UCP1, UCP2 and UCP3: are they true uncouplers of respiration?

The thermogenesis in brown adipose tissue (BAT) is due to the activity of a mitochondrial uncoupling protein (UCP1). This protein allows the protons pumped by the respiratory chain to re-enter the matrix without ATP synthesis. Therefore respiration is dramatically increased and produces only heat. The discovery of genes showing strong similarities with the UCP1 gene and expressed in other tissues raised the possibility that these proteins participate in the proton leak observed in mitochondria, and therefore participate in the regulation of energy expenditure. The recombinant expression of UCP1, UCP2 and UCP3 in yeast allows the comparison of the coupling state of yeast mitochondria in the presence or absence of these proteins.