UBE2F-SAG Axis–Mediated RHEB Neddylation: Mechanistic Insights into mTORC1 Activation and Liver Tumorigenesis

Study Background and Research Question

Neddylation is a post-translational modification in which the ubiquitin-like molecule NEDD8 is conjugated to lysine residues on substrate proteins. This process modulates protein stability, localization, and activity, and is catalyzed by a sequential cascade of E1, E2, and E3 enzymes. While neddylation of cullin family proteins is well-characterized, its role in non-cullin substrates—especially those involved in cancer-relevant signaling—remains less defined. The mechanistic target of rapamycin complex 1 (mTORC1) pathway, a critical regulator of cell growth and metabolism, is frequently upregulated in hepatocellular carcinoma (HCC) and other liver diseases. RHEB, a small GTPase, is a key activator of mTORC1, yet whether RHEB itself is regulated by neddylation was unknown. The reference study (Zhang et al., 2025) addresses this knowledge gap by investigating whether RHEB undergoes neddylation via the UBE2F-SAG axis, and how this modification impacts mTORC1 activity and liver tumorigenesis.

Key Innovation from the Reference Study

The central innovation of this work is the identification of RHEB as a direct substrate of neddylation by the UBE2F-SAG E2/E3 enzyme complex. The authors demonstrate that neddylation of RHEB at lysine 169 (K169) increases its lysosomal localization and GTP-binding affinity, thereby amplifying mTORC1 activity. This mechanistic link is further connected to pathological outcomes: modulating the neddylation pathway alters liver steatosis and tumorigenic potential in vivo. By elucidating the specific role of UBE2F-SAG–mediated RHEB neddylation, the study reveals a previously unrecognized regulatory layer of mTORC1 signaling in liver cancer biology.

Methods and Experimental Design Insights

The investigators employed a comprehensive combination of biochemical, molecular, cellular, and in vivo approaches to dissect the neddylation–mTORC1 axis. Key methodological highlights include:

• Use of CRISPR/Cas9-mediated knockout and RNAi to deplete UBE2F in cell lines and mouse liver, allowing assessment of downstream phenotypes related to mTORC1 signaling, cell growth, autophagy, and tumorigenesis.
• In vitro neddylation assays to confirm RHEB as a bona fide substrate of UBE2F-SAG, with site-directed mutagenesis pinpointing K169 as the critical conjugation site.
• Biochemical fractionation and immunofluorescence to examine subcellular localization of RHEB and its association with lysosomes, in relation to neddylation status.
• Immunoprecipitation and GTP-binding assays to measure the functional impact of RHEB neddylation on its activity and interaction with mTORC1 components.
• Liver-specific Ube2f knockout in Pten-deficient mice to model the interplay between neddylation, mTORC1 hyperactivation, steatosis, and tumorigenesis in vivo.
• Analysis of human HCC samples to correlate UBE2F expression and mTORC1 activity with patient survival.

Protocol Parameters

• UBE2F depletion: Use CRISPR/Cas9 or shRNA in hepatocyte cell lines or mouse liver; confirm by immunoblotting.
• In vitro neddylation assay: Recombinant RHEB (wild-type and K169R mutant) incubated with E1, UBE2F, SAG, and NEDD8 at 30°C for 1 hour.
• Subcellular localization: Immunofluorescence using anti-RHEB and lysosome markers; quantify lysosomal colocalization (e.g., Pearson’s coefficient).
• mTORC1 activity readouts: Immunoblot for phosphorylated S6K1 and 4EBP1 as canonical mTORC1 substrates.
• Animal models: Liver-specific Ube2f knockout crossed with Pten-null mice; monitor liver steatosis and tumor formation over 6–12 months.

Core Findings and Why They Matter

The authors provide compelling evidence that RHEB is neddylated at K169 by the UBE2F-SAG axis, and that this modification is functionally significant. Key findings include:

• Direct neddylation of RHEB: Biochemical assays show that RHEB, but not the K169R mutant, undergoes NEDD8 conjugation in a UBE2F/SAG-dependent manner.
• Enhanced lysosomal localization and GTP loading: Neddylated RHEB shows increased association with lysosomes and greater GTP-binding affinity, both of which potentiate mTORC1 activation.
• mTORC1 pathway modulation: Loss of UBE2F reduces mTORC1 activity (as measured by decreased phosphorylation of S6K1/4EBP1), impairs cell cycle progression, and promotes autophagy.
• In vivo relevance: Liver-specific Ube2f knockout in Pten-deficient mice leads to reduced steatosis and tumorigenesis, indicating a causal role in disease progression.
• Clinical association: Analysis of HCC patient samples reveals that elevated UBE2F expression and mTORC1 activity correlate with poorer survival, underscoring the translational relevance (Zhang et al., 2025).

This work positions the UBE2F-SAG–RHEB neddylation axis as a critical driver of mTORC1 hyperactivation and liver tumorigenesis, suggesting novel therapeutic targets for HCC and non-alcoholic fatty liver disease.

Comparison with Existing Internal Articles

Recent internal articles provide a complementary perspective on the technical challenges of studying neddylation and mTORC1 signaling, particularly with respect to protein purification and detection workflows. For example, the article "Strategic Tagging for Mechanistic Insight: X-press Tag Peptide" discusses how advanced N-terminal leader peptides, such as the X-press Tag Peptide, enable precise affinity purification and facilitate mechanistic studies of post-translational modifications, including neddylation. Similarly, "X-press Tag Peptide: Precision Tagging for Neddylation and mTORC1 Pathway Studies" highlights best practices for using protein purification tag peptides in workflows exploring signaling cross-talk and disease models. These resources emphasize the importance of reliable epitope tagging, antibody detection, and efficient purification platforms in dissecting complex post-translational regulatory networks, as exemplified by the UBE2F-SAG–RHEB axis described in the reference study.

Limitations and Transferability

While the study robustly demonstrates the functional importance of RHEB neddylation in hepatocytes and mouse models, several limitations should be noted:

• It remains to be established whether RHEB neddylation is similarly critical in other tissues or cancer types where mTORC1 is implicated.
• The broader substrate specificity of UBE2F-SAG in the liver context warrants further exploration, as does the potential for compensatory mechanisms upon UBE2F loss.
• Most mechanistic experiments rely on overexpression or knockout approaches, which may not fully recapitulate endogenous regulatory dynamics.
• Translation to clinical therapeutics will require validation in human models and further elucidation of off-target effects.

Nevertheless, the core mechanistic insights are well-supported and offer a foundation for targeted manipulation of the neddylation–mTORC1 axis in disease-relevant settings.

Research Support Resources

For researchers aiming to investigate neddylation or mTORC1 signaling in recombinant protein systems, robust N-terminal leader peptides are essential for efficient purification and detection. The X-press Tag Peptide (SKU A6010) from APExBIO is designed to support such workflows, offering a polyhistidine motif, Xpress epitope for anti-Xpress antibody detection, and an enterokinase cleavage site for streamlined affinity purification using ProBond resin. Its high solubility in DMSO and compatibility with protein purification in recombinant protein expression make it a practical tool for generating tagged proteins to probe post-translational modifications such as neddylation. For further technical guidance, researchers may refer to internal articles discussing best practices and troubleshooting for epitope tag–based purification in the context of mechanistic pathway studies.