Structural and functional analysis of the protein products derived from mutant fur alleles in an endoprotease-deficient Chinese hamster ovary cell strain.

The fur gene encodes the endoprotease, furin. We recently demonstrated mutations in both fur alleles in the mutant Chinese hamster ovary (CHO)-K1 strain, RPE.40, and hypothesized that these mutations were responsible for the endoprotease-deficient phenotype of these cells. We now present the structural and functional properties of three protein products derived from the mutant fur alleles. None of these protein products were able to process the precursor to von Willebrand factor, which is processed by wild-type furin. Pro-protein processing activity initially attributed to one of the mutant proteins was due to wild-type furin produced inadvertently from one of the expression constructs used in these experiments. None of the mutant proteins exhibited evidence of autocatalysis, consistent with the lack of activity versus the test substrate, and glycosylation patterns suggested at least two of them remained in the endoplasmic reticulum. These results confirm that RPE.40 cells are furin null mutants, as earlier evidence had suggested.