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A major role for tapasin as a stabilizer of the TAP peptide transporter and consequences for MHC class I expression

2003; Wiley; Volume: 33; Issue: 1 Linguagem: Inglês

10.1002/immu.200390029

1521-4141

Natalio Garbi, Neeraj Tiwari, Frank Momburg, Günter J. Hämmerling,

Ubiquitin and proteasome pathways

Abstract 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 3×10 4 TAP molecules per wild‐type splenocyte to about 1×10 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 butalso for optimization of the spectrum of bound peptides.