Newly synthesized class I heavy (H) chain/β2m heterodimers awaiting peptides in the endoplasmic reticulum are associated with the transporter associated with Ag processing (TAP). We present evidence here that calreticulin, but not calnexin, displays steady state association with class I/TAP complexes. To separate the ability of β2m to bind with TAP and calreticulin from that of H chain, we studied human cell lines that lack expression of β2m or H chain. Little if any H chain was detected in association with TAP and calreticulin in the β2m- cell line Daudi. By contrast, high levels of β2m are found with TAP and calreticulin in the H chain-deficient cell line LCL 721.221, even after preclearance of the trace amount of class IB protein expressed by this cell line. Thus, β2m appears to bind TAP in the absence of H chain, providing an elegant mechanism to retain β2m in the endoplasmic reticulum at the site of peptide loading. To investigate whether other molecules participate in the binding of β2m and H chain to TAP and calreticulin, we analyzed the deletion mutant cell line LCL 721.220, which lacks tapasin. In 721.220, TAP and calreticulin are not associated with each other. Also, in these cells, H chain/β2m are not associated with TAP, but H chain and a low level of β2m are associated with calreticulin. These results suggest that tapasin is an obligatory mediator of the assemblage of calreticulin/H chain/β2m with TAP.
|Number of pages||6|
|Journal||Journal of Immunology|
|State||Published - Mar 1 1997|