Table 1.

Chemoreceptors in model organisms operating via a metabotropic or an ionotropic mechanism.

	Metabotropic			Ionotropic
	RC/RK	GPCR	GR/OR	TRP	IR
E. coli	X	−	−	−	X
S. cerevisae	−	X	−	X	−
C. elegans	X	X	X	X	X
D. melanogaster	−	−	X*	X	X
M. musculus	X	X	−	X	−

RC/RK, receptor cyclase/receptor kinase; GPCR, G protein-coupled receptor; GR/OR, gustatory receptor, olfactory receptor; TRP, ion channel family named according to the first member to be discovered (“transient receptor potential” channel in Drosophila photoreceptors); IR, “ionotropic receptor”, a variant ionotropic glutamate receptor protein serving as olfactory receptor.

^*GR/ORs in insects are 7-TM proteins as GPCRs, but are inversely oriented in the membrane. They do not belong to the GPCR superfamily according to a bioinformatics analysis (Benton et al., 2006; Nordström et al., 2011), and form ionotropic receptors. X, receptor type reported for that species.