Rabs and other small GTPases in ciliary transport

The base of the primary cilium contains a zone of condensed membranes whose importance is not known. Here, we have studied the involvement of MAL, a tetraspanning protein that exclusively partitions into condensed membrane fractions, in the condensation of membranes at the ciliary base and investigated the importance of these membranes in primary cilium formation. We show that MAL accumulates at the ciliary base of epithelial MDCK cells. Knockdown of MAL expression resulted in a drastic reduction in the condensation of membranes at the ciliary base, the percentage of ciliated cells and the length of the cilia, but did not affect the docking of the centrosome to the plasma membrane or produce missorting of proteins to the pericentriolar zone or to the membrane of the remaining cilia. Rab8 (for which there are two isoforms, Rab8A and Rab8b), IFT88 and IFT20, which are important components of the machinery of ciliary growth, were recruited normally to the ciliary base of MAL-knockdown cells but were unable to elongate the primary cilium correctly. MAL, therefore, is crucial for the proper condensation of membranes at the ciliary base, which is required for efficient primary cilium extension.

Section: Discussionmentioning

confidence: 99%

The MAL protein is crucial for proper membrane condensation at the ciliary base, which is required for primary cilium elongation

Reales

Bernabé‐Rubio

Casares-Arias

et al. 2015

“…pathways (Emmer et al, 2010;Lim et al, 2011;Nachury et al, 2010;Pazour and Bloodgood, 2008). In particular, we focused on the possible mechanisms restricting localization of TAX-2, but not TAX-4, to the middle segments of AWB cilia.…”

Section: Journal Of Cell Sciencementioning

confidence: 99%

Cell- and subunit-specific mechanisms of CNG channel ciliary trafficking and localization inC. elegans

Wojtyniak

Brear

O'Halloran

et al. 2013

SummaryPrimary cilia are ubiquitous sensory organelles that concentrate transmembrane signaling proteins essential for sensing environmental cues. Mislocalization of crucial ciliary signaling proteins, such as the tetrameric cyclic nucleotide-gated (CNG) channels, can lead to cellular dysfunction and disease. Although several cis-and trans-acting factors required for ciliary protein trafficking and localization have been identified, whether these mechanisms act in a protein-and cell-specific manner is largely unknown. Here, we show that CNG channel subunits can be localized to discrete ciliary compartments in individual sensory neurons in C. elegans, suggesting that channel composition is heterogeneous across the cilium. We demonstrate that ciliary localization of CNG channel subunits is interdependent on different channel subunits in specific cells, and identify sequences required for efficient ciliary targeting and localization of the TAX-2 CNGB and TAX-4 CNGA subunits. Using a candidate gene approach, we show that Inversin, transition zone proteins, intraflagellar transport motors and a MYND-domain protein are required to traffic and/or localize CNG channel subunits in both a cell-and channel subunit-specific manner. We further find that TAX-2 and TAX-4 are relatively immobile in specific sensory cilia subcompartments, suggesting that these proteins undergo minimal turnover in these domains in mature cilia. Our results uncover unexpected diversity in the mechanisms that traffic and localize CNG channel subunits to cilia both within and across cell types, highlighting the essential contribution of this process to cellular functions.

“…ARF and RAB GTPases also play important roles in protein trafficking to and within cilia (Li and Hu, 2011;Lim et al, 2011), and the specific GTPases involved include ARF4, ARL3, ARL6, ARL13B, RAB8A and RAB8B, RAB10, RAB11, RAB23 and RAB28.…”

Section: Introductionmentioning

confidence: 99%

Regulation of ciliary retrograde protein trafficking by the Joubert syndrome proteins ARL13B and INPP5E

Nozaki

Katoh

Terada

et al. 2017

ARL13B (a small GTPase) and INPP5E (a phosphoinositide 5-phosphatase) are ciliary proteins encoded by causative genes of Joubert syndrome. We here showed, by taking advantage of a visible immunoprecipitation assay, that ARL13B interacts with the IFT46-IFT56 (IFT56 is also known as TTC26) dimer of the intraflagellar transport (IFT)-B complex, which mediates anterograde ciliary protein trafficking. However, the ciliary localization of ARL13B was found to be independent of its interaction with IFT-B, but dependent on the ciliary-targeting sequence RVEP in its C-terminal region. ARL13B-knockout cells had shorter cilia than control cells and exhibited aberrant localization of ciliary proteins, including INPP5E. In particular, in ARL13B-knockout cells, the IFT-A and IFT-B complexes accumulated at ciliary tips, and GPR161 (a negative regulator of Hedgehog signaling) could not exit cilia in response to stimulation with Smoothened agonist. This abnormal phenotype was rescued by the exogenous expression of wild-type ARL13B, as well as by its mutant defective in the interaction with IFT-B, but not by its mutants defective in INPP5E binding or in ciliary localization. Thus, ARL13B regulates IFT-A-mediated retrograde protein trafficking within cilia through its interaction with INPP5E.