RAB18 Orchestrates Signaling Protein Ciliary Homeostasis by Facilitating BBSome Diffusion through the Transition Zone for Ciliary Entry
The BBSome acts as an intraflagellar transport (IFT) cargo adaptor that connects transmembrane and membrane-anchored signaling proteins to the process of IFT within cilia. This function is critical for ciliary homeostasis maintenance of signaling proteins, allowing for successfully sensing and transducing extracellular stimuli inside the cell. It is established that, upon reaching the proximal ciliary region just above the transition zone (TZ), a diffusion barrier, via retrograde IFT, the BBSome, carrying signaling proteins, disengages from IFT and diffuses through the TZ for ciliary retrieval. This TZ passage event is promoted by the Arf-like 3 GTPase, which utilizes the BBSome autonomously of IFT association as its major effector. While it is known that the intact BBSome is recruited to the basal bodies before entering cilia, the precise mechanism by which it crosses the TZ to access cilia remains elusive. In our study on Chlamydomonas reinhardtii, we disclose that the Ras-associated binding (Rab) GTPase RAB18 is concentrated in a basal body region that overlaps with the BBSome. Positioned at this location, RAB18 in a GTP-bound state (RAB18-GTP) is anchored to the membrane, allowing it to diffuse into cilia. Throughout this process, the BBSome functions as a RAB18 effector, binding to RAB18-GTP and subsequently being recruited to diffuse through the TZ for ciliary entry in an IFT-independent manner. This collaboration between the BBSome and RAB18 is pivotal in maintaining the signaling protein phospholipase D ciliary homeostasis, providing a precise mechanistic understanding of the inward BBSome TZ passage essential for proper ciliary signaling. Significance statementAlthough certain ciliary signaling proteins cross the transition zone (TZ) for ciliary removal via the RABL2-ARL3 module-mediated outward BBSome TZ diffusion pathway, how the BBSome passes the TZ for ciliary entry remains unclear. Here, we show that RAB18, in its GTP-bound state, anchors to the membrane at the basal bodies. As a RAB18 effector, the BBSome is recruited to cross the TZ for ciliary entry via lateral transport between plasma and ciliary membranes. Once inside cilia, the BBSome integrates into anterograde IFT trains and subsequently functions as a phospholipase D (PLD) adaptor, ensuring PLD removal from cilia. This study highlights RAB18s pivotal role in facilitating BBSome entry into cilia, unveiling a regulatory mechanism crucial for maintaining ciliary signaling protein homeostasis.