Norrie Disease Protein (Norrin) Forms Disulfide-linked Oligomers Associated with the Extracellular Matrix

Abstract

COS-7 cells transfected with a DNA construct encoding the 133 amino acids in norrin plus six histidine residues at its carboxyl terminus were pulse-labeled with [35S]cysteine, and the labeled norrin was examined in cell lysates, the medium, and the extracellular matrix. SDS-gel electrophoresis under reducing conditions showed that the norrin expressed had an apparent Mr = 14,000 and was present only in cell lysates and the extracellular matrix. Under nonreducing conditions, most of the norrin in the extracellular matrix was oligomers that contained up to approximately 20 monomers. One of the major extracellular species of norrin under reducing conditions after cross-linking of norrin oligomers with bis(sulfosuccinimidyl)suberate had an apparent Mr = 28,000, consistent with covalent cross-linked dimers. Thus the covalently cross-linked dimers are key structural components of norrin oligomers. By site-directed mutagenesis, the codon for half-cystine 95 in norrin was changed to one encoding alanine. The norrin C95A found in the extracellular matrix of cells transfected with this mutant was the size of dimers, indicating that half-cystine 95 is involved in oligomer formation. The corresponding half-cystine residue in human prepro-von Willebrand factor is also involved in interchain disulfide bond formation, which is consistent with the sequence identity of the half-cystine residues in norrin and part of the half-cystine residues in a disulfide-rich domain of von Willebrand factor. Replacement of valine at residue 60 in norrin by glutamic acid, a mutation found in humans with a severe type of Norrie disease, results in a considerable reduction (50%) in the amount of norrin in the extracellular matrix of transfected COS-7 cells. Replacement of arginine at residue 121 by glutamine, which is associated with a less severe type of Norrie disease, results in a reduction in the amount of norrin R121Q in the extracellular matrix (26%). These studies suggest that norrin is a secreted protein that forms disulfide-bonded oligomers that are associated with the extracellular matrix upon secretion from cells. Moreover, the disulfide-rich motif of norrin and prepro-von Willebrand factor promotes interchain disulfide bond formation among polypeptides in which it is found.