Investigation into the role of Secisbp2 in re-coding of UGA codons in selenoproteins by global ribosomal footprinting

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Selenoproteins are essential for mammals. They contain the rare amino acid selenocysteine (Sec). Sec is the first known natural expansion of the genetic code. Incorporation of Sec in proteins requires the re-coding of a UGA (stop) codon. For re-coding, selenoprotein mRNAs contain the selenocysteine insertion sequence (SECIS) element in the 3(prime)-untranslated region. Mutations in SECIS-binding protein 2 (SECISBP2) in human lead to congenital defects of selenoprotein biosynthesis entailing neurological, endocrinological, and immunological symptoms. In the first funding period, we have generated and studied mouse models with mutations in Secisbp2. We have found that Secisbp2 does not only stimulate UGA re-coding, but carries an unexpected function by stabilizing selenoprotein mRNAs. In the next funding period, we will analyze selenoprotein biosynthesis on a global scale in unusual detail by using Ribosomal Profiling. We will follow four aims: (1) Determine the re-coding efficiency at UGA codons in the presence of not of Secisbp2. Using our novel mouse models, we will determine the influence of Secisbp2 on translation of UGA as Sec. (2) Do point mutations in Secisbp2 (targeting the SID-domain or L7Ae domain) alter the re-coding efficiency at UGA? This approach will allow us to understand how pathogenic point mutations affect Secisbp2 functions. (3) STOP does not equal STOP: What is the difference between stalling ribosomes lacking Secisbp2 or tRNA(Sec), respectively, with respect to UGA re-coding and mRNA surveillance pathways like Nonsense-Mediated Decay and No-Go Decay? (4) What is the exact mechanism of mRNA degradation, if Secisbp2 or tRNA(Sec) is lacking? This question will be approached by genetic manipulation in a hepatocyte cell model. We expect to gain fundamental insights into current questions of translation of non-canonical amino acids as well as into the normal role of No-Go Decay in mammals.