The surveillance of the structural fidelity of the proteome is of utmost importance to all cells. The endoplasmic reticulum (ER) is the organelle responsible for proper folding and delivery of proteins to the secretory pathway. It contains a sophisticated protein proofreading and elimination mechanism. Failure of this machinery leads to disease and, finally, to cell death. Elimination of misfolded proteins requires retrograde transport across the ER membrane and depends on the central cytoplasmic proteolytic machinery involved in cellular regulation: the ubiquitin–proteasome system. The basics of this process as well as recent advances in the field are reviewed.

SUMMARY The activity of RING finger ubiquitin ligases (E3) is dependent on their ability to facilitate transfer of ubiquitin from ubiquitin-conjugating enzymes (E2) to substrates. The G2BR domain within the E3 gp78 binds selectively and with high affinity to the E2 Ube2g2. Through structural and functional analyses, we determine that this occurs on a region of Ube2g2 distinct from binding sites for ubiquitin-activating enzyme (E1) and RING fingers. Binding to the G2BR results in conformational changes in Ube2g2 that affect ubiquitin loading. The Ube2g2:G2BR interaction also causes an ~ 50-fold increase in affinity between the E2 and RING finger. This results in markedly increased ubiquitylation by Ube2g2 and the gp78 RING finger. The significance of this G2BR effect is underscored by enhanced ubiquitylation observed when Ube2g2 is paired with other RING finger E3s. These findings uncover a mechanism whereby allosteric effects on an E2 enhance E2-RING finger interactions and consequently ubiquitylation.

Metastasis is the primary cause of mortality from cancer, but the mechanisms leading to metastasis are poorly understood. In particular, relatively little is known about metastasis in cancers of mesenchymal origins, which are known as sarcomas. Approximately ten proteins have been characterized as 'metastasis suppressors', but how these proteins function and are regulated is, in general, not well understood. Gp78 (also known as AMFR or RNF45) is a RING finger E3 ubiquitin ligase that is integral to the endoplasmic reticulum (ER) and involved in ER-associated degradation (ERAD) of diverse substrates. Here we report that expression of gp78 has a causal role in the metastasis of an aggressive human sarcoma and that this prometastatic activity requires the E3 activity of gp78. Further, gp78 associates with and targets the transmembrane metastasis suppressor, KAI1 (also known as CD82), for degradation. Suppression of gp78 increases KAI1 abundance and reduces the metastatic potential of tumor cells, an effect that is largely blocked by concomitant suppression of KAI1. An inverse relationship between these proteins was confirmed in a human sarcoma tissue microarray. Whereas most previous efforts have focused on genetic mechanisms for the loss of metastasis suppressor genes, our results provide new evidence for post-translational downregulation of a metastasis suppressor by its ubiquitin ligase, resulting in abrogation of its metastasis-suppressing effects.