MISFOLDED MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-I HEAVY-CHAINS ARE TRANSLOCATED INTO THE CYTOPLASM AND DEGRADED BY THE PROTEASOME

N-acetyl-L-leucyl-L-leucyl-L-norleucinal (LLnL), which reversibly inhi bits the proteasome in addition to other proteases, and a more specifi c irreversible inhibitor of the proteasome, lactacystin, were found to cause the accumulation of major histocompatibility complex (MHC) clas s I heavy chains in the cytosol of the beta(2)-microglobulin-deficient cell line Daudi and the TAP-deficient cell line .174. These cell line s, which are severely impaired in their ability to fold MHC class I he avy chain, showed an accumulation of soluble class I heavy chains at d ifferent rates over a period of hours in the presence of LLnL, The acc umulation of soluble class I heavy chains in the presence of either LL nL or lactacystin was easily revealed in Daudi and .174 but almost und etectable in a Daudi transfectant expressing beta(2)-microglobulin and in 45.1, the wild-type parent of .174, The soluble class I heavy chai n was also found to be devoid of its N-linked glycan and to be located in the cytosol, When the gene for ICP47, a herpes simplex virus prote in that blocks the translocation of peptides into the endoplasmic reti culum, was transfected into 45.1, a similar accumulation of soluble MH C class I heavy chain was detectable, These data suggest that in cells where the MHC class I molecule is unable to assemble properly, the mi sfolded heavy chain is removed from the endoplasmic reticulum to the c ytosol, deglycosylated, and degraded by the proteasome.