ScOPT1 and AtOPT4 function as proton-coupled oligopeptide transporters with broad but distinct substrate specificities

A group of OPTs (oligopeptide transporters) exclusively identified in plants and fungi are proposed to transport oligopeptides and derivatives of three to six amino acids in length, but their transport mechanisms and biological functions are poorly understood. We expressed the Saccharomyces cerevisiae (yeast) OPT ScOPT1 and five Arabidopsis thaliana AtOPTs in Xenopus laevis oocytes for two-electrode voltage-clamp studies. ScOPT1 produced inward currents in response to GSH or GSSG, the phytochelatin (PC) PC2 and oligopeptides including the tetrapeptide GGFL, but not KLGL. Inward currents were dependent on the external proton and substrate concentrations, with high affinity for both. This and the inward currents evoked by substrates with net negative charges showed that ScOPT1 is a proton-coupled transporter. ScOPT1 displayed highest apparent affinity for PC2, with small differences in the maximal current among substrates. Glutathione transport by any of the tested AtOPTs, including AtOPT6, was not detected in yeast growth complementation assays. With AtOPT4, initially only small KLGL-dependent currents were recorded in batches of oocytes showing high ScOPT1 expression. AtOPT4 expression was optimized by swapping the 5'-untranslated region with that of ScOPT1. AtOPT4 displayed a higher affinity for KLGL than ScOPT1 did for any peptide. AtOPT4-mediated KLGL transport was detectable at pH 5.0, but not at pH 6.0 or 7.0. Taken together, our results demonstrate that ScOPT1 and AtOPT4 are proton-coupled OPTs with broad but distinct substrate specificities and affinities.