Fig. 1. Metazoan monoamine transporters belonging to the SLC6 family of membrane transporters have diagnostic polypeptide motifs. These conserved motifs helped identify three different monoamine transporter types, iDAT, a dopamine transporter restricted to invertebrates, and two widely distributed metazoan transporters, MAT, a catecholamine/phenolamine monoamine transporter, and SERT, a serotonin transporter. Motifs 1, 2 and 3 are present in all three transporter types, whereas motif 4 is a C-terminal sequence restricted to the MAT-type catecholamine/phenolamine transporter. Motif 1 is mainly in transmembrane domain 1 (TMD1), motif 2 in the cytoplasmic loop (CL1) between TMDs 2 and 3 and motif 3 is in transmembrane domain 5 (TMD5). Amino acid residues shaded in grey are consensus residues; the aspartate residue (D) in motif 1 and the tryptophan residue (W) (rarely tyrosine, Y) in motif 3 are not found in related nutrient amino acid transporters (NATs). The amino acids (or site of missing amino acid in MAT) shaded in yellow are largely restricted to one transporter type. Amino acids shaded in blue or green are restricted to subsets within a transporter type. Abbreviations for the other amino acids follow the single-letter code. The positions of the primer sites used in RT-PCR screening for partial transporter sequences are shown above the alignment.