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  • Book
    Emily Lawson Egeler.
    Cellular maintenance of protein homeostasis , or proper protein folding, is essential for normal cellular function. Accumulation of misfolded protein can lead to disease and even cell death. The ubiquitin-proteasome system plays a central role in processing cellular proteins for degradation, but little is currently known about how misfolded cytosolic proteins are recognized by protein quality control machinery and targeted for degradation in mammalian cells. In the work presented here, we work with a model protein substrate that is unstable when expressed in cells, but stabilized by addition of a small cell-permeable ligand. Using purified protein, we show that unstable mutants are poorly folded or rapidly sampling unfolded states. The propensity to unfold in vitro correlates with observed instability when expressed in cells. We also show that these substrates are rapidly ubiquitinated en route to degradation at the proteasome in a ligand-dependent manner. We conducted a mass spectrometry screen to identify endogenous protein interactors with the model substrate, which identified a few annotated ubiquitin-proteasome system components. The findings presented herein correlate observed cellular instability to both the propensity of the protein to unfold and the extent of ubiquitination in the ligand-free state. We propose that this protein is a ligand-dependent substrate for the protein quality control machinery, providing an excellent substrate into further query the mechanisms of protein quality control.