C. elegans relies on its ability to sense and respond to its environment in order to survive. Chemosensation, the ability to detect chemicals, allows animals to move toward odorants that indicate a food source, and away from odorants that indicate danger. Seven transmembrane G protein-coupled receptors (GPCRs) are known to mediate chemosensation in organisms ranging from C. elegans to human. However, the biochemical pathways that regulate GPCR signaling are still being elucidated. G protein-coupled receptor kinases (GRKs) are a family of serine/threonine protein kinases that phosphorylate activated GPCRs. GRKs play an important role in receptor desensitization in mammals.
Ce-grk-2(W02B3.2) encodes a putative C. elegans homolog that is 65% identical to human GRK2 and 3. We identified a recessive mutant allele of
Ce-grk-2,
rt97, which renders animals unable to detect a variety of attractive and repellent volatile odorants. This suggests that
Ce-grk-2 plays a broad role in C. elegans chemosensation. The
Ce-grk-2 promoter::GFP expression as well as GRK-2 immunoreactivity is found widely in the nervous system. The
Ce-grk-2(
rt97) animals are partially defective in their response to high osmolarity, but they respond normally to touch. The
rt97 mutation corresponds to a T354I change in the kinase domain of Ce-GRK-2. We have found that the T354I mutation decreases GRK protein stability, both in C. elegans and in tissue culture. This mutation is also predicted to reduce kinase activity drastically. We also demonstrate that Ce-GRK-2 functions in the adult sensory neurons to mediate chemosensation. Finally, using cameleon (a genetically encoded, fluorescent calcium sensor) we show that calcium influx into the ASH neurons in response to high osmolarity and soluble repellent chemicals is perturbed in
Ce-grk-2(
rt97) animals. This suggests that
Ce-grk-2(
rt97) mutant animals are defective in detection of odorants, the first step of chemosensation. Analysis of
Ce-grk-2 is an important step in linking our understanding of receptor stimulation and regulation of chemosensory signaling in C. elegans.