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[A. Biochemistry/Molecular Biology] A-25

Autophagic Degradation of Inhibited Mammalian

Proteasomes Mediated by Ubiquitination and Aggresomal

Sequestration

Won Hoon Choi¹,³, Seo Hyeong Park¹,²,³, Jiseong Kim¹,²,³, Min Jae Lee¹,²*

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

Korea, ²Department of Biomedical Sciences, Seoul National University College of Medicine, Seoul 03080, Korea,

³BK21 FOUR Biomedical Science Program, Seoul National University College of Medicine, Seoul 03080, Korea

The 26S proteasome is a self-compartmentalized protease complex, one of whose crucial functions is protein quality
control. Multiple layers of regulatory systems elaborately modulate proteasomal activity, i.e., hydrolysis of

polyubiquitinated proteins. However, the mechanism of destruction of mammalian proteasomes is poorly
understood. We found that inactive 26S proteasomes are concentrated into an insoluble aggresome via HDAC6-

mediated retrograde transport. These proteasomes were colocalized with autophagic receptor SQSTM1 in a large
perinuclear inclusion body and were cleared through selective macroautophagy, linking aggresomal segregation to

proteaphagic degradation. This pathway might be counterbalanced with recovery of proteasomal activity and critical
for reducing cellular proteasomal stress. Structural alteration, changes in associated proteins, and polyubiquitination
on inactive proteasomes participated in the targeting mechanism to the aggresome and autophagosome. The CHIP

E3 Ub ligase polyubiquitinated purified human proteasomes in vitro mainly with Lys63-linked polyubiquitin chains.

CHIP silencing significantly delayed aggresome formation by inactive proteasomes. Thus, aggresomal sequestration
and autophagic degradation are crucial for proteasome quality control and overall mammalian protein homeostasis.