In the pathways for taste, smell and common chemical sense, there is a transduction of an external chemical signal into an intracellular signal in the receptor cell, usually a neuron or modified neuron. Chemical cues bind to the surface of the receptor cell and this interaction somehow results in increased neuronal activity or release of neurotransmitter. The transduction process between binding and electrical change is likely to involve internal second messengers, such as cyclic nucleotides, calcium or lipid metabolites. Indeed, there are human disease states in which the transduction processes are defective at the level of G proteins, those proteins that shuttle information between some receptors and the enzymes or ion channels that produce the second messengers. Paramecia are single celled animals that have been likened to swimming neurons and they serve as examples of chemoreceptor cells. The groundwork is laid to use Paramecium to explore the structure, function and distribution of its chemoreceptors and to explore the roles of internal calcium levels and the calcium extrusion pump that is implicated in producing the hyperpolarizing membrane conductance of Paramecium chemoreception. Molecular genetics will be used to clone and sequence the cAMP chemoreceptor gene. Antibodies against peptide sequences predicted from the gene sequence will be used to probe receptor structure and distribution and define functional domains, i.e. parts of the receptor that bind stimulus, span the membrane, interact intracellularly in signal transduction. To explore calcium as an intracellular signal in chemoreception, calcium levels will be measured with fluorescent dyes; the calcium ATPase pump will be characterized for enzyme activity in normal and mutant cells and will be partially purified in preparation for cloning. These studies are significant because very few chemoreceptors have been characterized and no common pattern has emerged from these few. Additionally, he calcium pump will be the first characterized protein moiety responsible for conductance changes in chemoreception and the first characterized Ca ATPase that qualifies as a calcium homeostasis pump in Paramecium.
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