The quantity of naturally processed peptides stably bound by HLA-A*0201 is significantly reduced in the absence of tapasin.
Tapasin plays a critical role in promoting peptide binding by major histocompatibility complex (MHC) class I molecules in the endoplasmic reticulum. In its absence, cell surface expression of most allotypes is significantly reduced. Two exceptions are HLA-A*0201 and HLA-B*2705. In this study, the repertoire of peptides bound endogenously by these allotypes in the absence of tapasin was examined and stability of the HLA class I/peptide complexes assessed. Similar quantities of peptides were recovered from B*2705 complexes expressed in the absence and presence of tapasin and the composition of the peptide pools were not radically different. However, the stability of B*2705 molecules expressed at the surface of tapasin-deficient cells was found to be reduced which suggests there are subtle changes to the peptide repertoire. The impact of the absence of tapasin was more dramatic for A*0201. Although equivalent levels of cell surface A*0201 are expressed in the presence and absence of tapasin, very little A*0201 glycoprotein was recovered from tapasin-deficient cells suggesting the complexes readily dissociate. Consistent with reduced stability, A*0201 complexes were found to be rapidly lost from the surface of tapasin-deficient cells. Analysis of the small quantity of endogenously bound peptides recovered from A*0201 expressed in the absence of tapasin revealed a complex mixture typical of A*0201 molecules expressed in normal cells. Therefore these molecules are unable to exploit the alternative supply of TAP-independent A*0201-binding peptides present in the endoplasmic reticulum. Loading of A*0201 with peptides from both TAP-dependent and TAP-independent sources is significantly compromised without tapasin.
Barber
L D
LD
Nuffield Department of Clinical Medicine, John Radcliffe Hospital, Oxford, United Kingdom. l.barber@rfc.ucl.ac.uk
Howarth
M
M
Bowness
P
P
Elliott
T
T
D000954:Antigens, Surface / Q000378:metabolism; D017920:Antiporters / Q000378:metabolism*; D002478:Cells, Cultured; D005434:Flow Cytometry; D015234:HLA-A Antigens / Q000378:metabolism*; D015789:HLA-A2 Antigen; D015235:HLA-B Antigens / Q000378:metabolism; D006801:Humans; D007136:Immunoglobulins / Q000378:metabolism*; D026901:Membrane Transport Proteins; D011485:Protein Binding / Q000276:immunology
D000954:Antigens, Surface; D017920:Antiporters; D015234:HLA-A Antigens; C435939:HLA-A*02:01 antigen; D015789:HLA-A2 Antigen; D015235:HLA-B Antigens; D007136:Immunoglobulins; D026901:Membrane Transport Proteins; C107762:tapasin