Substrate specificity and transport mode of the proton-dependent amino acid transporter mPAT2

The PAT2 transporter has been shown to act as an electrogenic proton/amino acid symporter. The PAT2 cDNA has been cloned from various human, mouse and rat tissues and belongs to a group of four genes (pat1 to pat4) with PAT3 and PAT4 still resembling orphan transporters. The first immunolocalization studies demonstrated that the PAT2 protein is found in the murine central nervous system in neuronal cells with a proposed role in the intra and/or intercellular amino acid transport. Here we provide a detailed analysis of the transport mode and substrate specificity of the murine PAT2 transporter after expression in Xenopus laevis oocytes, by electrophysiological techniques and flux studies. The structural requirements to the PAT2 substrates - when considering both low and high affinity type substrates - are similar to those reported for the PAT1 protein with the essential features of a free carboxy group and a small side chain. For high affinity binding, however, PAT2 requires the amino group to be located in an alpha-position, tolerates only one methyl function attached to the amino group and is highly selective for the L-enantiomers. Electrophysiological analysis revealed pronounced effects of membrane potential on proton binding affinity, but substrate affinities and maximal transport currents only modestly respond to changes in membrane voltage. Whereas substrate affinity is dependent on extracellular pH, proton binding affinity to PAT2 is substrate-independent, favouring a sequential binding of proton followed by substrate. Maximal transport currents are substrate-dependent which suggests that the translocation of the loaded carrier to the internal side is the rate-limiting step.