Structural Basis for Membrane Targeting of the BBSome by ARL6

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 C. 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 providing a molecular rationale for patient pathologies.