title: Evolutionary conservation of circulating soluble low density lipoprotein receptor-related protein-like ("LRP-like") molecules.
abstract: Low density lipoprotein receptor family members characteristically bind 39-kDa receptor associated protein (RAP). Soluble forms of these receptors have been described in humans including the 515/85-kDa dimeric receptor, low density lipoprotein receptor-related protein (LRP/alpha2MR), which is involved in multiple processes including lipoprotein and protease metabolism. Here we demonstrate evolutionary conservation in the generation of these soluble RAP-binding proteins of high molecular weight, by identifying their presence in mammalian, avian, and reptilian sera as well as in the circulating haemolymph of a mollusc. Sera extracted on immobilized RAP, produced bands at approximately 500 kDa in radiolabeled ligand blots by using the LRP/alpha2MR-specific ligand, Pseudomonas exotoxin A (PEA). These findings suggest that circulating RAP-binding proteins with high molecular weight in vertebrates share features of LRP/alpha2MR (LRP-like molecules). RAP-binding molecules in the mammalian serum extracts were further characterized as LRP/alpha2MR homologues in Western blots by using antibodies against the 515-kDa alpha-chain of LRP/alpha2MR. Western blots of mammalian serum extracts using two monoclonal antibodies recognizing the 85-kDa transmembrane beta-chain suggested that a portion of the beta-chain's ectodomain remains associated with the alpha-chain, but the beta-chain's intracellular carboxy terminus is absent. These results are consistent with evolutionary conservation in the generation, composition, and ligand-binding ability of soluble LRP-like receptors and suggest that their presence is a necessary aspect of the receptor's function.
mesh_terms: D000828:Animal Population Groups / Q000097:blood; D000818:Animals; D000911:Antibodies, Monoclonal / Q000097:blood; D015153:Blotting, Western; D002352:Carrier Proteins / Q000097:blood; D004586:Electrophoresis; D005098:Exotoxins; D006458:Hemolymph / Q000737:chemistry; D006801:Humans; D007128:Immunoglobulin Fragments / Q000097:blood; D007074:Immunoglobulin G; D008024:Ligands; D026503:Low Density Lipoprotein Receptor-Related Protein-1; D008974:Mollusca / Q000737:chemistry; D011549:Pseudomonas / Q000276:immunology; D011971:Receptors, Immunologic / Q000097:blood*; D012104:Reptiles / Q000378:metabolism; D012995:Solubility