Fig. 3. Sequence analysis of XORs. (A) Amino acid substitutions in receptor sequences of class I receptors. Substituted amino acids are highlighted in black. Transmembrane domains are shaded in grey. A high number of amino acid substitutions can be found in the TM4, EC2 and TM5 domains (not shown). (B) Class-specific sequence motifs in ORs from Xenopus laevis. XB107, XB239, XBb238, XB242 represent class I receptors, whereas XB178, XB180, XB177, XB350, XB352 and XB154 are class II receptors. The extracellular domains 2 (EC2) and 3 (EC3), as well as the intracellular domains 2 (IC2) and 3 (IC3) are shown. Class-specific amino acid locations, constituting physico-chemically comparable amino acids, are highlighted in black (class I-specific) and light grey (class II-specific). Connecting transmembrane regions, as well as N- and C-terminal domains, are omitted and symbolized by dashes.