This thesis aims to elucidate the molecular mechanism of indoleamine-induced encystment in dinoflagellates. Indoleamines such as melatonin and 5-methoxytryptamine (5MT) dose- and time-dependently induced encystment in many species of dinoflagellates. Such responses are reversible and liand-selective. Together, partial sequence homologous to the third intracellular loop of a G protein-coupled receptor (GPCR) was cloned, suggesting the presence of GPCRs in dinoflagellates. The involvement of G protein activation in encystment is suggested by the observation that mastoparan induced transient encystment. Immunodetection of Gα-like proteins and cloning of Gα-like partial sequences implicate the presence of novel G proteins in dinoflagellates. Depletion of extracellular Ca²⁺ significantly dim...[ Read more ]

This thesis aims to elucidate the molecular mechanism of indoleamine-induced encystment in dinoflagellates. Indoleamines such as melatonin and 5-methoxytryptamine (5MT) dose- and time-dependently induced encystment in many species of dinoflagellates. Such responses are reversible and liand-selective. Together, partial sequence homologous to the third intracellular loop of a G protein-coupled receptor (GPCR) was cloned, suggesting the presence of GPCRs in dinoflagellates. The involvement of G protein activation in encystment is suggested by the observation that mastoparan induced transient encystment. Immunodetection of Gα-like proteins and cloning of Gα-like partial sequences implicate the presence of novel G proteins in dinoflagellates. Depletion of extracellular Ca²⁺ significantly diminished the indoleamine-induced encystment. Ca²⁺ ionophores and Ca²⁺-ATPase inhibitors can mimick the indoleamine-induced encystment. Inactive indoleamines can antagonise 5MT-induced Ca²⁺ influx. Together, these results suggest that Ca²⁺ acts as a second messenger for encystment. Indoleamines were shown to stimulate a phospholipase C pathway in dinoflagellates, leading to the formation of inositol phosphates (IF). The indoleamine-stimulated IP formation was demonstrated to be associated with encystment by a profile of indoleamines tested. Moreover, CGP 52608-induced encystment suggests that nuclear receptor is also involved in indoleamine-induced encystment. In conclusion, this thesis demonstrated the involvement of receptor and G protein signaling in indoleamine-induced encystment in dinoflagellates.