Unraveling the Intricate Cargo-BBSome Coupling Mechanism at the Ciliary Tip

Abstract

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 Chlamydomonas ARL13 only in a GTP-bound form (ARL13GTP) anchors to the membrane for diffusing into cilia. Upon entering cilia, ARL13 undergoes GTPase cycle for shuttling between the ciliary membrane (ARL13GTP) and matrix (ARL13GDP). To achieve this goal, the ciliary membrane-anchored BBS3GTP binds and activates the ciliary matrix-residing ARL13GDP as an ARL13 guanine nucleotide exchange factor. At the ciliary tip, ARL13GTP binds and recruits the ciliary matrix-residing and post-remodeled BBSome as an ARL13 effector to anchor to the ciliary membrane. This makes the BBSome spatiotemporally become available for the ciliary membrane-tethered phospholipase D (PLD) to couple with. Afterward, ARL13GTP hydrolyzes GTP for releasing the PLD-laden BBSome to load onto retrograde IFT trains. According to this model, hedgehog signaling defects associated with ARL13b and BBS3 mutations in humans could be satisfactorily explained, providing us a mechanistic understanding behind BBSome-cargo coupling required for proper ciliary signaling. Significance statementCertain signaling proteins export in a BBSome-dependent manner from cilia via retrograde IFT. As IFT cargoes, their coupling with the BBSome is mediated by ARL13 and BBS3. Here, we report that ARL13 in a GTP-bound state binds the membrane for diffusing into cilia followed by running GTPase cycle with BBS3 acting as an ARL13-specific guanine nucleotide exchange factor. ARL13GTP recruits the IFT-shed BBSome as its effector to anchor to the membrane of the ciliary tip, making it spatiotemporally available for the ciliary membrane-tethered PLD to bind. Afterward, ARL13 GTP hydrolysis releases the PLD-laden BBSome for retrograde IFT train loading. ARL13 and BBS3 facilitate the cargo-BBSome coupling in such a way providing a regulatory mechanism for signaling protein export from cilia.