UBE2O is a quality control factor for orphans of multiprotein complexes

Area of Science:

• Molecular and Cellular Biology
• Protein Homeostasis
• Ubiquitination Pathways

Background:

• Nascent proteins assemble into multiprotein complexes, requiring precise stoichiometry.
• Mismatches in subunit synthesis lead to unassembled proteins, termed orphans.
• Mechanisms for selective orphan elimination and protein homeostasis are not fully understood.

Purpose of the Study:

• To investigate the mechanism by which unassembled proteins (orphans) are selectively eliminated.
• To identify the role of the ubiquitin-conjugating enzyme UBE2O in targeting protein complex orphans.
• To elucidate how UBE2O maintains protein homeostasis through the degradation of unassembled protein subunits.

Main Methods:

• Identified substrate recognition motifs (basic and hydrophobic patches) on unassembled proteins.
• Demonstrated UBE2O's direct ubiquitination activity without a separate ubiquitin ligase.
• Analyzed UBE2O's role in targeting unassembled α-globin in reticulocytes and ribosomal proteins in nonreticulocytes.

Main Results:

• UBE2O directly recognizes specific structural features on unassembled proteins.
• UBE2O mediates ubiquitination of unassembled α-globin in reticulocytes and ribosomal proteins in nonreticulocytes.
• UBE2O functions as a single protein incorporating both substrate recognition and ubiquitin transfer.

Conclusions:

• UBE2O is a critical, self-contained quality control factor for multiprotein complexes.
• UBE2O selectively targets and ubiquitinates orphans, facilitating their degradation.
• This mechanism is essential for maintaining cellular protein homeostasis.