Characteristic features and ligand specificity of the two olfactory receptor classes from Xenopus laevis

Amphibia have two classes of olfactory receptors (ORs), class I (fish-like receptors) and class II (mammalian-like receptors). These two receptor clas ses correspond to the two classes identified in other vertebrates, and amph ibians thus provide a unique opportunity to compare olfactory receptors of both classes in one animal species, without the constraints of evolutionary distance between different vertebrate orders, such as fish and mammals. We therefore identified the complete open reading frames of class I and class II ORs in Xenopus laevis. In addition to allowing a representative compari son of the deduced amino acid sequences between both receptor classes, we w ere also able to perform differential functional analysis. These studies re vealed distinct class-specific motifs, particularly in the extracellular lo ops 2 and 3, which might be of importance for the interaction with odorants , as well as in the intracellular loops 2 and 3, which might be responsible for interactions with specific G-proteins. The results of functional expre ssion studies in Xenopus oocytes, comparing distinct receptor types, suppor t the idea that class I receptors are activated by water-soluble odorants, whereas class II receptors are activated by volatile compounds.