guanosine-triphosphate and Bardet-Biedl-Syndrome

Certain ciliary transmembrane and membrane-tethered signaling proteins migrate from the ciliary tip to base via retrograde intraflagellar transport (IFT), essential for maintaining their ciliary dynamics to enable cells to sense and transduce extracellular stimuli inside the cell. During this process, the BBSome functions as an adaptor between retrograde IFT trains and these signaling protein cargoes. The Arf-like 13 (ARL13) small GTPase resembles ARL6/BBS3 in facilitating these signaling cargoes to couple with the BBSome at the ciliary tip prior to loading onto retrograde IFT trains for transporting towards the ciliary base, while the molecular basis for how this intricate coupling event happens remains elusive. Here, we report that

Topics: Bardet-Biedl Syndrome; Cilia; Flagella; Guanosine Triphosphate; Hedgehog Proteins; Humans; Membrane Proteins; Protein Transport

Topics: Bardet-Biedl Syndrome; Cell Cycle Proteins; Cilia; Cytoskeletal Proteins; Flagella; GTP Phosphohydrolases; Guanosine Triphosphate; Humans; Membrane Proteins; Protein Transport

The BBSome is a coat-like ciliary trafficking complex composed of proteins mutated in Bardet-Biedl syndrome (BBS). A critical step in BBSome-mediated sorting is recruitment of the BBSome to membranes by the GTP-bound Arf-like GTPase ARL6. We have determined crystal structures of Chlamydomonas reinhardtii ARL6-GDP, ARL6-GTP and the ARL6-GTP-BBS1 complex. The structures demonstrate how ARL6-GTP binds the BBS1 β-propeller at blades 1 and 7 and explain why GTP- but not GDP-bound ARL6 can recruit the BBSome to membranes. Single point mutations in the ARL6-GTP-BBS1 interface abolish the interaction of ARL6 with the BBSome and prevent the import of BBSomes into cilia. Furthermore, we show that BBS1 with the M390R mutation, responsible for 30% of all reported BBS disease cases, fails to interact with ARL6-GTP, thus providing a molecular rationale for patient pathologies.

Topics: ADP-Ribosylation Factors; Bardet-Biedl Syndrome; Chlamydomonas reinhardtii; Cilia; Crystallography, X-Ray; Guanosine Triphosphate; Humans; Microtubule-Associated Proteins; Models, Molecular; Plant Proteins; Point Mutation; Protein Conformation; Protein Transport; Recombinant Proteins