RECEPTOR-ASSOCIATED PROTEIN AND MEMBERS OF THE LOW-DENSITY-LIPOPROTEIN RECEPTOR FAMILY SHARE A COMMON EPITOPE - AN EXTENDED MODEL FOR THE DEVELOPMENT OF PASSIVE HEYMANN NEPHRITIS

Heymann nephritis is an experimental rat model for human membranous gl omerulonephritis. Two target antigens have been identified in the prox imal tubule brush border of rat kidneys. One of them is megalin, a 600 -kDa membrane protein that belongs to the family of low density lipopr otein receptor (LDLR)-related proteins. The other one is receptor-asso ciated protein (RAP), a polypeptide of 40 kDa that associates with mem bers of the LDLR family. Here we show that antibodies produced against recombinant human RAP strongly crossreact with the chicken oocyte rec eptor for very low density Lipoprotein and vitellogenin (LR8), and wit h two other members of the LDLR family, LDLR-related protein and megal in. The interaction of this antibody with LR8 showed binding character istics exactly as those demonstrated for the physiological ligands of this receptor, in that binding of the antibody: (i) is Ca2+-dependent; (ii) is abolished by unfolding of the cysteine-rich binding domain by reduction; and (iii) interferes with the binding of very low density Lipoprotein and vitellogenin. Immunopurification of the LR8-specific s ubpopulation of the polyclonal antiserum yielded an IgG fraction stron gly reacting with LR8 as well as with RAP. Using recombinant fragments of RAP and peptide mapping, the cross-reacting epitope(s) could be na rrowed down to three short sequences (5-7 residues) in the COOH-termin al part of the protein. After immunization with RAP, anti-LR8 antibodi es and anti-RAP antibodies arise simultaneously, indicating that the r eceptor-specific activity is not due to anti-idiotypic antibodies. The se findings suggest the existence of a common epitope(s) on RAP and me mbers of the LDL receptor family Based on these results, we present an extended molecular model for the development of passive Heymann nephr itis.