An essential function of tapasin in quality control of HLA-G molecules.
Tapasin plays an important role in the quality control of major histocompatibility complex (MHC) class I assembly, but its precise function in this process remains controversial. Whether tapasin participates in the assembly of HLA-G has not been studied. HLA-G, an MHC class Ib molecule that binds a more restricted set of peptides than class Ia molecules, is a particularly interesting molecule, because during assembly, it recycles between the endoplasmic reticulum (ER) and the cis-Golgi until it is loaded with a high affinity peptide. We have taken advantage of this unusual trafficking property of HLA-G and its requirement for high affinity peptides to demonstrate that a critical function of tapasin is to transform class I molecules into a high affinity, peptide-receptive form. In the absence of tapasin, HLA-G molecules cannot bind high affinity peptides, and an abundant supply of peptides cannot overcome the tapasin requirement for high affinity peptide loading. The addition of tapasin renders HLA-G molecules capable of loading high affinity peptides and of transporting to the surface, suggesting that tapasin is a prerequisite for the binding of high-affinity ligands. Interestingly, the "tapasin-dependent" HLA-G molecules are not empty in the absence of tapasin but are in fact associated with suboptimal peptides and continue to recycle between the ER and the cis-Golgi. Together with the finding that empty HLA-G heterodimers are strictly retained in the ER and degraded, our data suggest that MHC class I molecules bind any available peptides to avoid ER-mediated degradation and that the peptides are in turn replaced by higher affinity peptides with the aid of tapasin.
Park
Boyoun
B
Graduate School of Biotechnology, Korea University, Seoul 136-701, Korea.
Ahn
Kwangseog
K
D016475:3T3 Cells; D000818:Animals; D000941:Antigens / Q000737:chemistry; D017920:Antiporters / Q000502:physiology*; D015153:Blotting, Western; D002462:Cell Membrane / Q000378:metabolism; D019281:Dimerization; D004305:Dose-Response Relationship, Drug; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005434:Flow Cytometry; D006056:Golgi Apparatus / Q000378:metabolism; D006680:HLA Antigens / Q000378:metabolism*; D059951:HLA-G Antigens; D015395:Histocompatibility Antigens Class I / Q000378:metabolism*; D006801:Humans; D007136:Immunoglobulins / Q000502:physiology*; D007700:Kinetics; D008024:Ligands; D026901:Membrane Transport Proteins; D051379:Mice; D008856:Microscopy, Fluorescence; D008861:Microsomes / Q000378:metabolism; D010455:Peptides / Q000737:chemistry; D011233:Precipitin Tests; D011487:Protein Conformation; D013696:Temperature
D000941:Antigens; D017920:Antiporters; D006680:HLA Antigens; D059951:HLA-G Antigens; D015395:Histocompatibility Antigens Class I; D007136:Immunoglobulins; D008024:Ligands; D026901:Membrane Transport Proteins; D010455:Peptides; C107762:tapasin