Structural and functional study of a conserved region in the uncoupling protein UCP1: the three matrix loops are involved in the control of transport.

It has been reported that the region 261-269 of the uncoupling protein from brown adipose tissue mitochondria, UCP1, has an important role in the control of its proton translocating activity. Thus the deletion of residues Phe267-Lys268-Gly269 leads to the loss of the nucleotide regulation of the protein, while the complete deletion of the segment leads to the formation of a pore. The region displays sequence homology with the DNA-binding domain of the estrogen receptor. The present report analyzes the structure, by NMR and circular dichroism, of a 20 amino acid residue peptide containing the residues of interest. We demonstrate that residues 263-268 adopt an alpha-helical structure. The helix is at the N-terminal end of the sixth transmembrane domain. The functional significance of this helix has been examined by site-directed mutagenesis of the protein expressed recombinantly in yeasts. Alterations in the structure or orientation of the region leads to an impairment of the regulation, by nucleotides and fatty acids, of the transport activity. UCP1 is one member of the family formed by the carriers of the mitochondrial inner membrane. The family is characterized by a tripartite structure with three repeated segments of about 100 amino acid residues. Two of the mutations have also been performed in the first and second matrix loops and the effect on UCP1 function is very similar. We conclude that the three matrix loops contribute to the formation of the gating domain in UCP1 and propose that they form a hydrophobic pocket that accommodates the purine moiety of the bound nucleotide. Copyright 1999 Academic Press.