Spatial regulation of ARL3 and GSF-mediated lipidated cargo trafficking in the primary cilium.

Lipid-modified proteins are essential for ciliary signaling and structure, but their hydrophobic modifications hinder cytosolic transport and selective delivery. GDI-like solubilizing factors (GSFs), such as PDE6D and UNC119A/B, bind lipid moieties to shield cargo proteins and enable diffusion. However, the mechanisms that govern spatially restricted cargo release-particularly at the primary cilium-are not fully elucidated yet. In this Review, we highlight the central role of the small G protein ARL3 and its regulators in mediating selective release of lipidated cargoes. We discuss ARL13B, a ciliary-localized guanine nucleotide exchange factor (GEF) for ARL3, and BART, a co-GEF that enhances ARL3 activation by relieving autoinhibition. In contrast, RP2, a GTPase-activating protein (GAP) at the ciliary base, likely inactivates ARL3 outside the cilium, establishing a spatial ARL3·GTP gradient that restricts cargo release. Additional specificity arises from ARL2 exclusion from the cilium, differential GSF-cargo binding affinities, and putative docking platforms such as RPGR. Disruption of this pathway is implicated in ciliopathies, including Joubert syndrome. Current models and recent findings provide a framework for understanding spatial GTPase signaling in ciliary transport.