ubiquitin conjugating enzyme E2, J2 isoform 1; ubiquitin conjugating enzyme 6; yeast UBC6 homolog

Intravesicular, secreted, and plasma membrane proteins typically require assistance for proper folding. When transcribed into the endoplasmic reticulum, proteins are folded with the aid of chaperones, isomerases and foldases. But if this fails, misfolded proteins are shuttled to the cytoplasm for proteasomal degradation, through a ubiquitylation-coupled process. Sec61 translocation pores are used for translation-coupled import and are surprisingly also used for ubiquitylation-coupled export. Two ubiquitin conjugating enzymes have been tightly associated with the translocation export apparatus: ubc6p (ube2j1/ncube1) and ubc7p (ube2g1/E217K). The former has a carboxy-terminal transmembrane region, which crosses the endoplasmic reticulum membrane, with the conjugating domain exposed to the cytoplasm and a short peptide in the lumen of compartment. In contrast, ubc7p does not have membrane targeting sequence but associates with ubc6p through cue-domain-containing proteins. Analysis of the genome further revealed an additional E2: ube2j2/ncube2, related to ubc6p with similar unique active site residues and endoplasmic reticulum directing, carboxy-terminal transmembrane anchor (Lester et al., 2000; Lenk et al., 2002b). Biological roles of these proteins may be reflected in their distinct tissue distribution: while ubc6p is found only in lymphoid tissue; ube2j2 is ubiquitously expressed.
Because ubc6p and ubc7p are necessary for retrograde transport of lumenal substrates out of the ER, export and ubiquitylation are considered mechanistically integrated. Typically, mutants of ubc6p or ubc7p cause accumulation of misfolded proteins in the endoplasmic reticulum (Biederer et al., 1996; Hitchcock et al., 2003). Similarly, manipulation of ube2j2/ncube2 levels in mammalian cells affects endoplasmic reticulum associated protein degradation of TCR and CFTR (Lenk et al., 2002a). Notably unclear is the mechanism by which retrograde transport and ubiquitylation are coupled. The ubc6p/ubc7p pair has also been implicated in ubiquitylation of cytoplasmic unfolded proteins, and that the interaction between ubc and unfolded protein has been hypothesized to be E3 ligase independent. Various lines of evidence suggest that short stretches of residues that are both strongly hydrophobic and strongly basic, which occur predominantly in domain cores, are degradation signals (Gilon et al., 2000).

We have determined the high resolution structure of human ube2j2. It adopts the typical alpha/beta fold seen in other UBC structures with the following notable features. The loop connecting the catalytic cysteine with the central alpha helix, including Leu-111 to Thr-120, is disordered in our structure. Although this region is one of the most divergent elements seen in ubc domains, it has been implicated in substrate selectivity and may represent a site of ube2j2 that could interact with degradation signals. Dysfunction of the ubc6p/ubc7p associated degradation machinery may be at the root of various pathologies, including diabetes, and may be an attractive pharmaceutical target.