The ASCC2 CUE domain contacts adjacent ubiquitins to recognize K63-linked polyubiquitin

Alkylation of DNA and RNA is a potentially toxic lesion that can result in mutations and cell death. In response to alkylation damage, K63-linked polyubiquitin chains are assembled that localize the ALKBH3-ASCC repair complex to damage sites in the nucleus. The protein ASCC2, a subunit of the ASCC complex, selectively binds K63-linked polyubiquitin chains using its CUE domain, a type of ubiquitin-binding domain that typically binds monoubiquitin and does not discriminate among different polyubiquitin linkage types. We report here that the ASCC2 CUE domain selectively binds K63-linked diubiquitin by contacting both the distal and proximal ubiquitin. Whereas the ASCC2 CUE domain binds the distal ubiquitin in a manner similar to that reported for other CUE domains bound to a single ubiquitin, the contacts with the proximal ubiquitin are unique to ASCC2. The N-terminal portion of the ASCC2 α1 helix, including residues E467 and S470, contributes to the binding interaction with the proximal ubiquitin of K63-linked diubiquitin. Mutation of residues within the N-terminal portion of the ASCC2 α1 helix decreases ASCC2 recruitment in response to DNA alkylation, supporting the functional significance of these interactions during the alkylation damage response.

Expression of CUE domain containing 2 protein in serous ovarian cancer tissue: predicting disease-free and overall survival of patients

Objective The aim of this study was to predict disease-free (DFS) and overall (OS) survival of cancer patients through expression of CUE domain containing 2 (CUEDC2) protein. Methods In this retrospective study, we investigated CUEDC2 expression in 75 serous ovarian cancer tissues and 34 tubal fimbria tissues by immunohistochemistry. Chemoresistance was analyzed using clinical follow-up data. Results CUEDC2 expression scores were 1.35 ± 0.60, 1.54 ± 0.57, 1.78 ± 0.71, and 2.13 ± 0.27 for International Federation of Gynecology and Obstetrics (FIGO) stages I, II, III, and IV tissues, respectively, indicating that CUEDC2 expression increased with stage and that scores differed between patients with early and advanced cancers. We found no differences in CUEDC2 expression for tissues with low, medium, and high differentiation. CUEDC2 expression was unrelated to patient age, pathological grade, or presence or absence of lymph node metastasis, but was related to tumor stage. For CUEDC2-positive patients, median DFS and OS survival were 32.6 and 54.3 months, respectively. For CUEDC2-negative patients, median DFS and OS were 51.9 and 63.5 months, respectively. Expression of CUEDC2 was correlated with DFS but not OS. Conclusion CUEDC2 is highly expressed in ovarian cancer tissues and is related to tumor stage and DFS.

A Ubiquitin-Binding Domain that Does Not Bind Ubiquitin

Ubiquitylation, the post-translational linkage of ubiquitin moieties to lysines in target proteins, helps regulate a myriad of biological processes. Ubiquitin, and sometimes ubiquitin-homology domains, are recognized by ubiquitin-binding domains, including CUE domains. CUE domains are thus generally thought to function exclusively by mediating interactions with ubiquitylated proteins. The chromatin remodeler, SMARCAD1, interacts with KAP1, a transcriptional corepressor. We show that the SMARCAD1-KAP1 interaction is direct and involves the first SMARCAD1 CUE domain (CUE1) and the RBCC domain of KAP1. A structural model of the minimal KAP1 RBCC-SMARCAD1 CUE1 complex based on X-ray crystallography analysis is presented. Remarkably, the CUE1 domain, which resembles a canonical CUE domain, recognizes 2 clusters of exposed hydrophobic residues on KAP1, but these are presented in the context of a coiled-coil domain, not in a structure resembling ubiquitin. Together, these data challenge the well-established dogma that CUE domains exclusively recognize the ubiquitin-fold.

Ubiquitin binding by the CUE domain promotes endosomal localization of the Rab5 GEF Vps9

Vps9 and Muk1 are guanine nucleotide exchange factors (GEFs) in Saccharomyces cerevisiae that regulate membrane trafficking in the endolysosomal pathway by activating Rab5 GTPases. We show that Vps9 is the primary Rab5 GEF required for biogenesis of late endosomal multivesicular bodies (MVBs). However, only Vps9 (but not Muk1) is required for the formation of aberrant class E compartments that arise upon dysfunction of endosomal sorting complexes required for transport (ESCRTs). ESCRT dysfunction causes ubiquitinated transmembrane proteins to accumulate at endosomes, and we demonstrate that endosomal recruitment of Vps9 is promoted by its ubiquitin-binding CUE domain. Muk1 lacks ubiquitin-binding motifs, but its fusion to the Vps9 CUE domain allows Muk1 to rescue endosome morphology, cargo trafficking, and cellular stress-tolerance phenotypes that result from loss of Vps9 function. These results indicate that ubiquitin binding by the CUE domain promotes Vps9 function in endolysosomal membrane trafficking via promotion of localization.