Adaptor protein complex 1 facilitates ciliary localization of serotonin receptor type 6

The primary cilium, an immotile protrusion of vertebrate cells, detects chemical and mechanical stimuli in the extracellular milieu and transduces them into the cell body, thereby contributing to cellular development and homeostasis. In the mammalian brain, serotonin receptor type 6 (Htr6) and other specific G protein-coupled receptors (GPCRs) localize preferentially to primary cilia and function in ciliary chemical detection; however, the molecular mechanism by which a subset of GPCRs is transported to primary cilia has not been fully elucidated. In the present study, we demonstrate that a region in the fourth intracellular domain of Htr6 (Htr6 i4) is sufficient for ciliary localization. In yeast, the interaction of this region with the C-terminal region of γ1-Adaptin, a subunit of adaptor protein complex 1 (AP-1), was identified. The interaction between Htr6 and γ1-Adaptin was confirmed by immunoprecipitation analysis using HEK293T cells. The preference for ciliary localization of Htr6 and Htr6 i4 was significantly decreased by ablation of γ1-Adaptin in hTERT RPE-1 cells, which was rescued by exogenous expression of γ1-Adaptin. Furthermore, Htr6 and Htr6 i4 showed reduced localization to primary cilia in γ1-Adaptin-depleted cultured hippocampal neurons compared with control neurons. These results indicate that the ciliary localization of Htr6 is facilitated by AP-1-mediated membrane trafficking.