Bardet–Biedl syndrome 3 protein promotes ciliary exit of the signaling protein phospholipase D via the BBSome

Certain ciliary signaling proteins couple with the BBSome, a conserved complex of Bardet–Biedl syndrome (BBS) proteins, to load onto retrograde intraflagellar transport (IFT) trains for their removal out of cilia in Chlamydomonas reinhardtii. Here, we show that loss of the Arf-like 6 (ARL6) GTPase BBS3 causes the signaling protein phospholipase D (PLD) to accumulate in cilia. Upon targeting to the basal body, BBSomes enter and cycle through cilia via IFT, while BBS3 in a GTP-bound state separates from BBSomes, associates with the membrane, and translocates from the basal body to cilia by diffusion. Upon arriving at the ciliary tip, GTP-bound BBS3 binds and recruits BBSomes to the ciliary membrane for interacting with PLD, thus making the PLD-laden BBSomes available to load onto retrograde IFT trains for ciliary exit. Therefore, BBS3 promotes PLD exit from cilia via the BBSome, providing a regulatory mechanism for ciliary signaling protein removal out of cilia.