The bulk of UCP3 expressed in yeast cells is incompetent for a nucleotide regulated H+ transport.

The impact of uncoupling protein (UCP) 1, UCP3 and UCP3s expressed in yeast on oxidative phosphorylation, membrane potential and H+ transport is determined. Intracellular ATP synthesis is inhibited by UCP3, much more than by UCP1, while similar levels of UCP3 and UCP1 exist in the mitochondrial fractions. Measurements of membrane potential and H+ efflux in isolated mitochondria show that, different from UCP1, with UCP3 and UCP3s there is a priori a preponderant uncoupling not inhibited by GDP. The results are interpreted to show that UCP3 and UCP3s in yeast mitochondria are in a deranged state causing uncontrolled uncoupling, which does not represent their physiological function.