Ubc7p is a ubiquitin-conjugating enzyme (E2) that functions with endoplasmic reticulum (ER)-resident ubiquitin ligases (E3s) to promote endoplasmic reticulum-associated degradation (ERAD). Ubc7p only functions in ERAD if bound to the ER surface by Cue1p, a membrane-anchored ER protein. The role of Cue1p was thought to involve passive concentration of Ubc7p at the surface of the ER. However, our biochemical studies of Ubc7p suggested that Cue1p may, in addition, stimulate Ubc7p E2 activity. We have tested this idea and found it to be true both in vitro and in vivo. Ubc7p bound to the soluble domain of Cue1p showed strongly enhanced in vitro ubiquitination activity, both in the presence and absence of E3. Cue1p also enhanced Ubc7p function in vivo, and this activation was separable from the established ER-anchoring role of Cue1p. Finally, we tested in vivo activation of Ubc7p by Cue1p in an assay independent of the ER membrane and ERAD. A chimeric E2 linking Ubc7p to the Cdc34p/Ubc3p localization domain complemented the cdc34-2 TS phenotype, and co-expression of the soluble Cue1p domain enhanced complementation by this chimeric Ubc7p E2. These studies reveal a previously unobserved stimulation of Ubc7p E2 activity by Cue1p that is critical for full ERAD and that functions independently of the well known Cue1p anchoring function. Moreover, it suggests a previously unappreciated mode for regulation of E2s by Cue1p-like interacting partners.
[Show abstract][Hide abstract] ABSTRACT: Peroxisomal biogenesis is an ubiquitin-dependent process because the receptors required for the import of peroxisomal matrix proteins are controlled via their ubiquitination status. A key step is the monoubiquitination of the import receptor Pex5p by the ubiquitin-conjugating enzyme (E2) Pex4p. This monoubiquitination is supposed to take place after Pex5p has released the cargo into the peroxisomal matrix and primes Pex5p for the extraction from the membrane by the mechano-enzymes Pex1p/Pex6p. These two AAA-type ATPases export Pex5p back to the cytosol for further rounds of matrix protein import. Recently, it has been reported that the soluble Pex4p requires the interaction to its peroxisomal membrane-anchor Pex22p to display full activity. Here we demonstrate that the soluble C-terminal domain of Pex22p harbours its biological activity and that this activity is independent from its function as membrane-anchor of Pex4p. We show that Pex4p can be functionally fused to the trans-membrane segment of the membrane protein Pex3p, which is not directly involved in Pex5p-ubiquitination and matrix protein import. However, this Pex3(N)-Pex4p chimera can only complement the double-deletion strain pex4Δ/pex22Δ and ensure optimal Pex5p-ubiquitination when the C-terminal part of Pex22p is additionally expressed in the cell. Thus, while the membrane-bound portion Pex22(N)p is not required when Pex4p is fused to Pex3(N)p, the soluble Pex22(C)p is essential for peroxisomal biogenesis and efficient monoubiquitination of the import receptor Pex5p by the E3-ligase Pex12p in vivo and in vitro. The results merge into a picture of an ubiquitin-conjugating complex at the peroxisomal membrane consisting of three domains: the ubiquitin-conjugating domain (Pex4p), a membrane-anchor domain (Pex22(N)p) and an enhancing domain (Pex22(C)p), with the membrane-anchor domain being mutually exchangeable, while the Ubc- and enhancer-domains are essential.
PLoS ONE 08/2014; 9(8):e105894. DOI:10.1371/journal.pone.0105894 · 3.23 Impact Factor
"One strategy is to assemble ubiquitin chains on the active site of an E2 enzyme, and then transfer these preassembled ubiquitin chains to substrates (Li et al, 2007). Ubiquitin chain assembly on E2 active site has been reported for several E2s (Haldeman et al, 1997; Cao et al, 2007; Ravid & Hochstrasser, 2007; Bazirgan & Hampton, 2008), but the best characterized example is the ER-associated E2 Ube2g2 (Li et al, 2007, 2009), which acts in conjugation with the RING (Really Interesting New Gene) domain E3 gp78 to ubiquitinate and degrade many misfolded ER proteins (Fang et al, 2001; Song et al, 2005; Christianson et al, 2011). gp78 can rapidly assemble ubiquitin chains on the active site of Ube2g2 and transfer preassembled chains to a substrate (Li et al, 2007). "
[Show abstract][Hide abstract] ABSTRACT: Cellular adaptation to proteotoxic stress at the endoplasmic reticulum (ER) depends on Lys48-linked polyubiquitination by ER-associated ubiquitin ligases (E3s) and subsequent elimination of ubiquitinated retrotranslocation products by the proteasome. The ER-associated E3 gp78 ubiquitinates misfolded proteins by transferring preformed Lys48-linked ubiquitin chains from the cognate E2 Ube2g2 to substrates. Here we demonstrate that Ube2g2 synthesizes linkage specific ubiquitin chains by forming an unprecedented homodimer: The dimerization of Ube2g2, mediated primarily by electrostatic interactions between two Ube2g2s, is also facilitated by the charged ubiquitin molecules. Mutagenesis studies show that Ube2g2 dimerization is required for ER-associated degradation (ERAD). In addition to E2 dimerization, we show that a highly conserved arginine residue in the donor Ube2g2 senses the presence of an aspartate in the acceptor ubiquitin to position only Lys48 of ubiquitin in proximity to the donor E2 active site. These results reveal an unanticipated mode of E2 self-association that allows the E2 to effectively engage two ubiquitins to specifically synthesize Lys48-linked ubiquitin chains.
The EMBO Journal 01/2014; 33(1). DOI:10.1002/embj.201385315 · 10.43 Impact Factor
"Consistently, we found that this linkage specificity, mediated by Ubc7p, is required for degradation of the ERAD model substrate CPY* in vivo. As previously published, the ligase-associated factor Cue1p is not only required for recruitment of Ubc7p to the ER membrane in vivo but also stimulates polyubiquitin chain formation in vitro (Bazirgan and Hampton, 2008; Kostova et al., 2009). A small C-terminal Ubc7p-binding region in Cue1p promotes a basic activation of Ubc7p-dependent ubiquitin conjugation. "
[Show abstract][Hide abstract] ABSTRACT: Ubiquitin-binding domains (UBDs) differentially recognize ubiquitin (ub) modifications. Some of them specifically bind mono-ub, as has been shown for the CUE domain. Interestingly, so far no significant ubiquitin binding has been observed for the CUE domain of yeast Cue1p. Cue1p is receptor and activator of the ubiquitin-conjugating enzyme Ubc7p. It integrates Ubc7p into endoplasmic reticulum (ER) membrane-bound ubiquitin ligase complexes, and thus, it is crucial for ER-associated protein degradation (ERAD). Here we show that the CUE domain of Cue1p binds ubiquitin chains, which is pivotal for the efficient formation of K48-linked polyubiquitin chains in vitro. Mutations that abolish ubiquitin binding by Cue1p affect the turnover of ERAD substrates in vivo. Our data strongly imply that the CUE domain facilitates substrate ubiquitylation by stabilizing growing ubiquitin chains at the ERAD ubiquitin ligases. Hence, we demonstrate an unexpected function of a UBD in the regulation of ubiquitin chain synthesis.
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