A major role for tapasin as a stabilizer of the TAP peptide transporter and consequences for MHC class I expression.
Tapasin is a member of the MHC class I loading complex where it bridges the TAP peptide transporter to class I molecules. The main role of tapasin is assumed to be the facilitation of peptide loading and optimization of the peptide cargo. Here, we describe another important function for tapasin. In tapasin-deficient (Tpn(-/-)) mice the absence of tapasin was found to have a dramatic effect on the stability of the TAP1/TAP2 heterodimeric peptide transporter. Steady-state expression of TAP protein was reduced more than 100-fold from about 3 x 10(4) TAP molecules per wild-type splenocyte to about 1 x 10(2) TAP per Tpn(-/-) splenocyte. Thus, a major function of murine tapasin appears to be the stabilization of TAP. The low amount of TAP moleculesin Tpn(-/-) lymphocytes is likely to contribute to the severe impairment of MHC class I expression. Surprisingly, activation of Tpn(-/-) lymphocytes yielded strongly enhanced class I expression comparable to wild-type levels, although TAP expression remained low and in the magnitude of several hundred molecules per cell. The high level of class I on activated Tpn(-/-) cells depended on peptides generated by the proteasome as indicated by blockade with the proteasome-specific inhibitor lactacystin. Lymphocyte activation induced an increase in ubiquitinated proteins that are cleaved into peptides by the proteasome. These findings suggest that in the presence of a large peptide pool in the cytosol, a small number of TAP transporters is sufficient to translocate enough peptides for high class I expression. However, these class I molecules were less stable than those of wild-type cells, indicating that tapasin is not only required for stabilization of TAP but also for optimization of the spectrum of bound peptides.
Garbi
Natalio
N
Deutsches Krebsforschungszentrum Heidelberg, Abteilung Molekulare Immunologie, Heidelberg, Germany.
Tiwari
Neeraj
N
Momburg
Frank
F
Hämmerling
Günter J
GJ
D000071181:ATP Binding Cassette Transporter, Subfamily B, Member 2; D000071450:ATP Binding Cassette Transporter, Subfamily B, Member 3; D018528:ATP-Binding Cassette Transporters / Q000378:metabolism*; D000818:Animals; D017920:Antiporters / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D001692:Biological Transport; D016923:Cell Death; D002460:Cell Line; D003546:Cysteine Endopeptidases / Q000378:metabolism; D017353:Gene Deletion; D005786:Gene Expression Regulation; D006207:Half-Life; D015395:Histocompatibility Antigens Class I / Q000235:genetics*; D006801:Humans; D007136:Immunoglobulins / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D008213:Lymphocyte Activation; D026901:Membrane Transport Proteins; D051379:Mice; D018345:Mice, Knockout; D009097:Multienzyme Complexes / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex; D025801:Ubiquitin / Q000378:metabolism
D000071181:ATP Binding Cassette Transporter, Subfamily B, Member 2; D000071450:ATP Binding Cassette Transporter, Subfamily B, Member 3; D018528:ATP-Binding Cassette Transporters; D017920:Antiporters; D015395:Histocompatibility Antigens Class I; D007136:Immunoglobulins; D026901:Membrane Transport Proteins; D009097:Multienzyme Complexes; C073119:TAP1 protein, human; C486171:Tap1 protein, mouse; C489516:Tap2 protein, mouse; D025801:Ubiquitin; C107762:tapasin; C071692:TAP2 protein, human; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex