Molecular dissection of the ubiquitin-independent degradation signal at the unstructured amino terminal end of human thymidylate synthase

• Main Author: Melo Carlos, Sandra Patricia
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2009
• Subjects: Biochemistry; Molecular biology
• ISBN: 1109545886; 9781109545883

The proteasome regulates the turnover of many cellular proteins. Typically, proteins are targeted to and recognized by the proteasome through the covalent attachment of a small protein called ubiquitin. However, in recent years, several proteasomal substrates have been shown to be degraded efficiently without requirement of this modification, revealing additional yet poorly understood recognition mechanisms, and expanding the possible repertoire of substrates targeted by the proteasome. One of these ubiquitin-independent substrates is the human DNA metabolic enzyme thymidylate synthase (TS). Previous studies showed that the degradation of TS is mediated by an intrinsically disordered 27-residue region at the N-terminal end of the protein, and that this region, in cooperation with an α-helix formed by the next 15 residues, functions as a degron, i.e., it is capable of destabilizing a structurally unrelated protein to which it is N-terminally fused. However, the specific amino acids or chemical characteristics of the N-terminus required for its degron activity have not been identified. The current work uses a combination of biochemical, proteomics and molecular biology techniques to comprehensively characterize the N-terminal degradation signal of human TS. It was found that a free α-amino group and positive charge provided by a basic arginine dipeptide 10 residues away from the N-terminus are distinct signatures of TS's degradation signal. Both elements are required to promote degradation of TS, and together with the other amino acids contained within the first 15 residues, impose directionality of proteolysis from the N-towards the C-terminal end. N-terminal truncation molecules lacking these 15 residues exhibit aberrant proteasomal degradation through an alternative mechanism occurring from the C- towards the N-terminus. The studies presented in this work reveal the molecular basis for the degradation activity exhibited by the unstructured N-terminus of human TS.