The uncoupling protein dimer can form a disulfide cross-link between the mobile C-terminal SH groups.

Isolated uncoupling protein (UCP) can be cross-linked, by various disulfide-forming reagents, to dimers. The best cross-linking is achieved with Cu2+-phenanthroline oxidation. Because cross-linking is independent of UCP concentration and prevented by SDS addition, a disulfide bridge must be formed between the two subunits of the native dimer. Cross-linking is prevented by SH reagent and reversed by SH-reducing reagents. In mitochondria, cross-linking of UCP with disulfide-forming agents is even more efficient than in isolated state. It proves that UCP is a dimer in mitochondria, before isolation. Disulfide-bridge formation does not inhibit GTP-binding to UCP. Cross-linked UCP re-incorporated in proteoliposomes either before or after cross-linking fully retains the H1-transport function. Rapid cross-linking by membrane impermeant reagents indicates a surface localization of the C-terminus in soluble UCP and projection to the outer surface in mitochondria. Intermolecular disulfide-bridge formation in a dimer requires juxtaposition of identical cysteines at the twofold symmetry axis. A rigid juxtaposition of cysteines is unlikely, unless intended for a native disulfide bridge. The absence of such a bridge in UCP suggests that juxtaposition of cysteines is generated by high mobility. In order to localize the cysteine involved, cross-linked UCP was cleaved by BrCN. The CB-7 C-terminal peptide, which contains cysteines at positions 287 and 304, disappears. Limited trypsinolytic cleavage, previously shown to occur at Lys-292, removed cross-linking in UCP both in the solubilized and mitochondrially bound state. The cleaved C-terminal peptide of 11 residues contains only cystein-304 which, thus, should be the only one (out of 7 cysteines in UCP) involved in the S-S bridge formation. Obviously, the C-terminal location of the cysteine, because of its high mobility, permits juxtapositioning for cross-linking. This agrees with predictions from hydrophobicity analysis that the last 14 residues in UCP protrude from the membrane.