Photoaffinity labeling of the nucleotide‐binding site of the uncoupling protein from hamster brown adipose tissue

Abstract

The nucleotide binding center of the uncoupling protein from brown adipose tissue (UCP) was probed by photoaffinity labeling with 8‐azido‐ATP. The isolated dimeric UCP in non‐ionic detergent was used. 8‐azido‐ATP binds to UCP with a K_d= 3 μM, i.e. with an only threefold lower affinity than ATP and a maximum number of binding sites of about 12 μmol/g protein corresponding to about 1 mol/mol dimer UCP. UCP is rapidly degraded by ultraviolet radiation, and therefore only near ultraviolet and visible light can be used for photoaffinity labeling. The total covalent incorporation is shown to be dependent on the concentration of azido‐ATP and on competing phospholipids. The specific, i.e. ATP‐sensitive incorporation only to the binding site depends on the presence of cysteine. With CNBr cleavage the 8‐azido‐[γ‐³²P]ATP insertion within the primary structure was located by identifying ATP‐sensitive labeled peptides in SDS/PAGE. A major specific 8‐azido‐ATP incorporation was found by autoradiography in the smallest CNBr fragments. Identification of the radioactive peptides was difficult since 8‐azido‐ATP insertion causes a distinct shift in the gels from the stained peptides. Identification was possible by specific disulfide formation at the C‐terminal within the UCP dimer which only removed the CB7 (CB, CNBr fragment) portion of the low‐molecular‐mass peptides but did not move the radioactive band. This excludes the C‐terminal CB7 and identifies the labeled peptide as CB6. Also, limited tryptic cleavage of intact UCP at Lys293 did not remove the radioactivity. Cleavage of tryptophanes support localization of 8‐azido‐ATP between residues 173–280 which includes CB6. Solid‐phase sequencing of the labeled CB6 both after serine lactone and carboxyl coupling suggest incorporation into Thr260. These results indicate that the adenine‐binding site is within the third domain of the tripartite UCP structure at a putative hydrophilic channel which can be assessed both from the cytosol and matrix of mitochondria.