The Terminal Catalytic Complex of Selenocysteine Synthesis and Its Role in Neurodegenerative Disorders

Fidelity of mRNA translation is vital for the structure and function of proteins and for the maintenance of physiological processes in living organisms. Mutations in human transfer RNAs (tRNAs), tRNA modifying enzymes, translational factors, ribosomal proteins, and aminoacyl-tRNA synthetases (AaRSs) and related enzymes cause tumorigenesis, Diamond-Blackfan anemia, type 2 diabetes and a myriad of neurological disorders and diseases. Particularly intriguing is the importance of the integrity of gene translation for the development of the human brain. Recently, four mutations in the human gene encoding the terminal catalytic enzyme of selenocysteine synthesis, O-phosphoseryl-tRNASec:selenocysteinyl-tRNASec synthase (SepSecS), have been causatively linked to the development of severe neurological disorders in children. The affected individuals experienced progressive atrophy in the cerebrum and cerebellum, severe spasticity, profound mental retardation and rarely lived past 12-13 years of age. In spite of the detailed genetic studies and clinical profiles, the mechanism by which these disorders develop and the role SepSecS plays in the process are not clear. Herein, we determined the architecture, stoichiometry and arrangement in solution of the SepSecS-tRNASec binary complex, which is critical for synthesis of selenocysteine and selenoproteins, and hence for the integrity of the human selenoproteome. We also present, for the first time, a detailed biophysical characterization of the pathological SepSecS mutants and propose a rational mechanism that governs the development of brain pathologies. Our results further our understanding of the role of SepSecS in selenoprotein synthesis, extend the list of neurological disorders elicited by protein and enzyme aggregation, and suggest that mutations in components of translational machinery could induce both early and late onset neurological disorders and diseases. The findings presented in this doctoral work will facilitate future in vitro and cell based experiments aimed at deciphering the exact sequence of events that lead to the developmental failures observed in patients harboring detrimental SEPSECS mutations.