Human mitochondrial uncoupling protein 3 functions as a metabolite transporter

Since its discovery, a major debate about mitochondrial uncoupling protein 3 (UCP3) has been whether its metabolic actions result primarily from mitochondrial inner membrane proton transport, a process that decreases respiratory efficiency and ATP synthesis. However, UCP3 expression and activity are induced by conditions that would seem at odds with inefficient 'uncoupled' respiration, including fasting and exercise. Here, we demonstrate that the bacterially expressed human UCP3, reconstituted into liposomes, catalyses a strict exchange of aspartate, malate, sulphate and phosphate. The R282Q mutation abolishes the transport activity of the protein. Although the substrate specificity and inhibitor sensitivity of UCP3 display similarity with that of its close homolog UCP2, the two proteins significantly differ in their transport mode and kinetic constants. We show that recombinant human uncoupling protein 3 (UCP3), refolded and reconstituted into liposomes, efficiently transports aspartate and malate, and, to a lesser extent, phosphate and sulphate. Unlike UCP2, UCP3 cannot catalyse unidirectional substrate transport and exhibits an approximately sevenfold higher transport affinity for aspartate compared with UCP2. The R282Q mutation in the sixth alpha-helix completely abolishes the transport activity of UCP3.image