A single enzyme with two opposing catalytic active sites?

Ubiquitin is one of the most abundant proteins in eukaryotes; by modifying other proteins, it functions mainly as a proteasomal degradation signal. Substrate tagging by ubiquitin on lysine residues of a target is mediated by a cascade consisting of an activating enzyme (E1), a conjugating enzyme (E2) and a ligase (E3). Ubiquitination is a reversible process by deubiquitinating enzymes (DUBs).

Ubiquitin C’ terminal hydrolase-L1 (UCH-L1) is a DUB abundantly expressed in neurons (estimated at 1-5% of total soluble proteins in neurons), yet its biological function in neuronal cells remains a mystery.

Preliminary work in our lab and data in the literature observed loss-of-function or down-regulation of UCH-L1 in Alzheimer`s patients’ brains, mice, and human neuronal models of Alzheimer’s disease. The high level of UCH-L1 in neuronal cells and its critical link to suppressing the accumulation of A-beta42 raises the hypothesis that UCH-L1 plays a unique role in prolonging neuronal health.

In addition to its well-studied hydrolase activity as a DUB, a report pointed to a cryptic ligase activity of UCH-L1. We dissected the mechanism of UCH-L1 and show, for the first time, a new active site that is responsible for the ligation activity. Moreover, the two active sites work independently of each other.

We are now ironing out which of the two distinct activities is required for its role in suppressing A-beta pathology. Investigating the biochemical mechanisms of UCHL1 dual enzymatic activities helps to uncover the mystery of its unique function in neurons.