In eukaryotes, the ubiquitin-proteasome pathway (UPP) regulates almost every cellular process
including gene transcription, DNA repair, cell cycle progression, cell development and apoptosis.
Deregulation of this pathway is involved in the pathophysiology of numerous human diseases
including cancer and neurodegenerative diseases. There are two major processes in the ubiquitin-
proteasome pathway: protein ubiquitination/deubiquitination and proteasomal degradation. Our lab is interested in using biochemical, biophysical and cell biological approaches to study the
molecular mechanisms by which the UPP mediates protein degradation and cell signaling
progression. Our study could reveal novel roles of the UPP in the pathogenesis of cancer and
Parkinson disease.
1) The protein degradation machinery of the 26S proteasome. The 26S proteasome is a 2.5
MDa large protein complex that is responsible for degradation of the majority of intracellular
proteins in human cells. The 26S proteasome is consisted of two subcomplexes: the 19S
regulatory complex (also call PA700) and the 20S proteasome. The 19S regulatory complex
(20 subunits) possesses several enzymatic activities that catalyze protein unfolding, substrate
deubiquitination and ATP hydrolysis. The 20S proteasome has a four ring-stacked structure
arranged as αββα. Each α and β ring contains seven different proteins. Three β subunits
have distinct peptidase activities. Access to these catalytic sites is connected with a narrow
substrate translocation channel. Only the 26S proteasome degrades polyubiquitinated
proteins and efficient degradation requires orchestrated actions of all proteasome activities.
We are investigating how substrate unfolding couples with other activities to drive efficient
degradation; how different polyubiquitin chain topologies affect proteasomal degradation;
and how posttranslational modifications regulate proteasomal activity.
2) Cellular regulation by deubiquitination. Protein ubiquitination is catalyzed by a cascade of
enzymatic reactions involving a ubiquitin-activating enzyme (E1), a ubiquitin-conjugating
enzyme (E2) and a ubiquitin-protein ligase (E3). The E3 enzymes control substrate
specificity and more than 600 E3s have been identified in human cells. In cells, protein
ubiquitination and deubiquitination are very dynamic; human cells contain about one
hundred of deubiquitinating enzymes (DUBs) that act to antagonize the process of protein
ubiquitination. Recent studies have shown that protein ubiquitination/deubiquitination are
key mediators for many cellular activities including cell cycle progression and DNA repair.
Usp9x (~290 kDa) is one of the largest deubiquitinating enzymes in human cells. We are
investigating how Usp9x mediates cell signaling pathways in response to growth factor
stimuli and how its deubiquitination activity contributes to cancer cell progression.
3) Aggregation of alpha-synuclein (αSyn) in Parkinson disease (PD). Formation of intracellular
αSyn aggregates (called Lewy Bodies) is a pathological hallmark of PD and several other
neurodegenerative diseases. αSyn mutations were identified in rare familial PD, providing
direct genetic evidence that links αSyn aggregation to PD pathogenesis. αSyn in a highly
soluble protein enriched in neurons. How αSyn turns into insoluble aggregates in PD brains
is not clear, but impaired mitochondrial and proteasomal functions are thought to be the two
major factors that contribute to PD pathogenesis. We are investigating how the UPP mediates
αSyn ubiquitination/degradation/aggregation; how αSyn aggregates impair cellular signaling
pathways. By collaboration with Dr. Eisenmesser in my department, we are using NMR to
probe αSyn aggregation intermediates, which will provide structural insights for understanding the initiation of αSyn aggregation and thus, helps to develop small molecules
that inhibit αSyn aggregation.
Representative Publications:
Representative Publications
Zhang, N-Y., Tang Z. Y. and Liu, C-W., (2008) “alpha-synuclein Protofibrils Inhibit 26S
Proteasome-mediated Protein Degradation, understanding the cytotoxicity of protein protofibrils
in neurodegenerative diseases pathogenesis”. J. Biol. Chem., 283, 20288-20298.
Liu, C-W., Li, X-H., Thompson, D., Wooding, K., Chang, T-L., Tan, Z., Yu H., Thomas, P. J.,
and DeMartino, G. N., (2006) “ATP Binding and ATP Hydrolysis Play Distinct Roles in the
Function of 26S Proteasome.” Molecular Cell, 24, 39-50.
Liu, C-W., Giasson B. I., Lewis K., Lee V. M., DeMartino G. N., Thomas P. J., (2005) “A
Precipitating Role for Truncated α-synuclein and the Proteasome in α-synuclein Aggregation,
Implications for Pathogenesis of Parkinson Disease.” J. Biol. Chem., 280, 22670-22678.
Liu, C-W., Corboy, M., DeMartino, D.N., and Thomas, P.J. (2003) “Endoproteolytic Activity of
Proteasome.” Science 299, 408-411.
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