The human factor H-related protein 4 (FHR-4). A novel short consensus repeat-containing protein is associated with human triglyceride-rich lipoproteins

A novel apoprotein of an apparent molecular mass of 86 kDa in its unreduced form was identified in human triglyceride-rich lipoproteins. This protein was purified and the amino acid sequence of six proteolytic fragments was found to overlap with that of the factor H-related proteins. In parallel we identified the cDNA encoding a new complement factor H-related protein, termed FHR-4. The sequences of the new apoprotein overlapped with that of the FHR-4 protein. Similar to the previously described factor H-related proteins, FHR-4 contains a hydrophobic signal sequence followed by a stretch of five repetitive elements termed short consensus repeats. Recombinant FHR-4 protein was expressed in the baculovirus system and has an apparent molecular mass of 42 kDa. In addition a 84-kDa dimeric form of the recombinant FHR-4 was detected. Using an immunoaffinity column with antibodies raised against the recombinant FHR-4, we isolated a 86-kDa protein from human plasma. The different molecular mass of the recombinant FHR-4 and the dimeric FHR-4 in plasma is due to different carbohydrate moieties. The 86-kDa plasma protein and the novel apolipoprotein had identical mobility on SDS-polyacrylamide gel electrophoresis analysis and reacted with antisera raised against the reFHR-4 and the purified apoprotein. In conclusion, we have identified a novel factor H-related protein, FHR-4, in human plasma and demonstrate that this protein is present in triglyceride-rich lipoproteins in a dimeric form. This observation provides an intriguing new aspect on possible function(s) of this novel protein and the other factor H-related proteins.