Interaction with newly synthesized and retained proteins in the endoplasmic reticulum suggests a chaperone function for human integral membrane protein IP90 (calnexin).
A cDNA clone encoding the human endoplasmic reticulum (ER) resident protein IP90 was isolated and sequenced. It predicts a transmembrane protein with a large ER luminal region showing sequence similarity to calreticulin and a short cytoplasmic domain containing a COOH-terminal RKPRRE sequence that may be relevant to its retention in the ER. It is 95% homologous to the canine ER membrane phosphorprotein called pp90 or calnexin (Wada, I., Rindress, D., Cameron, P. H., Ou, W.-J., Doherty, J. J., II, Louvard, D., Bell, A. W., Dignard, D., Thomas, D. Y., and Bergeron, J. J. M. (1991) J. Biol. Chem. 266, 19599-19610). Previously, in lymphocytes, we have characterized IP90 as a protein associated with partially assembled multichain proteins including the T cell receptor, the membrane immunoglobulin, and the heavy chain of the major histocompatibility complex class I molecules (Hochstenbach, F., David, V., Watkins, S., and Brenner, M. B. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 4734-4738). Here, we show that within a short metabolic labeling period, IP90 associates transiently with many different newly synthesized proteins. However, in a T cell line that cannot assemble a complete T cell receptor because it lacks the alpha subunit, the unassembled T cell receptor beta chains, which are retained in the ER, remain associated with IP90 throughout a prolonged chase time period. Together, these data offer further evidence suggesting that IP90 may act in assisting protein assembly and/or in the retention within the ER of unassembled protein subunits.
David
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V
Department of Rheumatology and Immunology, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts 02115.
Hochstenbach
F
F
Rajagopalan
S
S
Brenner
M B
MB
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002135:Calcium-Binding Proteins / Q000235:genetics* / Q000378:metabolism*; D037281:Calnexin; D002460:Cell Line; D003001:Cloning, Molecular; D004285:Dogs; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005260:Female; D015723:Gene Library; D006360:Heat-Shock Proteins / Q000235:genetics / Q000378:metabolism*; D015395:Histocompatibility Antigens Class I / Q000235:genetics; D006801:Humans; D046911:Macromolecular Substances; D008565:Membrane Proteins / Q000235:genetics*; D008715:Methionine / Q000378:metabolism; D008969:Molecular Sequence Data; D010750:Phosphoproteins / Q000235:genetics / Q000378:metabolism; D010920:Placenta / Q000378:metabolism; D011247:Pregnancy; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D011947:Receptors, Antigen, B-Cell / Q000235:genetics; D011948:Receptors, Antigen, T-Cell / Q000235:genetics; D017386:Sequence Homology, Amino Acid; D013601:T-Lymphocytes / Q000276:immunology / Q000502:physiology; D014162:Transfection
D002135:Calcium-Binding Proteins; D006360:Heat-Shock Proteins; D015395:Histocompatibility Antigens Class I; D046911:Macromolecular Substances; D008565:Membrane Proteins; D010750:Phosphoproteins; D012333:RNA, Messenger; D011947:Receptors, Antigen, B-Cell; D011948:Receptors, Antigen, T-Cell; D037281:Calnexin; D008715:Methionine