This ubiquitin C-terminal hydrolase (UCH) family includes UCH-L1 and UCH-L3, the two members sharing around 53% sequence identity as well as conserved catalytic residues. Both enzymes hydrolyze carboxyl terminal esters and amides of ubiquitin (Ub). UCH-L1, in dimeric form, has additional enzymatic activity as a ubiquitin ligase. It is highly abundant in the brain, constituting up to 2% of total protein, and is expressed exclusively in neurons and testes. Abnormal expression of UCH-L1 has been shown to correlate with several forms of cancer, including several primary lung tumors, lung tumor cell lines, and colorectal cancers. Mutations in the UCH-L1 gene have been linked to susceptibility to and protection from Parkinson's disease (PD); dysfunction of the hydrolase activity can lead to an accumulation of alpha-synuclein, which is linked to Parkinson's disease (PD), while accumulation of neurofibrillary tangles is linked to Alzheimer's disease (AD). UCH-L3 hydrolyzes isopeptide bonds at the C-terminal glycine of either Ub or Nedd8, a ubiquitin-like protein. It can also interact with Lys48-linked Ub dimers to protect them from degradation while inhibiting its hydrolase activity at the same time. Unlike UCH-L1, neither dimerization nor ligase activity have been observed for UCH-L3. It has been shown that levels of Nedd8 and the apoptotic protein p53 and Bax are elevated in UCH-L3 knockout mice upon cryptorchid injury, possibly contributing to profound germ cell loss via apoptosis.
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