IDENTIFICATION OF A CELL-LINE THAT EXPRESSES A CELL-SURFACE AND A SOLUBLE FORM OF THE GP330 RECEPTOR-ASSOCIATED PROTEIN (RAP) HEYMANN NEPHRITIS ANTIGENIC COMPLEX

gp330 is a large glycoprotein located in clathrin-coated pits at the s urface of the glomerular and proximal tubule epithelia in the rat kidn ey. It was originally identified as the target of autoimmune antibodie s in Heymann nephritis (HN) and has since been shown to be a member of the low density lipoprotein receptor gene family and to form a stable association with receptor-associated protein (RAP), which together co nstitute the HN antigen complex (HNAC). Progress in defining the norma l functions of gp330 as well as the molecular mechanisms of HN has bee n hampered by the lack of an available kidney cell line that expresses this protein. We here report the identification of a rat yolk sac car cinoma cell line (L2) that synthesizes HNAC and expresses it in coated pits at the cell surface. gp330 and RAP from L2 cells are immunologic ally identical to their kidney counterparts, and peptide maps of gp330 yielded identical peptide fragments. Characterization of the cell lin e revealed that there are 3.3 x 10(4) gp330 molecules per L2 cell and that the cells produce a soluble form of gp330 that is released into t he medium. Heparin ligand blot analysis demonstrated that RAP but not gp330 binds heparin. By heparin affinity chromatography, gp330 and RAP copurify, indicating that the glycosaminoglycan binding site within R AP is accessible when the subunit is complexed with gp330. These resul ts indicate that the L2 cell line provides a valid and useful model fo r studies on the function of HNAC and the pathogenesis of HN.