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A high-molecular-weight complex of membrane proteins BAP29/BAP31 is involved in the retention of membrane-bound IgD in the endoplasmic reticulum.

B cell antigen receptors (BCRs) are multimeric transmembrane protein complexes comprising membrane-bound immunoglobulins (mIgs) and Ig-alpha/Ig-beta heterodimers. In most cases, transport of mIgs from the endoplasmic reticulum (ER) to the cell surface requires assembly with the Ig-alpha/Ig-beta subunits. In addition to Ig-alpha/Ig-beta, mIg molecules also bind two ER-resident membrane proteins, BAP29 and BAP31, and the chaperone heavy chain binding protein (BiP). In this article, we show that neither Ig-alpha/Ig-beta nor BAP29/BAP31 nor BiP bind simultaneously to the same mIgD molecule. Blue native PAGE revealed that only a minor fraction of intracellular mIgD is associated with high-molecular-weight BAP29/BAP31 complexes. BAP-binding to mIgs was found to correlate with ER retention of chimeric mIgD molecules. On high-level expression in Drosophila melanogaster S2 cells, mIgD molecules were detected on the cell surface in the absence of Ig-alpha/Ig-beta. This aberrant transport was prevented by coexpression of BAP29 and BAP31. Thus, BAP complexes contribute to ER retention of mIg complexes that are not bound to Ig-alpha/Ig-beta. Furthermore, the mechanism of ER retention of both BAP31 and mIgD is not through retrieval from a post-ER compartment, but true ER retention. In conclusion, BAP29 and BAP31 might be the long sought after retention proteins and/or chaperones that act on transmembrane regions of various proteins.

Pubmed ID: 12886015

Authors

  • Schamel WW
  • Kuppig S
  • Becker B
  • Gimborn K
  • Hauri HP
  • Reth M

Journal

Proceedings of the National Academy of Sciences of the United States of America

Publication Data

August 19, 2003

Associated Grants

None

Mesh Terms

  • Animals
  • Antigens, CD
  • Antigens, CD79
  • Binding Sites
  • CHO Cells
  • COS Cells
  • Cell Line
  • Cricetinae
  • Dimerization
  • Drosophila melanogaster
  • Endoplasmic Reticulum
  • Immunoglobulin D
  • Intracellular Membranes
  • Macromolecular Substances
  • Membrane Proteins
  • Mice
  • Molecular Weight
  • Receptors, Antigen, B-Cell
  • Recombinant Fusion Proteins