The binding protein of corticotropin-releasing factor: Ligand-binding site and subunit structure

Corticotropin-releasing factor (CRF), recognized as an important stress factor, binds to a CRF receptor and a CRF-binding protein (CRFBP) that represents a reservoir of endogenous CRF. Although CRFBP was observed to dimerize, at least in part, the ligand was found to be exclusively bound to the monomer—as indicated by photoaffinity labeling. We localized the CRF binding site by using photoaffinity labeling in combination with different mass spectrometric techniques. The amino acid residues Arg-23 and Arg-36 of CRFBP were identified as the sites of photoincorporation of monofunctional and bifunctional photoprobes designed on the basis of the amino acid sequence of human/rat CRF(6–33). It was, therefore, concluded that the sequence of amino acid residues 23–36 of CRFBP is involved in ligand binding. Our data are in support of an antiparallel alignment of the photoprobe with the amino acid residues 23–36 of the CRFBP monomer.