Page 4 - T. Protein modification and regulation

[T. Protein modification and regulation-2]

Mammalian Proteaphagy Involves STUB1-Mediated

Ubiquitination, Aggresomal Sequestration, and Autophagic

Degradation of Inactive Proteasomes

Won Hoon Choi¹˙², Taerim Oh¹˙², Min Jae Lee¹˙²˙*

¹Department of Biochemistry and Molecular Biology, Seoul National University College of Medicine, Seoul 03080,

Korea, ²Biomedical Sciences, Seoul National University Graduate School, Seoul 03080, Korea

The 26S proteasome, a self-compartmentalized protease complex, plays a crucial role in protein quality control.
Multiple levels of regulatory systems modulate proteasomal activity for substrate hydrolysis. However, the

destruction mechanism of mammalian proteasomes is poorly understood. We found that inactive 26S proteasomes
are sequestered into the insoluble aggresome via HDAC6- and dynein-mediated transport. These proteasomes

colocalized with the autophagic receptor SQSTM1 and cleared through selective macroautophagy, linking
aggresomal segregation to autophagic degradation. This proteaphagic pathway was counterbalanced with the

recovery of proteasomal activity and was critical for reducing cellular proteasomal stress. Changes in associated
proteins and polyubiquitination on defective proteasomes participated in the targeting mechanism to the

aggresome and autophagosome. The STUB1 E3 Ub ligase specifically ubiquitinated human proteasomes, mainly via
Lys63-linked chains. Genetic and chemical inhibition of STUB1 activity significantly impaired proteasome processing

and reduced resistance to proteasomal stress. These data demonstrate that aggresomal sequestration is the crucial
upstream event for proteasome quality control and overall protein homeostasis in mammals.