Reconstitution of recombinant uncoupling proteins: UCP1, -2, and -3 have similar affinities for ATP and are unaffected by coenzyme Q10.

The successful development of recombinant expression and reconstitution protocols has enabled a detailed study of the transport properties and regulation of the uncoupling proteins (UCP). We optimized conditions of isolation and refolding of bacterially expressed uncoupling proteins and reexamined the transport properties and regulation of bacterially expressed UCP1, -2, and -3 reconstituted in liposomes. We show for the first time that ATP inhibits UCP1, -2, and -3 with similar affinities. The Ki values for ATP inhibition were 50 microm (UCP1), 70 microm (UCP2), and 120 microm (UCP3) at pH 7.2. These affinities for ATP are similar to those obtained with native UCP1 isolated from brown adipose tissue mitochondria (Ki = 65 microm at pH 7.2). The Vmax values for proton transport were also similar among the UCPs, ranging from 8 to 20 micromol.min(-1).mg(-1), depending on experimental conditions. We also examined the effect of coenzyme Q on fatty acid-catalyzed proton flux in liposomes containing recombinant UCP1, -2, and -3. We found that coenzyme Q had no effect on the fatty acid-dependent proton transport catalyzed by any of the UCPs nor did it affect nucleotide regulation of the UCPs. We conclude that coenzyme Q is not a cofactor of UCP-mediated proton transport.