REGULATION OF SELECTIVE PROTEIN-DEGRADATION IN THE ENDOPLASMIC-RETICULUM BY REDOX POTENTIAL

Recent studies show that the endoplasmic reticulum (ER) contains prote ases, but it is not understood how these enzymes are regulated. In thi s report we study the selective ER degradation of the subunits (alphab etagammadeltaepsilonzeta) of the T-cell antigen receptor (TCR). When a nalyzed in vivo, unassembled subunits of the TCR fail to reach the Gol gi apparatus and show a differential sensitivity to degradation after synthesis. The alpha, beta, and delta subunits are degraded rapidly, w hile gamma, epsilon, and zeta are stable. To study the regulation of p roteolysis in more detail, beta, gamma, delta, and epsilon subunits we re expressed alone in fibroblasts and their selective degradation anal yzed in vitro. The beta and delta chains were degraded in the complete absence of vesicular transport, indicating their degradation in the E R membrane compartment. Proteolysis was unaffected by GTPgammaS (guano sine 5'-O-(thiotriphosphate)), EDTA, or depletion of ATP. They and eps ilon subunits were stable under the same in vitro conditions, indicati ng that the assay reconstituted selective protein degradation within t he ER. Furthermore, the results showed that the gamma and epsilon subu nits did not escape degradation by being transported from the ER to pr e-Golgi, or cis-Golgi, membrane compartments. Structural determinants of ER degradation contained within the membrane anchor of the TCR beta subunit were only active in permeabilized cells when reducing agents were added to the assay. Surprisingly, reducing conditions disrupted t he regulation of ER proteolysis and induced rapid ER degradation of th e stable CD3 gamma subunit and of a control interleukin 2 receptor chi mera. Taken together, the results indicated that the ER membrane compa rtment regulates the selective degradation of newly synthesized protei ns. Importantly, the stability of proteins retained in the ER was high ly sensitive to redox conditions. It is possible that the redox buffer within the ER lumen may regulate ER protein degradation in vivo.