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UFD4 lacking the proteasome-binding region catalyses ubiquitination but is impaired in proteolysis
Youming Xie1, Alexander Varshavsky
1Division of Biology, California Institute of Technology, Pasadena, California 91125, USA.
Abstract:
The ubiquitin system recognizes degradation signals of protein substrates through E3-E2 ubiquitin ligases, which produce a substrate-linked multi-ubiquitin chain. Ubiquitinated substrates are degraded by the 26S proteasome, which consists of the 20S protease and two 19S particles. We previously showed that UBR1 and UFD4, two E3 ligases of the yeast Saccharomyces cerevisiae, interact with specific proteasomal subunits. Here we advance this analysis for UFD4 and show that it interacts with RPT4 and RPT6, two subunits of the 19S particle. The 201-residue amino-terminal region of UFD4 is essential for its binding to RPT4 and RPT6. UFD4(DeltaN), which lacks this N-terminal region, adds ubiquitin to test substrates with apparently wild-type activity, but is impaired in conferring short half-lives on these substrates. We propose that interaction of a targeted substrate with the 26S proteasome involves contacts of specific proteasomal subunits with the substrate-bound ubiquitin ligase, with the substrate-linked multi-ubiquitin chain and with the substrate itself. This multiple-site binding may function to slow down dissociation of the substrate from the proteasome and to facilitate the unfolding of substrate through ATP-dependent movements of the chaperone subunits of the 19S particle.
Insights
The yeast E3 ligase UFD4 binds to the 26S proteasome via RPT4 and RPT6 subunits. This interaction is crucial for targeting proteins for degradation, ensuring their short half-lives.
Area of Science:
- Cellular Biology
- Biochemistry
- Molecular Biology
Background:
- The ubiquitin-proteasome system (UPS) is essential for protein homeostasis.
- E3 ubiquitin ligases target substrates for degradation by the 26S proteasome.
- Specific E3 ligases interact with proteasomal subunits.
Purpose of the Study:
- To investigate the interaction between the E3 ligase UFD4 and the 26S proteasome.
- To identify the specific proteasomal subunits involved in UFD4 binding.
- To elucidate the role of UFD4-proteasome interaction in protein degradation.
Main Methods:
- Yeast two-hybrid assays to study protein-protein interactions.
- Co-immunoprecipitation to confirm binding.
- Analysis of ubiquitination and protein half-life assays using wild-type and mutant UFD4.
Main Results:
- UFD4 interacts with RPT4 and RPT6, subunits of the 19S regulatory particle of the 26S proteasome.
- The N-terminal 201 residues of UFD4 are essential for binding to RPT4 and RPT6.
- While UFD4 lacking the N-terminus can ubiquitinate substrates, it is impaired in conferring short half-lives.
Conclusions:
- The interaction between UFD4 and the 19S proteasome is mediated by specific subunits (RPT4, RPT6) and UFD4's N-terminus.
- This interaction is critical for efficient substrate degradation and regulation of protein half-life.
- A multi-site binding model is proposed for substrate recognition and processing by the 26S proteasome.