The relationship between the intracellular stability of tyrosine hydroxylase and the neurodegeneration on Parkison's disease

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The abnormality of ubiquitin-proteasome system caused by the mutations of alpha-synuclein has been implicated in the pathogenesis of Parkinson's disease. Recently, tyrosine hydroxylase (TH) was found to be a substrate for conjugation to ubiquitin in a reconstituted in vitro system and to be partially degraded by proteasomes in a reticulocyte lysate system. The N-terminus of TH is supposed to be located on the surface of the molecule, and it is presumed to possess flexible conformation susceptible to proteases such as calpain. Therefore, the N-terminus of TH is thought to be a target region for intracellular proteolysis such as occurs in the ubiquitin-proteasome system. Collectively, these results suggest that the N-terminus of TH might play a critical role in the stability of the TH molecule as well as in the regulation of TH catalytic activity in cells.In this research, an experimental scheme was designed in order to prove the influence of the N-terminus on the stability of the TH molecule. At first, we clarified that the deletion of the N-terminus region augmented the intracellular stability of the human TH type 1 molecule and that the accumulation of dopamine in the cells producing the N-terminus-deleted mutants can be attributed to augmented stabilization of the TH molecule. Moreover, computer-assisted analysis of the spatial configuration of human TH type 1 identified 5 newly recognized PEST motifs, the target sequence for proteasome, and one of which was located in the N-terminus sequence of Met^1-Lys^<12>. We believe that this study provides some information to understand the stability of TH protein in mammalian cells and/or the pathogenesis of Parkinson's disease.