Reference: Bioi Bull 183: 147-154. (August, 1992) Cilia from Abalone Larvae Contain a Receptor-Dependent G Protein Transduction System Similar to that in Mammals GREGORY T. BAXTER AND DANIEL E. MORSE* Department of Biological Sciences and the Marine Biotechnology Center, University of California. Santa Barbara. California 93106 Abstract. Lysine and related diamino acids amplify (fa-cilitate) the response to inducers of metamorphosis in lar-vae of the marine mollusk Ha/iotis 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 ex-hibit sodium-independent binding of the facilitating di-amino 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 recip-rocally regulate one another, suggesting that the chemo-sensor may be a member of the rhodopsin-like, G protein-coupled transmembrane receptor superfamily. Previous analyses of mRNAs from the larval cilia revealed a se-quence coding for a G protein with high homology to G q from mammalian brain, and another sequence coding for a protein homologous to G,/G . Similarities between this system, other chemosensory signal transduction pathways, and mechanisms of neuronal long-term potentiation are Received 30 March 1992; accepted 18 May 1992. Abbreviations: G protein, guanine nucleotide binding protein; GABA. y-aminobutyric acid; GppNHp, guanosine 5' (0,7-imido) triphosphate; PKC. protein kinase C; ASW. artificial seawater. SDS, sodium dodecyl sulfate; PAGE, polyacrylamide gel electrophoresis; DAPA, L-a./j-di-aminopropionic acid; GDP-/3-S, guanosine 5'-0-[/5-thio] diphosphate; PLC. phospholipase C; Tris. tris-hydroxymethylaminomethane. * Author to whom all correspondence should be addressed. evident. Because the receptors and transducers controlling settlement and metamorphosis in Haliotis and other ma-rine invertebrate larvae appear homologous to compo-nents controlling neuronal activity, cellular proliferation, and differentiation in mammals, characterization of the molecules controlling metamorphosis may help in the de-sign of new regulators useful in medicine. Introduction The molecular components and mechanisms control-ling the settlement and metamorphosis of marine inver-tebrate larvae in response to chemical signals from the environment are strikingly similar, in some cases, to those that mediate responses to hormones, transmitters, and other signals regulating neuronal activity, behavior and cellular differentiation in mammals. These processes in the larvae, like their counterparts in mammals, also exhibit far greater complexity than first realized. We present here recent findings on one such system from a molluscan larva, and discuss these data in the context of recently discovered sequence homologies and other evidence for relatedness to mammalian receptor-transducer pathways in molluscan and coral larval systems. Larvae of the marine gastropod mollusk Haliotis ru-fescens (red abalone) are induced to settle from the plank-ton and metamorphose in response to exogenous 7-ami-nobutyric acid (GABA)'-mimetic peptides found on the surfaces of specific algae; GABA and GABA analogs also induce this metamorphosis (Morse el al. 1979, 1980; Morse and Morse, 1984; Morse, 1985, 1990). These mor-phogenic inducers are recognized by externally accessible chemosensory receptors that have been characterized by radioligand binding and competition studies (Trapido-Rosenthal and Morse, 1986a). The induction of meta-147
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