Comparison of the caprine (Ca-), bovine (Bo-), human (Hu-), murine (Mu-) and rat (Ra-) α subunits amino acid sequences. The letters in the top row identify the constitutive blocks: putative signal peptide (s), extracellular domain (e), transmembrane region (t), cytoplasmic tail (c), I-domain (i), MIDAS motif (m) and divalent cation binding motifs (d). Black, dark grey and light grey columns represent identity among 6, 5 and 4 species, respectively. Cysteine residues (#) and potential N-glycosylation sites (*) are marked at the bottom of the sequences. The important Glu-332 residue (+) and the Gln-743 residue which is absent in the smaller allele (=) are identified.