title: Regulated membrane protein entry into flagella is facilitated by cytoplasmic microtubules and does not require IFT.
abstract: The membrane protein composition of the primary cilium, a key sensory organelle, is dynamically regulated during cilium-generated signaling [1, 2]. During ciliogenesis, ciliary membrane proteins, along with structural and signaling proteins, are carried through the multicomponent, intensely studied ciliary diffusion barrier at the base of the organelle [3-8] by intraflagellar transport (IFT) [9-18]. A favored model is that signaling-triggered accumulation of previously excluded membrane proteins in fully formed cilia [19-21] also requires IFT, but direct evidence is lacking. Here, in studies of regulated entry of a membrane protein into the flagellum of Chlamydomonas, we show that cells use an IFT-independent mechanism to breach the diffusion barrier at the flagellar base. In resting cells, a flagellar signaling component [22], the integral membrane polypeptide SAG1-C65, is uniformly distributed over the plasma membrane and excluded from the flagellar membrane. Flagellar adhesion-induced signaling triggers rapid, striking redistribution of the protein to the apical ends of the cells concomitantly with entry into the flagella. Protein polarization and flagellar enrichment are facilitated by cytoplasmic microtubules. Using a conditional anterograde IFT mutant, we demonstrate that the IFT machinery is not required for regulated SAG1-C65 entry into flagella. Thus, integral membrane proteins can negotiate passage through the ciliary diffusion barrier without the need for a motor.
authors.affiliation: Department of Cell Biology, University of Texas Southwestern Medical School, 6000 Harry Hines Boulevard, Dallas, TX 75390-9039, USA.
authors.forename: Olivier
authors.initials: O
authors.lastname: Belzile
authors.forename: Carmen I
authors.initials: CI
authors.lastname: Hernandez-Lara
authors.forename: Qian
authors.initials: Q
authors.lastname: Wang
authors.forename: William J
authors.initials: WJ
authors.lastname: Snell
mesh_terms: D002462:Cell Membrane / Q000378:metabolism*; D016825:Chlamydomonas reinhardtii / Q000235:genetics / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism; D005407:Flagella / Q000378:metabolism*; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D008870:Microtubules / Q000378:metabolism*; D009154:Mutation; D010446:Peptide Fragments / Q000235:genetics / Q000378:metabolism; D021381:Protein Transport; D015398:Signal Transduction
chemical_list: D008565:Membrane Proteins; D010446:Peptide Fragments
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