Details of the T3 receptor beta complex in comparison with determinants of the MCT8 transport channel. A) The crystal structure of the T3 receptor beta ligand binding domain (PDB code 3GWS  backbone-ribbon) reveals detailed insights into the binding mode of this hormone. Surrounded by hydrophobic (green) and aromatic (cyan) amino acids, T3 (magenta) binding is characterized by H-bonds to an arginine and a histidine. According to the assumption of analogy for T3 binding (T3 receptor, A) to different proteins, such specific complementary residues might be generally mandatory for hormone binding/transport. B) In the MCT8 homology model two histidines (H192, H415) and three positively charged residues (R445, K418, R301) are predicted to point inside the putative substrate transport channel (sticks with labels). Further amino acids in the transport channel are also highlighted by sticks: orange – hydrophilic residues without charges, magenta – negatively charged residues.