C. elegans chemotaxes toward many different volatile odorants detected by the AWA and AWC neurons. After prolonged exposure to an attractive odorant, adaptation can occur as revealed by loss of chemotaxis to that compound. We are interested in olfactory adaptation to the odorants (such as isoamyl alcohol, benzaldehyde, and butanone) sensed by the AWC neurons. Previous work has shown that mutations in the gene
osm-9 diminish adapation to a subset of odorants sensed by the AWC neurons but do not affect initial chemotaxis to those odorants (Colbert and Bargmann, Neuron 14, 1-20). However,
osm-9 mutations do abolish chemotaxis toward odorants (such as diacetyl) sensed by the AWA neurons. In addition,
osm-9 mutants are deficient in osmotic avoidance and the nose touch response. The
osm-9 gene has been cloned (Colbert and Bargmann, in preparation). The 938 amino acid protein predicted from the cDNA sequence shows weak homology to the Drosophila transient receptor potential (trp) protein, a calcium channel involved in adaptation in the visual system. Both proteins contain ankyrin repeat domains (which may participate in protein-protein interactions) and six transmembrane segments. Translational fusions of
osm-9 to the GFP coding region revealed expression in the AWA, OLQ and ADL cells. The six alleles of
osm-9 behave identically in behavioural assays, but have different molecular lesions.
osm-9(
ky10) and
osm-9(
n1603) are nonsense mutations that truncate the protein at different points in the N-terminus before the ankyrin repeats and transmembrane domains.
osm-9(
ky161) is a nonsense mutation that truncates the protein in an ankyrin repeat but before the transmembrane domains.
osm-9(
n1601) is an internal deletion that removes a large part of the N-terminus.
osm-9(
n1516) and
osm-9(
n2743) are missense mutations that change different single amino acids in the ankyrin repeats (Colbert and Bargmann, in preparation). Which cells require Osm-9 to perform their normal functions? To answer this question, we are currently tagging the protein with GFP at the C-terminus and plan to make antibodies to a portion of the protein to investigate further the expression pattern and the subcellular localization of Osm-9. Additionally, using an AWA specific promoter, we are trying to determine whether expression of
osm-9 in the AWA neurons is sufficient to rescue the adapation, chemotaxis, or osmotic avoidance defects. What role does Osm-9 play in chemotaxis to the AWA-sensed odorants and adaptation to the AWC-sensed odorants? It is possible that mutations in proteins which interact with Osm-9 will show chemotaxis and/or adaptation defects and will provide a context in which to understand the function of Osm-9. Therefore, to identify possible interacting proteins, the missense mutants with changes in the ankyrin repeats will be used in screens attempting to suppress the strongest phenotype of the
osm-9 mutants, the AWA diacetyl chemotaxis defect. Alternatively, to identify other proteins involved in adaptation, direct screens for mutants deficient in adapting to isoamyl alcohol may be performed. We are also investigating the role of other similar proteins in the nervous system. The C. elegans genome sequencing project has identified two genes with predicted protein products more closely related to the trp protein. We are using expression and reverse genetic methods to investigate the functions of these genes.