Cytosolic degradation of T-cell receptor alpha chains by the proteasome

The T-cell antigen receptor (TCR) is an hetero-oligomeric membrane complex composed of at least seven transmembrane polypeptide chains that has served as a model for the assembly and degradation of integral membrane proteins in the endoplasmic reticulum (ER), Unassembled TCR alpha chains fail to mature to the Golgi apparatus and are rapidly degraded by a non-lysosomal ''ER degradation'' pathway that has been proposed to be autonomous to the ER. In these studies we show that the degradation of core glycosylated TCR alpha is blocked by N-acetyl-L-leucyl-L-leucyl-L-norleucinal (ALLN) and lactacystin, implicating the proteasome in ER degradation, Either acute or chronic treatment of TCR alpha-transfected cells with proteasome inhibitors cause the core-glycosylated TCR alpha chains to progressively shift to an similar to 28-kDa form that lacks N-linked oligosaccharides and the N-terminal signal peptide. The susceptibility of this 28-kDa species to extravesicular protease indicates that it is not protected by the ER membrane and, hence, cytoplasmic, These data suggest a model in which TCR alpha chains that are translocated across the membrane, core-glycosylated, but fail to assemble are dislocated back to the cytoplasm for degradation by cytoplasmic proteasomes. Our data also suggest that covalent modification of TCR alpha with ubiquitin is not required for its degradation.