The olfactory system of C. elegans is ideally suited to study specificity of G-protein coupled signaling. Olfaction is mediated by two pairs of neurons, AWA and AWC. These neurons express multiple odorant receptors, enabling C. elegans to detect and discriminate between many odorants. Furthermore, six G subunits are expressed in these neurons: GPA-2, -3, -5, -6, -13 and ODR-3 (Jansen ea 1999 Nat Gen 21,414). To understand how they function, we study the different processes that are required for normal odorant responses. Here we focus on their role in odorant detection and in the establishment of AWC cell fate.We raised antibodies against all six G subunits to determine their site of action. This showed that all, except GPA-6, are localized to the ciliated endings of AWA and/or AWC, suggesting a direct role in the detection of odorants. Some were also localized to the axon. Next, we determined their role in odorant detection by testing chemotaxis of animals with loss-of-function mutations in one or more G subunits. This confirmed that ODR-3 forms the main signaling route and is sufficient to mediate olfactory perception (Roayaie ea 1998 Neuron 20,55). GPA-3 is redundant to ODR-3 and can also be sufficient for normal perception. GPA-5 is a negative regulator and GPA-2 can be both a negative and a positive regulator. GPA-13 influences signaling mostly in the context of GPA-2.The candidate odorant receptor
str-2 is expressed in either the left or the right AWC neuron, indicating a functional difference between the two AWC neurons. The initial decision for this left-right asymmetry was shown to be regulated by axon contact, followed by Ca2+ and MAPK signaling (Troemel ea 1999 Cell 99,387; Sagasti ea 2001 Cell 105,221). The genes involved in these pathways include the cytoskeleton/axon guidance genes
unc-44 and
unc-33, the Ca2+ channel subunit
unc-36, the CaM kinase II
unc-43 and the MAPKKK
nsy-1. We decided to study the role of the G subunits in regulating
str-2 expression. For this purpose,
str-2::GFP expression was determined in different G mutant backgrounds. This showed that loss-of-function of the G subunits does not affect the asymmetry of
str-2 expression, but does affect its initiation and maintenance. We found that
gpa-2,
gpa-5,
gpa-6 and
odr-3 function in a pathway parallel to the Ca2+/MAPK signaling pathway formed by
unc-36,
unc-43 and
nsy-1 to initiate
str-2 expression. Similarly,
gpa-5 acts in parallel to
unc-44, but not
unc-33. In addition, we found that
gpa-6 and
odr-3 may serve to maintain
str-2 expression.