LOCALIZATION OF THE DISULFIDE BONDS IN THE NH2-TERMINAL DOMAIN OF THECELLULAR RECEPTOR FOR HUMAN UROKINASE-TYPE PLASMINOGEN-ACTIVATOR - A DOMAIN-STRUCTURE BELONGING TO A NOVEL SUPERFAMILY OF GLYCOLIPID-ANCHORED MEMBRANE-PROTEINS

The receptor for human urokinase-type plasminogen activator (uPAR) is synthesized as a 313-residue-long polypeptide containing 28 cysteine r esidues, the pattern of which defines three homologous repeats within the protein. These entities are believed to represent a novel protein domain structure, of which the NH2-terminal domain of uPAR can be cova lently cross-linked to the epidermal growth factor-like module of urok inase after receptor-ligand interaction. The NH2-terminal domain of a recombinant, soluble uPAR derivative, labeled with S-35!cysteine, was isolated after limited proteolysis with chymotrypsin. The four disulf ide bonds present within this domain were assigned by a combination of plasma desorption mass spectrometry, amino acid composition, and sequ ence analyses of peptides generated by trypsin, endoproteinase Asp-N, and thermolysin. The following disulfide bond structure was determined : Cys3-Cys24, Cys6-Cys12, Cys17-Cys45, and Cys71-Cys76. Similar cystei ne pairing is likely to be found within other members of this protein superfamily, i.e. the membrane inhibitor of reactive lysis, Ly-6, and the remaining two domains of uPAR. However, an additional pair of cyst eines present within these domains probably forms a fifth disulfide bo nd.