A UbcH5/ubiquitin noncovalent complex is required for processive BRCA1-directed ubiquitination

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Biology

Background:

• Protein ubiquitination is a crucial post-translational modification regulating eukaryotic cell functions.
• The ubiquitination pathway involves Ub-activating (E1), Ub transfer (E2), and Ub ligase (E3) enzymes.
• E2 enzymes, characterized by a conserved alpha/beta-fold core, are central to ubiquitin transfer.

Purpose of the Study:

• To investigate noncanonical ubiquitin binding by the UbcH5 E2 enzyme family.
• To elucidate the structural basis of noncovalent ubiquitin interaction with E2 enzymes.
• To determine the functional consequences of this interaction on polyubiquitin chain synthesis.

Main Methods:

• Solution structure determination of the UbcH5c/ubiquitin noncovalent complex.
• Biochemical assays to assess polyubiquitin chain formation.
• Analysis of ubiquitination reactions catalyzed by UbcH5c in the presence of BRCA1.

Main Results:

• The UbcH5 E2 enzymes bind ubiquitin noncovalently via a surface distinct from the active site.
• This noncovalent interaction facilitates the self-assembly of activated UbcH5c-ubiquitin molecules.
• Self-assembly enhances the processive formation of polyubiquitin chains, particularly in BRCA1-directed ubiquitination.

Conclusions:

• E2 enzymes possess mechanisms beyond their active sites to regulate ubiquitination.
• Noncovalent ubiquitin binding and self-assembly represent a novel regulatory mode for E2 enzymes.
• This finding provides new insights into the regulation of polyubiquitin chain synthesis and its role in cellular processes, including cancer-related pathways involving BRCA1.