The roles of the co-chaperone SGTA/Sgt2, the BAG6 complex and E3 Ubiquitin Ligase RNF126 in cytosolic quality control

Student thesis: Doctoral ThesisDoctor of Philosophy


Eukaryotic cells rely on quality control mechanisms to sustain protein homeostasis by regulating protein folding, targeting and degradation. These mechanisms involve the recognition of exposed hydrophobic regions of membrane proteins that have mislocalized to the cytosol to prevent them from misfolding and aggregation. In mammals, the heterotrimeric BAG6 complex, composed of three proteins, BAG6, UBL4A and TRC35, work closely with the co-chaperone SGTA to triage hydrophobic protein clients ensuring their delivery to the ER or to the proteasomal degradation pathway. Recently, RNF126 has been identified as the BAG6-dependent E3 ligase, which ubiquitinates lysine residues on BAG6 associated substrates. However, the decision-making process in classifying hydrophobic protein clients for degradation or rescue are unclear. Therefore, the characterisation of proteins involved in triage system is critical to understanding the mechanisms of protein sorting. The work presented here provides insights into functions of the co-chaperone SGTA and the E3 ubiquitin ligase, RNF126, within the BAG6 quality control module. It also focuses on the additional role of the co-chaperone SGTA and its yeast homologue Sgt2, which is the interaction with molecular chaperones such as Hsp70 (Ssa1 in yeast) and Hsp90 (Hsc82 in yeast) through the central tetratricopeptide repeat (TPR) domain. The solution structures of the N-terminal dimerisation domain of SGTA and N-terminal zinc finger motif of RNF126 E3 ligase are presented in Chapter 3 and 4 respectively. The interactions of both RNF126 and SGTA with the UBL domains of BAG6 and UBL4A are also characterised. This includes the structural models of complexes of RNF126_NZF and SGTA_NT with UBLs using NMR spectroscopy and HADDOCK (Chapters 3 and 4). Chapter 5 contains the x-ray structures of Sgt2_TPR and its complex with the extreme C-terminal of Ssa1. In addition, it shows that Sgt2_TPR interacts with C-terminal fragments of Ssa1, Hsc82 and Ybr137wp (a protein whose function is yet to be elucidated) in the similar binding mode and with comparable binding affinities. The binding studies were performed using biophysical methods, such as isothermal titration calorimetry and microscale thermophoresis. Together, this work aims to extend our understanding of the quality control mechanism in yeast and mammals, by providing molecular details of the components of the SGTA/BAG6 complex quality control module.
Date of Award2018
Original languageEnglish
Awarding Institution
  • King's College London
SupervisorJames McDonnell (Supervisor) & Rivka Isaacson (Supervisor)

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