTPase Rab8 interacts with VAMP-3 to regulate the delivery of recycling T-cell receptors to the immune synapse. J Cell Sci 128(14):2541–2552
Szalinski CM et al (2014) VAMP7 modulates ciliary biogenesis in kidney cells. PLoS One 9(1):e86425
Mazelova J et al (2009) Syntaxin 3 and SNAP-25 pairing, regulated by omega-3 docosahexaenoic acid, controls the delivery of rhodopsin for the biogenesis of cilia-derived sensory organelles, the rod outer segments. J Cell Sci 122(Pt 12):2003–2013
Baker SA et al (2008) The outer segment serves as a default destination for the trafficking of membrane proteins in photoreceptors. J Cell Biol 183(3):485–498
Lu Q et al (2015) Early steps in primary cilium assembly require EHD1/EHD3-dependent ciliary vesicle formation. Nat Cell Biol 17(3):228–240
Hunnicutt GR et al (1990) Cell body and flagellar agglutinins in Chlamydomonas reinhardtii: the cell body plasma membrane is a reservoir for agglutinins whose migration to the flagella is regulated by a functional barrier. J Cell Biol 111(4):1605–1616
Milenkovic L et al (2009) Lateral transport of Smoothened from the plasma membrane to the membrane of the cilium. J Cell Biol 187(3):365–374
Rosenbaum JL, Witman GB (2002) Intraflagellar transport. Nat Rev Mol Cell Biol 3(11):813–825
Ou G et al (2007) Sensory ciliogenesis in Caenorhabditis elegans: assignment of IFT components into distinct modules based on transport and phenotypic profiles. Mol Biol Cell 18(5):1554–1569
Lechtreck KF et al (2009) The Chlamydomonas reinhardtii BBSome is an IFT cargo required for export of specific signaling proteins from flagella. J Cell Biol 187(7):1117–1132
Marfori M et al (2011) Molecular basis for specificity of nuclear import and prediction of nuclear localization. Biochim Biophys Acta 9:1562–1577
Twyffels L et al (2014) Transportin-1 and Transportin-2: protein nuclear import and beyond. FEBS Lett 588(10):1857–1868
Soniat M, Chook YM (2015) Nuclear localization signals for four distinct karyopherin-beta nuclear import systems. Biochem J 468(3):353–362
Fan S et al (2004) Polarity proteins control ciliogenesis via kinesin motor interactions. Curr Biol 14(16):1451–1461
Kovacs JJ et al (2008) Beta-arrestin-mediated localization of smoothened to the primary cilium. Science 320(5884):1777–1781
Ghossoub R et al (2013) Septins 2, 7 and 9 and MAP4 colocalize along the axoneme in the primary cilium and control ciliary length. J Cell Sci 126(Pt 12):2583–2594
Spiliotis ET (2010) Regulation of microtubule organization and functions by septin GTPases. Cytoskeleton (Hoboken) 67(6):339–345
Calvert PD et al (2006) Light-driven translocation of signaling proteins in vertebrate photoreceptors. Trends Cell Biol 16(11):560–568
Ellenberg J et al (1997) Nuclear membrane dynamics and reassembly in living cells: targeting of an inner nuclear membrane protein in interphase and mitosis. J Cell Biol 138(6):1193–1206
Larkins CE et al (2011) Arl13b regulates ciliogenesis and the dynamic localization of Shh signaling proteins. Mol Biol Cell 22(23):4694–4703
Lim YS, Tang BL (2015) A role for Rab23 in the trafficking of Kif17 to the primary cilium. J Cell Sci 128(16):2996–3008
Stenmark H (2009) Rab GTPases as coordinators of vesicle traffic. Nat Rev Mol Cell Biol 10(8):513–525
Yoshimura S et al (2007) Functional dissection of Rab GTPases involved in primary cilium formation. J Cell Biol 178(3):363–369
Knodler A et al (2010) Coordination of Rab8 and Rab11 in primary ciliogenesis. Proc Natl Acad Sci USA 107(14):6346–6351
Nachury MV et al (2007) A core complex of BBS proteins cooperates with the GTPase Rab8 to promote ciliary membrane biogenesis. Cell 129(6):1201–1213
Westlake CJ et al (2011) Primary cilia membrane assembly is initiated by Rab11 and transport protein particle II (TRAPPII) complex-dependent trafficking of Rabin8 to the centrosome. Proc Natl Acad Sci USA 108(7):2759–2764
Follit JA et al (2010) The cytoplasmic tail of fibrocystin contains a ciliary targeting sequence. J Cell Biol 188(1):21–28
Ward HH et al (2011) A conserved signal and GTPase complex are required for the ciliary transport of polycystin-1. Mol Biol Cell 22(18):3289–3305
Zhang B et al (2015) GSK3beta-Dzip1-Rab8 cascade regulates ciliogenesis after mitosis. PLoS Biol 13(4):e1002129
Hattula K et al (2002) A Rab8-specific GDP/GTP exchange factor is involved in actin remodeling and polarized membrane transport. Mol Biol Cell 13(9):3268–3280
Murga-Zamalloa CA et al (2010) Interaction of retinitis pigmentosa GTPase regulator (RPGR) with RAB8A GTPase: implications for cilia dysfunction and photoreceptor degeneration. Hum Mol Genet 19(18):3591–3598
Babbey CM et al (2010) Rab10 associates with primary cilia and the exocyst complex in renal epithelial cells. Am J Physiol Renal Physiol 299(3):F495–F506
Lumb JH, Field MC (2011) Rab23 is a flagellar protein in Trypanosoma brucei. BMC Res Notes 4:190
Sato T et al (2014) Rab8a and Rab8b are essential for several apical transport pathways but insufficient for ciliogenesis. J Cell Sci 127(Pt 2):422–431
Eggenschwiler JT et al (2001) Rab23 is an essential negative regulator of the mouse Sonic hedgehog signalling pathway. Nature 412(6843):194–198
Bangs F, Anderson KV (2017) Primary cilia and mammalian hedgehog signaling. Cold Spring Harb Perspect Biol 9(5):a028175
Ungricht R, Kutay U (2015) Establishment of NE asymmetry-targeting of membrane proteins to the inner nuclear membrane. Curr Opin Cell Biol 34:135–141
Burns LT, Wente SR (2012) Trafficking to uncharted territory of the nuclear envelope. Curr Opin Cell Biol 24(3):341–349
Acknowledgements
We apologize to all authors, whose work could not be cited due to space limitations. This work is supported by the following Ministry of Education (Singapore) Grants to L.L.: AcRF Tier 2 MOE2015-T2-2-073 and AcRF Tier1 RG132/15 and AcRF Tier1 RG48/13.
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Lu, L., Madugula, V. Mechanisms of ciliary targeting: entering importins and Rabs. Cell. Mol. Life Sci. 75, 597–606 (2018). https://doi.org/10.1007/s00018-017-2629-3
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DOI: https://doi.org/10.1007/s00018-017-2629-3