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Identification of Megalin as the Sole Rat Kidney Sialoglycoprotein Containing Poly 2,8 Deaminoneuraminic Acid

MARTIN ZIAK^*, DONTSCHO KERJASCHKI, MARILYN G. FARQUHAR and JÜRGEN ROTH^*

^* Division of Cell and Molecular Pathology, Department of Pathology, University of Zürich, Zürich, Switzerland
Institute of Pathological Anatomy, University of Vienna, Vienna, Austria
Division of Cellular and Molecular Medicine, University of California at San Diego, La Jolla, California.

Correspondence to Jürgen Roth, Division of Cell and Molecular Pathology, Department Pathology, University of Zürich, CH-8091 Zürich, Switzerland. Phone: 41 1 255 50 90; Fax: 41 1 255 44 07; E-mail: juergen.roth{at}pty.usz. ch

Abstract. Recently, poly 2,8 deaminoneuraminic acid (poly 2,8 KDN) was demonstrated in various embryonic and adult mammalian tissues. This study reports the purification and characterization of the single poly 2,8 KDN-bearing glycoprotein from rat kidney. Amino acid sequences of proteolytic fragments shared homology with megalin, a member of the LDL receptor family. Immunochemical analysis supported this finding, since immunoprecipitated poly 2,8 KDN-bearing glycoprotein was immunoreactive with anti-megalin antibodies in Western blotting and conversely immunoprecipitated megalin was immunoreactive with the monoclonal anti-poly 2,8 KDN antibody. Furthermore, receptor-associated protein affinity-purified megalin reacted with the anti-poly 2,8 KDN antibody. By immunoelectron microscopy, labeling for both poly 2,8 KDN and megalin coincided in the brush border, endocytic invaginations and vesicles, and apical dense tubules of proximal convoluted tubules. Immunoreactivity for poly 2,8 KDN on purified megalin was abolished by ß-elimination reaction but not by N-glycosidase F treatment. These data identified megalin as the sole glycoprotein of rat kidney, which contains poly 2,8 KDN present on O-glycosidically linked oligosaccharides. Furthermore, this study shows that megalin carries N-glycosidically linked hybrid and complex-type oligosaccharides terminating with sialic acid. Both poly 2,8 KDN and sialic acids on megalin may contribute to the binding of Ca²⁺ and cationic ligands.

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