Ubiquitin C-Terminal Hydrolase L1 (UCHL1), Beyond Hydrolysis

Abstract

Ubiquitin C-terminal hydrolase L1 (UCHL1) is a component of the ubiquitin-proteasome system (UPS) linked to neurodegeneration. Despite its exceptionally high abundance in neurons, UCHL1's precise role remains unclear. This review critically examines the proposed functions of UCHL1 and the challenges to understanding its role in neuronal cells. While UCHL1 hydrolyzes small adducts from the C-terminus of ubiquitin, its occluded active site limits the range of possible substrates and restricts its activity as an efficient deubiquitinase (DUB). These constraints, alongside the paucity of identified substrates, challenge the centrality of this proposed role. We also explore the potential of UCHL1 acting as a ubiquitin ligase and its nonenzymatic role in stabilizing mono-ubiquitin by preventing its lysosomal degradation. By highlighting the unresolved complexities surrounding UCHL1, this perspective proposes several approaches to elucidate UCHL1's significance in the brain.

Keywords: Alzheimer's disease; deubiquitinases; neurodegeneration; proteostasis; ubiquitin; ubiquitin ligases.

FIGURE 1

Sequence and structure alignment of UCH subfamily members. (a) Schematic representation of UCH sequences and alignment of the active site crossover loop. (b) 3D structure alignment of UCH catalytic domains (in gray), highlighting differences in the crossover loops of UCHL1 (PDB: 3WK5, green), UCHL3 (PDB:1XD3, yellow), and UCHL5 (PDB:4UEL, blue).

FIGURE 2

Re‐arrangement of the UCHL1 active site upon ubiquitin binding. A comparison of the active site triad (Cys90, His161, and Asp176) positions in the apo‐enzyme (PDB: 2ETL, blue) and upon ubiquitin binding (PDB: 3KW5, gray), obtained by superposition of the two PDBs in ChimeraX, shows that ubiquitin binding triggers a re‐organization of the side chains of these essential residues. The most notable shift was observed for His161, which moves to a position where it can impact the nucleophilicity of the active cysteine.

FIGURE 3

A schematic representation of UCHL1's various activities: In its monomeric form, (left) UCHL1 performs hydrolysis, cleaving the small extension from the C‐terminus of ubiquitin through the active cysteine at position 90. Additionally, UCHL1 exhibits a high affinity for binding free mono‐ubiquitin. According to a proposed model [20], the dimeric form of UCHL1 (right) carries out an ATP‐independent ligation activity, where the first unit binds to free mono‐ubiquitin, and the second binds to the elongated ubiquitin C‐terminus. The C90 cysteine cleaves this linkage, and the energy released produces di‐ubiquitin. This figure was created by Biorender.