The proteasomal degradation signal FAT10 modulates substrate stability to accelerate proteasomal degradation

The proteasome machinery regulates the levels of most cellular proteins. However, the factors determining rate of proteasomal degradation remain poorly understood. A rate-limiting step of degradation is the mechanical unfolding of substrates by proteasome ATPases before they enter the core peptidase chamber. Posttranslational modification with the protein FAT10 serves as a proteasomal degradation signal. FAT10 degradation is distinct from the Ubiquitin-proteasome degradation pathway since FAT10 is degraded with the substrate, whereas Ubiquitin is recycled. Hence, both FAT10 and FAT10lyated substrates unfold at the proteasome, and their unfolding kinetics are critical to the degradation rate. Here, we have characterized the thermodynamics and kinetics of unfolding FAT10 and FAT10 conjugated substrate proteins. Our results highlight that fundamental electrostatic interactions of the Ubiquitin fold are absent in FAT10, resulting in a conformationally flexible and malleable native state with low free energy of unfolding. FAT10 substantially modulates substrate stability and induces partially unfolded forms in the substrate to accelerate its unfolding and degradation. The substrate’s energy landscape and the FAT10 binding site determine the change in free energy of the substrate. These results highlight a strong dependence of proteasomal degradation rate on the plasticity of proteasomal signal FAT10. ### Competing Interest Statement The authors have declared no competing interest.