Chemical modifications were used to identify some of the functionally important amino acid residues of the potato plant uncoupling protein (StUCP). The proton-dependent swelling of potato mitochondria in K(+)-acetate in the presence of linoleic acid and valinomycin was inhibited by mersalyl (K(i) = 5 microM) and other hydrophilic SH reagents such as Thiolyte MB, iodoacetate and 5, 5'-dithio-bis-(2-nitrobenzoate), but not by hydrophobic N-ethylmaleimide. This pattern of inhibition by SH reagents was similar to that of brown adipose tissue uncoupling protein (UCP1). As with UCP1, the arginine reagent 2,3-butadione, but not N-ethylmaleimide or other hydrophobic SH reagents, prevented the inhibition of StUCP-mediated transport by ATP in isolated potato mitochondria or with reconstituted StUCP. The results indicate that the most reactive amino acid residues in UCP1 and StUCP are similar, with the exception of N-ethylmaleimide-reactive cysteines in the purine nucleotide-binding site.