Immunocytochemical and biochemical characterization of the Heymann nephritis antigenic complex in rat L2 yolk sac cells

Heymann nephritis in the rat is the most widely used model of human membranous glomerulonephritis. Glycoprotein (gp)330, a large (M(r) > 550,000) membrane-associated glycoprotein, has been identified as the main antigen in this autoimmune disease. Studies of gp330 and receptor-associated protein (RAP), its 44-kd subunit, have been restricted largely to rat kidney, as no stable cultured cell line has been available that expresses gp330. We have recently identified a rat yolk sac carcinoma cell line (L2) that expresses both gp330 and RAP. In this report, we have carried out detailed morphological, immunocytochemical, and biochemical studies characterizing the biosynthesis and localization of gp330 and RAP in the L2 rat yolk sac cell line. At the electron microscope level, the L2 cells are seen to be attached by cell junctions, and their predominant morphological features include extensive networks of rough endoplasmic reticulum (ER) and numerous clathrin-coated pits found on the cell membrane. By immunocytochemistry, gp330 was localized primarily to clathrin-coated pits at the cell surface, whereas RAP was localized predominantly to the lumen of the rough ER. Pulse-chase experiments indicated that gp330 spends a prolonged time maturing in the ER of L2 cells, as transport of gp330 to the Golgi complex (based on acquisition of endoglycosidase H resistance) is slow (t1/2 = 90 to 120 minutes). Gp330 reached the L2 cell surface beginning at 2 hours after synthesis, where it could be detected by cell surface immunoprecipitation. RAP was found to be an N-linked glycoprotein, and it remained endoglycosidase H-sensitive up to 4 hours after synthesis. Co-precipitation and co-sedimentation experiments demonstrated that gp330 and RAP form a large heterodimer (M(r) approximately 669,000) immediately after biosynthesis and are further assembled into a large hetero-oligomer in the ER. These findings demonstrate that the localization and the kinetics of assembly of gp330 and RAP into the Heymann nephritis antigenic complex are similar in both L2 cells and rat kidney. They also provide new information on the intracellular processing of these two molecules and their delivery to the cell surface. Thus, the L2 cell system should facilitate further characterization of the functions and interactions of gp330 and RAP, which may shed light on the cellular and molecular mechanisms of Heymann nephritis.