Poster Presentation The 47th Lorne Conference on Protein Structure and Function 2022

A biosensor to gauge protein homeostasis resilience differences in the nucleus compared to cytosol of mammalian cells   (#243)

Candice B Raeburn 1 , Angelique R Ormsby 1 , Chloe A Gerak 1 , Christian Makhoul 1 , Nagaraj S Moily 1 , Dezerae Cox 1 , Simon Ebbinghaus 2 , Alex Dickson 3 , Gawain McColl 4 , Danny M Hatters 1
  1. Universitiy of Melbourne, Parkville, VIC, Australia
  2. Physical and Theoretical Chemistry, Germany and Braunschweig Integrated centre of systems biology, Braunschweig, Germany
  3. Biochemistry and Molecular Biology , Michigan state university , East Lansing , Michigan, USA
  4. Melbourne Dementia Research Centre , Florey Institute of Neuroscience and Mental Health and University of Melbourne , Melbourne , Victoria , Australia

Protein homeostasis involves chaperones and other quality control machinery that guard proteins from inappropriate aggregation, which is a hallmark of neurodegenerative diseases. How the systems remain buffered under stress and in different cellular compartments remains incompletely understood.  We applied a biosensor system to explore how well quality control machinery, including the Hsp40 and Hsp70 proteins DNAJB1 and HSPA1A protects the misfolding and aggregation of a biosensor construct in the nucleus and cytosol of HEK293T cells, and what impact mutant huntingtin exon 1 protein (Httex1) aggregation has on these locations as well as regulating the activity of Hsp40 and Hsp70.  The biosensors read out measures of chaperone-holdase activity and inappropriate protein aggregation and were targeted to the nucleus with a nuclear localization sequence (NLS) or the cytosol with a Nuclear Export Sequence (NES). We found the biosensor was less protected from engaging with quality control, and inappropriate aggregation, in the nucleus compared to the cytosol and that these effects could be regulated by Hsp70-related machinery. Overexpressed NLS versus NES-tagged biosensors could sequester the endogenous pools of HSPA1A and DNAJB1 by enriching their levels in the nucleus or cytosol respectively.  Aggregation of mutant huntingtin exon 1 protein (Httex1) in the cytosol depleted the engagement of quality control with the biosensor in the cytosol, and appeared to also deplete the overall levels of DNAJB1 and HSPA1A in the cell, and particularly in the nucleus.  Unexpectedly, Httex1 aggregation also increased the apparent engagement of the biosensor with quality control machinery in the nucleus suggesting an independent implementation of “holdase” activity to DNAJB1 and HSPA1A.  Collectively these results suggest that proteostasis stress can trigger a rebalance of chaperone supply in different subcellular compartments through a dynamic network involving different chaperone-client interactions.