TABLE 1.

Conserved transmembrane amino acid residue in Class A GPCRs and bitter taste receptors (T2Rs)

Transmembrane helix	Highly conserved residue in Class A GPCRsa	T2Rsb
		Amino acid residue in T2R1	Humans [25]	Rat [31]	Mouse [29]	Total [188]
			%	%	%	%
I	Asn1.50	Asn-241.50	92	90.3	89.7	89.9 (19)
		Ile-271.53	92	83.9	82.8	85.6 (27)
II	Leu2.50	Leu-512.50	100	96.8	93.1	96.8 (6)
		Arg-552.54	96	96.8	96.6	93.6 (12)
		Asn-662.65	16	12.9	13.7	12.7 (164)
		Glu-742.73	4			2.1 (184)
III	Arg3.50	Asn-893.45	84	80.6	72.4	82.0 (34)
		Trp-943.50	100	96.8	96.6	97.3 (5)
		Leu-993.55	96	96.8	96.6	97.8 (4)
IV	Trp4.50	Leu-1254.50	88	83.9	82.8	88.8 (21)
V	Tyr5.50	Leu-1975.50	100	100	100	98.2 (3)
		Ser-2005.53	100	96.8	100	97.9 (4)
		Leu-2015.54	96	100	100	98.9 (2)
VI	Pro6.50	Phe-2336.50	100	87.1	93.1	93.1 (13)
VII	Pro7.50	His-2737.46	96	90.3	89.7	89.4 (20)
		Leu-2777.50	96	93.5	93.1	93.1 (13)
		Ile-2787.51	92	87.1	82.8	85.6 (27)

^a Numbers in superscript correspond to the Ballesteros and Weinstein (24) nomenclature for GPCRs.

^b The conserved amino acids from the sequence analysis were expressed as a percentage. A total of 188 T2R amino acid sequences were analyzed (shown in the last column). Only Asn-66 and Glu-74 are T2R1-specific residues. Numbers in brackets correspond to the total number of receptors analyzed, and numbers in parentheses in the last column correspond to number of receptors in which the amino acid residues are not conserved (e.g. in the case of Leu-51^2.50 of the 188 T2R amino acid sequences analyzed (brackets), this residue was absent in six sequences (parentheses)). Trp-94^3.50 and Leu-99^3.55 and similarly Leu-197^5.50 and Leu-201^5.54 show very close sequence conservation among the 188 T2Rs analyzed; however, Trp-94^3.50 and Leu-197^5.50 are 100% conserved in all the 25 human T2Rs and hence were designated as residues 0.50 in the respective TM helices.