VMAT gene sequence analysis demonstrates that 4 aspartic acid residues in the middle region of TMD I, VI, X, and XI and one Lysine residue in TMDII have highly conserved gene sequences, suggesting these residues play a critical role in transporter structure and function. Specifically, the residues Lys 139 and Asp 427 are thought to compose an ion pair that promotes high-affinity interaction with VMAT substrates and inhibitors. The Asp431 residue located in TMD XI is believed to be critical for amine transport, but does not interact with RES binding; this residue is thought to complete the substrate transport cycle.