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J Biol Chem, Vol. 274, Issue 10, 6374-6380, March 5, 1999

The Intrinsic Factor-Vitamin B₁₂ Receptor, Cubilin, Is Assembled into Trimers via a Coiled-coil -Helix

Anders Lindblom, Natascha Quadt^¶, Tracey Marsh, Daniel Aeschlimann^**, Matthias Mörgelin, Karlheinz Mann^§§, Patrik Maurer^¶, and Mats Paulsson^¶

From the  Department of Internal Medicine, Malmö General Hospital, Lund University, S-214 01 Malmö, Sweden, the ^¶ Institute for Biochemistry, Medical Faculty, University of Cologne, Joseph-Stelzmann-Strasse 52, D-50931 Cologne, Germany, the  M. E. Müller Institute for Biomechanics, University of Bern, CH-3010 Bern, Switzerland, the ^** Division of Orthopedic Surgery, University of Wisconsin, Madison, Wisconsin 53792, the  Department of Cell and Molecular Biology, Lund University, S-221 00 Lund, Sweden, and the ^§§ Department of Protein Chemistry, Max Planck Institute for Biochemistry, D-82152 Martinsried, Germany

A large protein was purified from bovine kidney, using selective extraction with EDTA to solubilize proteins anchored by divalent cation-dependent interactions. An antiserum raised against the purified protein labeled the apical cell surface of the epithelial cells in proximal tubules and the luminal surface of small intestine. Ten peptide sequences, derived from the protein, all matched the recently published sequences for rat (Moestrup, S. K., Kozyraki, R., Kristiansen, M., Kaysen, J. H., Holm Rasmussen, H., Brault, D., Pontillon, F., Goda, F. O., Christensen, E. I., Hammond, T. G., and Verroust, P. J. (1998) J. Biol. Chem. 273, 5235-5242) and human cubilin, a receptor for intrinsic factor-vitamin B₁₂ complexes, identifying the protein as bovine cubilin. In electron microscopy, a three-armed structure was seen, indicating an oligomerization of three identical subunits. This model was supported by the M_r values of about 1,500,000 for the intact protein and 440,000 for its subunits obtained by analytical ultracentrifugation. In a search for a potential assembly domain, we identified a region of heptad repeats in the N-terminal part of the cubilin sequence. Computer-assisted analysis supported the presence of a coiled-coil -helix between amino acids 103 and 132 of the human cubilin sequence and predicted the formation of a triple coiled-coil. We therefore conclude that cubilin forms a noncovalent trimer of identical subunits connected by an N-terminal coiled-coil -helix.

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