Observation of an E2 (Ubc9)-homodimer by crystallography

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Cell Biology

Background:

• Post-translational modifications by small ubiquitin-like modifiers (SUMO) regulate vital cellular functions.
• Aberrant SUMOylation is linked to severe diseases.
• Ubc9 (SUMO E2 enzyme) interaction with SUMO is crucial for poly-SUMO-2/3 chain formation.

Purpose of the Study:

• To elucidate the structural basis of poly-SUMO-2/3 chain formation.
• To investigate the role of Ubc9 homodimerization in SUMOylation.
• To characterize the interaction between Ubc9, RWD domain, and other SUMOylation factors.

Main Methods:

• X-ray crystallography to determine the structure of a Ubc9-RWD domain heterotrimer.
• Solution NMR studies to confirm Ubc9 homodimerization in solution.
• Site-directed mutagenesis and biochemical assays to assess the functional significance of Ubc9 dimerization.

Main Results:

• The crystal structure of an asymmetric Ubc9 homodimer complexed with the RWD domain of RWDD3 was solved.
• The Ubc9 homodimer interface involves the N-terminal region and a catalytic Cys-proximal surface.
• Biochemical data support the functional importance of the Ubc9 homodimer in poly-SUMO chain formation.

Conclusions:

• The identified Ubc9 homodimer structure provides a missing mechanistic link for poly-SUMO chain assembly.
• This structural insight is crucial for understanding SUMOylation regulation and its role in disease.
• The RWD domain acts as an interaction module facilitating Ubc9 dimerization.