E2-EPF UCP Possesses E3 Ubiquitin Ligase Activity via Its Cysteine 118 Residue

Breaking the Fourth Wall: Modulating Quaternary Associations for Protein Regulation and Drug Discovery

Protein-protein interactions (PPIs) are an effective means to orchestrate intricate biological processes required to sustain life. Approximately 650 000 PPIs underlie the human interactome; thus underscoring its complexity and the manifold signaling outputs altered in response to changes in specific PPIs. This minireview illustrates the growing arsenal of PPI assemblies and offers insights into how these varied PPI regulatory modalities are relevant to customized drug discovery, with a focus on cancer. First, known and emerging PPIs and PPI-targeted drugs of both natural and synthetic origin are categorized. Building on these discussions, the merits of PPI-guided therapeutics over traditional drug design are discussed. Finally, a compare-and-contrast section for different PPI blockers, with gain-of-function PPI interventions, such as PROTACS, is provided.

Ubiquitination of MAP1LC3B by pVHL is associated with autophagy and cell death in renal cell carcinoma

Von Hippel Lindau (VHL) expression is significantly decreased in high-grade RCC, and autophagy, which is involved in tumor growth, invasion, differentiation, and metastasis, is activated in various human cancers. However, the relationship of autophagy and VHL in tumor progression remains controversial. Here, we showed that the expression levels of VHL and microtubule-associated protein 1 light chain 3B (MAP1LC3B, LC3B) were inversely correlated with various tumor grades of RCC tissues. pVHL was found to possess the LIR motif within a beta domain that interacted with MAP1LC3B and ubiquitinated it. The L101A VHL mutant failed to interact with MAP1LC3B, thereby failing to induce ubiquitination. MAP1LC3B-mediated autophagy was inhibited by functional pVHL and the ubiquitination of MAPLC3B was implicated in autophagy-induced cell death. We screened various autophagy inducers to determine the physiological function of the inhibition of LC3B-mediated autophagy by pVHL using VHL-deficient and VHL-expressing cell lines. MLN9708, a proteasome inhibitor, potently induced autophagy via the induction of MAP1LC3B and sensitized the cell to autophagy-mediated cell death in VHL-deficient and VHL-mutant (L101A) cells. In conclusion, our results showed that pVHL interacts with MAPL1LC3B and inhibits LC3B-mediated autophagy via MAP1LC3B ubiquitination. Furthermore, the activation of autophagy by the proteasome inhibitor MLN9708 induced cell death, indicating that MLN9708 can be used for VHL-deficient RCC therapy.

Hepatitis C Virus Downregulates Ubiquitin-Conjugating Enzyme E2S Expression To Prevent Proteasomal Degradation of NS5A, Leading to Host Cells More Sensitive to DNA Damage

Hepatitis C virus (HCV) infection may cause chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma. HCV exploits cellular machineries to establish persistent infection. We demonstrate here that ubiquitin-conjugating enzyme E2S (UBE2S), a member of the ubiquitin-conjugating enzyme family (E2s), was downregulated by endoplasmic reticulum stress caused by HCV in Huh7 cells. UBE2S interacted with domain I of HCV NS5A and degraded NS5A protein through the Lys11-linked proteasome-dependent pathway. Overexpression of UBE2S suppressed viral propagation, while depletion of UBE2S expression increased viral infectivity. Enzymatically inactive UBE2S C95A mutant exerted no antiviral activity, suggesting that ubiquitin-conjugating enzymatic activity was required for the suppressive role of UBE2S. Chromatin ubiquitination plays a crucial role in the DNA damage response. We showed that the levels of UBE2S and Lys11 chains bound to the chromatin were markedly decreased in the context of HCV replication, rendering HCV-infected cells more sensitive to DNA damage. These data suggest that HCV counteracts antiviral activity of UBE2S to optimize viral propagation and may contribute to HCV-induced liver pathogenesis.IMPORTANCE Protein homeostasis is essential to normal cell function. HCV infection disturbs the protein homeostasis in the host cells. Therefore, host cells exert an anti-HCV activity in order to maintain normal cellular metabolism. We showed that UBE2S interacted with HCV NS5A and degraded NS5A protein through the Lys11-linked proteasome-dependent pathway. However, HCV has evolved to overcome host antiviral activity. We demonstrated that the UBE2S expression level was suppressed in HCV-infected cells. Since UBE2S is an ubiquitin-conjugating enzyme and this enzyme activity is involved in DNA damage repair, HCV-infected cells are more sensitive to DNA damage, and thus UBE2S may contribute to viral oncogenesis.