Post-Translational Modifications

The Ubiquitin-Proteasome System (UPS)

Summary

The ubiquitin-proteasome system is the primary pathway for targeted protein degradation in eukaryotes, utilizing a cascade of E1-E2-E3 enzymes to attach polyubiquitin chains that mark substrates for destruction by the 26S proteasome.

Key Points

  • 1Ubiquitination uses an E1-E2-E3 cascade with E3 ligases providing substrate specificity
  • 2K48-linked polyubiquitin chains are the canonical signal for proteasomal degradation
  • 3The 26S proteasome unfolds and degrades ubiquitinated substrates in an ATP-dependent manner
  • 4Over 600 E3 ligases and 100 DUBs create a highly regulated degradation network
  • 5UPS dysfunction is central to cancer and neurodegeneration; inhibitors are FDA-approved drugs

# The Ubiquitin-Proteasome System (UPS)

The ubiquitin-proteasome system (UPS) is the primary mechanism for controlled protein degradation in eukaryotic cells. By selectively targeting proteins marked with polyubiquitin chains, the UPS regulates virtually every cellular process, from cell cycle progression to immune responses and proteostasis.

The Ubiquitin Molecule

Structure

  • 76-amino acid protein, highly conserved across eukaryotes
  • Compact β-grasp fold with flexible C-terminal tail
  • Seven lysine residues (K6, K11, K27, K29, K33, K48, K63) for chain formation
  • N-terminal methionine (M1) also serves as chain linkage point

    • The Ubiquitin Code

    Different chain types signal different fates:

    - K48 chains: Proteasomal degradation (canonical)

    - K11 chains: Cell cycle regulation, degradation

    - K63 chains: Signaling, autophagy, DNA repair (non-degradative)

    - M1 (linear) chains: NF-κB signaling

    - Mixed/branched chains: Complex regulatory signals

    The Ubiquitination Cascade

    E1: Ubiquitin-Activating Enzymes

  • ATP-dependent activation of ubiquitin C-terminus
  • Forms high-energy thioester with E1 active-site cysteine
  • Only 2 E1 enzymes in humans (UBA1, UBA6)
  • Rate-limiting for overall ubiquitination capacity

    • E2: Ubiquitin-Conjugating Enzymes

  • Receive ubiquitin from E1 via transthioesterification
  • ~40 E2 enzymes in humans
  • Determine chain topology (K48 vs K63 specificity)
  • Some E2s work in heterodimeric pairs (UBE2N-UBE2V1)

    • E3: Ubiquitin Ligases

  • Confer substrate specificity
  • >600 E3 ligases in humans
  • Two major mechanistic classes:

    • #### RING E3 Ligases

  • Act as scaffolds bringing E2~Ub and substrate together
  • Direct transfer from E2 to substrate
  • Examples: SCF complexes, APC/C

    • #### HECT/RBR E3 Ligases

  • Form obligate thioester intermediate with ubiquitin
  • Transfer ubiquitin from E3 to substrate
  • Examples: E6AP (HECT), Parkin (RBR)

    • Substrate Recognition

    Degrons

    Sequence or structural features recognized by E3 ligases:

    - N-degron: N-terminal residue determines stability (N-end rule)

    - Phosphodegrons: Phosphorylation creates E3 binding site

    - Oxygen-dependent degrons: HIF-1α hydroxylation-VHL recognition

    Cullin-RING Ligases (CRLs)

    Modular E3 architecture:

    - Cullin scaffold: CUL1-7 family members

    - RING protein: RBX1/ROC1

    - Adaptor proteins: Connect cullin to substrate receptor

    - Substrate receptors: >400 different receptors provide specificity

    The 26S Proteasome

    Architecture

    - 20S core particle: Barrel-shaped proteolytic chamber

    - α-rings: Gatekeeping subunits

    - β-rings: Catalytic subunits (β1, β2, β5 activities)

    - 19S regulatory particle:

    - Lid: Deubiquitination (Rpn11)

    - Base: ATPase ring for unfolding and translocation

    Proteolytic Mechanism

  • Ubiquitin chain recognition by 19S receptors (Rpn10, Rpn13)
  • Deubiquitination and ubiquitin recycling
  • ATP-dependent substrate unfolding
  • Translocation into 20S core
  • Processive degradation to peptides (3-25 residues)

    • Deubiquitinating Enzymes (DUBs)

    Functions

  • Ubiquitin recycling from substrates
  • Chain editing (trimming, remodeling)
  • Rescue proteins from degradation
  • ~100 DUBs in humans across 7 families

    • Key Examples

    - USP7: Stabilizes p53 and MDM2

    - USP14/UCH37: Proteasome-associated DUBs

    - CYLD: Negative regulator of NF-κB

    UPS in Disease

    Cancer

  • p53 degradation by MDM2
  • CRL dysregulation in various malignancies
  • Proteasome inhibitors as therapy (Bortezomib)

    • Neurodegeneration

  • Impaired UPS in Parkinson's (Parkin mutations)
  • Accumulation of ubiquitinated inclusions
  • Proteasome overwhelmed by aggregation-prone proteins

    • Therapeutic Targeting

    - Proteasome inhibitors: Bortezomib, Carfilzomib for myeloma

    - PROTACs: Bifunctional molecules recruiting E3s to neo-substrates

    - Molecular glues: Lenalidomide-type degraders

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