Structural insights into the chaperone-dependent ubiquitin/proteasome protein degradation pathway

Abstract

Maintaining the integrity of the proteome is essential for cell viability. The native state of proteins, although energetically favoured, is in precarious balance with destabilizing mutations and stress conditions that perturb cell proteostasis and trigger aging, pathological states, and even cell death. Maintenance of protein homeostasis includes parallel and interconnected strategies that focus on folding, degrading or accumulating potentially dangerous species in different compartments. Molecular chaperones not only prevent misfolding of their substrates, but can also drive them towards degradation. In the case of chaperones Hsp70 and Hsp90, this is achieved through the co-chaperone CHIP, which can act as an E3 ligase and direct the Hsp70:substrate complex to substrate degradation. Using electron microscopy and image processing, we characterized the mechanism of Hsp40-mediated substrate transfer to Hsp70. Using the same techniques and a chimeric p53 protein as substrate, we characterized the complex formed with Hsp70, the interaction between Hsp70 and its co-chaperone CHIP, and the complex formed by the chaperone, the co-chaperone and the substrate. A similar approach was used to describe the degradation pathway controlled by chaperone Hsp90. All this structural information allows us to propose a model of chaperone interaction with the co-chaperone CHIP and substrate ubiquitination in the ubiquitin/proteasome pathway.