Tyrosine 112 is essential for organic cation transport by the plasma membrane monoamine transporter

Plasma membrane monoamine transporter (PMAT) is a polyspecific organic cation transporter in the solute carrier 29 (SLC29) family. Previous studies suggested that the major substrate recognition sites are located within transmembrane domains (TM) 1-6, and interaction of PMAT with organic cations may involve aromatic residues. In this study, we analyzed the roles of tyrosine and tryptophan residues located within TM1-6 with a goal of identifying potential residues involved in substrate recognition and translocation. The six tyrosines and one tryptophan in this region were each mutated to alanine followed by analysis of the mutant's membrane localization and transport activity toward 1-methyl-4-phenylpyridinium (MPP(+)), serotonin (5-HT), and dopamine. Two mutants, Y85A and Y112A, exhibited normal cell surface expressions but lost their transport activities toward organic cations. At position Y85, aromatic substitution with phenylalanine or tryptophan fully restored organic cation transport activity. Interestingly, at position Y112, phenylalanine substitution was not allowed. Tryptophan substitution at Y112 partially restored transport activity toward 5-HT and dopamine but severely impaired MPP(+) transport. Detailed kinetic analyses revealed that tryptophan substitution at Y85 and Y112 affected the apparent binding affinity (K(m)) and maximal transport velocity (V(max)) in a substrate-dependent manner. Together, our data suggest that Y85 and Y112 are important molecular determinants for PMAT function, and Y112 is indispensable for optimal interaction with organic cation substrates. Our analyses also suggest the involvement of transmembrane domains 1 and 2 in forming the substrate permeation pathway of PMAT.