PGP9.5/UCHL1

Ubiquitin C-terminal hydrolase 1 (UCHL1) has several other names, such as ubiquitin carboxyl esterase L1, ubiquitin thiolesterase, neuron-specific protein PGP9.5 and Park5. The protein is extremely abundant, and was estimated to be present at a concentration of 200-500 micrograms/g wet weight, representing a major protein component of neuronal cytoplasm (1). This has been claimed to represent 1-2% of total brain protein.

The activity is also important to remove ubiquitin from partially degraded proteins, allowing the ubiquitin monomer to be recycled. Regulation of the ubiquitin pathway is very important and many disease states are associated with defects in this pathway. Point mutations in the UCHL1 gene are associated with some forms of human Parkinson's disease. Recent studies suggest that UCHL1 also has a ubiqutinyl ligase activity, being able to couple ubiquitin monomers by linking the C-terminus of one with lysine 63 of the other. Since UCHL1 is heavily expressed in neurons, antibodies to UCHL1 can be used to identify neurons in histological sections and in tissue culture. The great abundance of this protein in neurons means that it is released from neurons in large amounts following injury or degeneration, so the detection of UCHL1 in CSF and other bodily fluids can be used as a biomarker. UCHL1 was also discovered as a gene mutated in some rare familial forms of Parkinson's disease. Interestingly a common allelic variant of UCHL1, the S18Y polymorphism is actually protective against Parkinson's disease.  It is also a marker for cells of the diffuse neuroendocrine system and their tumors.