The localization of signaling molecules such as G protein-coupled receptors (GPCRs) to primary cilia is essential for correct signal transduction. Detailed studies over the past decade have begun to elucidate the diverse sequences and trafficking mechanisms that sort and transport GPCRs to the ciliary compartment. However, a systematic analysis of the pathways required for ciliary targeting of multiple GPCRs in different cell types in vivo has not been reported. Here we describe the sequences and proteins required to localize GPCRs to the cilia of the AWB and ASK sensory neuron types in Caenorhabditis elegans. We find that GPCRs expressed in AWB or ASK utilize conserved and novel sequences for ciliary localization, and that the requirement for a ciliary targeting sequence in a given GPCR is different in different neuron types. Consistent with the presence of multiple ciliary targeting sequences, we identify diverse proteins required for ciliary localization of individual GPCRs in AWB and ASK. In particular, we show that the TUB-1 Tubby protein is required for ciliary localization of a subset of GPCRs, implying that defects in GPCR localization may be causal to the metabolic phenotypes of tub-1 mutants. Together, our results describe a remarkable complexity of mechanisms that act in a protein-and cellspecific manner to localize GPCRs to cilia, and suggest that this diversity allows for precise regulation of GPCR-mediated signaling as a function of external and internal context.
SIGNALING molecules must be precisely localized to specific subcellular domains to optimize detection and transduction of external stimuli. G protein-coupled receptors (GPCRs) comprise a large family of transmembrane signaling proteins that directly bind and transduce a range of cues including photons, odorants, neurotransmitters, and peptides (Pierce et al. 2002;Kato and Touhara 2009;Demaria and Ngai 2010;Sung and Chuang 2010;Chamero et al. 2012;Frooninckx et al. 2012;Montell 2012;Bathgate et al. 2013). Regulation of GPCR function, including regulation of membrane targeting and trafficking to specific subcellular regions, is a major contributor to the tuning of signaling efficacy and fidelity (e.g., Deretic et al. 1995;Ango et al. 2000;Xia et al. 2003;Esseltine et al. 2012;. However, much remains to be understood regarding the mechanisms by which GPCR trafficking and membrane localization are regulated.GPCR-mediated signal transduction in many cellular contexts requires these proteins to be localized to specialized microtubule-based primary cilia. For example, in photoreceptors and olfactory neurons, efficient sensory signal transduction is mediated via localization of rhodopsin and olfactory receptors, together with other signaling molecules, to photoreceptor outer segments and olfactory neuron cilia, respectively (Insinna and Besharse 2008;Berbari et al. 2009;Pifferi et al. 2010;Deretic and Wang 2012). Receptors in the Hedgehog (Hh) morphogen signaling pathway such as the Smoothened and Patched transmembrane proteins, and the G...