Cilia from Abalone Larvae Contain a Receptor-Dependent G Protein Transduction System Similar to that in Mammals

Lysine and related diamino acids amplify (facilitate) the response to inducers of metamorphosis in larvae of the marine mollusk Haliotis rufescens. Previous studies showed that a cholera toxin-sensitive G protein transduces the lysine signal via a diacylglycerol-dependent pathway. We have isolated and partially purified larval cilia that may be involved in recognizing the facilitating chemical signals. These isolated cilia provide an open or porous membrane-associated sensory system that is uniquely tractable for in vitro analyses of chemosensory signal transduction. The cilia contain receptors that exhibit sodium-independent binding of the facilitating diamino acids. The binding strength for lysine and related diamino acids in vitro is correlated with the effectiveness of these ligands as facilitators in vivo. The cilia contain a cholera toxin-sensitive G protein functionally coupled to the lysine receptor. The receptor and the G protein reciprocally regulate one another, suggesting that the chemosensor may be a member of the rhodopsin-like, G proteincoupled transmembrane receptor superfamily. Previous analyses of mRNAs from the larval cilia revealed a sequence coding for a G protein with high homology to Gq from mammalian brain, and another sequence coding for a protein homologous to Gi/Go. Similarities between this system, other chemosensory signal transduction pathways, and mechanisms of neuronal long-term potentiation are evident. Because the receptors and transducers controlling settlement and metamorphosis in Haliotis and other marine invertebrate larvae appear homologous to components controlling neuronal activity, cellular proliferation, and differentiation in mammals, characterization of the molecules controlling metamorphosis may help in the design of new regulators useful in medicine.