The G protein alpha Chaperone and Guanine-Nucleotide Exchange Factor RIC-8 Regulates Cilia Morphogenesis in Caenorhabditis elegans Sensory Neurons

Heterotrimeric G ({beta}{gamma}) proteins are canonical transducers of G-protein-coupled receptor (GPCR) signaling and play critical roles in communication between cells and their environment. Many GPCRs and heterotrimeric G proteins localize to primary cilia and modulate cilia morphology via mechanisms that are not well understood. Here, we show that RIC-8, a cytosolic guanine nucleotide exchange factor (GEF) and chaperone for G protein subunits, shapes cilia membrane morphology in a subset of Caenorhabditis elegans sensory neurons. Consistent with its role in ciliogenesis, C. elegans RIC-8 localizes to cilia in different sensory neuron types. Using domain mutagenesis, we demonstrate that while the GEF function alone is not sufficient, both the GEF and G-interacting chaperone motifs of RIC-8 are required for its role in cilia morphogenesis. We identify ODR-3 as the RIC-8 G client and demonstrate that RIC-8 functions in the same genetic pathway with another component of the non-canonical G protein signaling AGS-3 to shape cilia morphology. Notably, despite severe defects in AWC cilia morphology, ags-3 null mutants exhibit normal chemotaxis toward benzaldehyde unlike odr-3 mutant animals. Collectively, our findings describe a novel function for the evolutionarily conserved protein RIC-8 and non-canonical RIC-8-AGS-3-ODR-3 signaling in cilia morphogenesis and uncouple G ODR-3 functions in ciliogenesis and olfaction. Author SummaryPrimary cilia are specialized cellular compartments that mediate communication between cells and their environment. In humans, deficits in cilia assembly and function lead to genetic diseases called ciliopathies. While the molecular machinery that sends out and receives major signaling cues is present inside cilia of different cell types, the mechanisms by which these signaling factors modulate cilia structure or cilia-dependent cellular functions are not well understood. In this study, we identify a new role for the RIC-8-AGS-3-ODR-3 signaling module in formation of specialized ciliary structures in C. elegans neurons. Our findings provide new insight into mechanisms of cilia assembly and highlight how the same signaling component (i.e. ODR-3) can be differentially regulated to mediate distinct cellular functions (cilia formation versus olfactory behavior).