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E2/E3-mediated assembly of lysine 29-linked polyubiquitin chains
L D Mastrandrea1, J You, E G Niles
1Department of Biochemistry, School of Medicine and Biomedical Sciences, State University of New York, Buffalo, New York 14214, USA.
The Journal of Biological Chemistry
|September 10, 1999
Summary
Researchers identified enzymes that assemble unanchored polyubiquitin chains, including Lys-29 linkages. These findings suggest distinct pathways for ubiquitin-ubiquitin versus substrate-ubiquitin ligation in cells.
Area of Science:
- Biochemistry
- Molecular Biology
- Cell Biology
Background:
- Polyubiquitin chains, particularly Lys-48 linkages, target proteins for proteasomal degradation.
- Mechanisms of polyubiquitin chain assembly and the functions of non-Lys-48 linkages remain largely unknown.
Purpose of the Study:
- To investigate the enzymatic components responsible for assembling unancharred polyubiquitin chains.
- To characterize the assembly of Lys-29-linked polyubiquitin chains and compare it to Lys-48-linked chains.
Main Methods:
- Reconstitution of polyubiquitin chain assembly using purified ubiquitin-activating enzyme (E1), ubiquitin-conjugating enzyme (E2, UbcH5A), and a ubiquitin-protein ligase (E3) from rabbit reticulocyte lysate.
- Kinetic analysis of diubiquitin synthesis and ubiquitin conjugation to different ubiquitin chain lengths.
Main Results:
- Identified enzymatic components in rabbit reticulocyte lysate capable of assembling unanchored Lys-29-linked polyubiquitin chains.
- The same E3 preparation also catalyzed the assembly of unanchored Lys-48-linked polyubiquitin chains.
- Kinetic studies showed similar Michaelis constants (Km) for acceptor ubiquitin in both Lys-29 and Lys-48 diubiquitin synthesis, with Km values comparable to cellular free ubiquitin concentrations.
Conclusions:
- Distinct enzymatic pathways may exist for ubiquitin-ubiquitin ligation versus substrate-ubiquitin ligation.
- The identified enzymes and their kinetic properties provide insights into the regulation of polyubiquitin chain formation.