What is the primary mechanism by which ubiquitin marks proteins for degradation in the proteasome pathway?

A Ubiquitin binds to the protein and recruits an E3 ligase
B Ubiquitin is attached to the protein through an isopeptide bond ✓
C Ubiquitin competes with other proteins for binding sites on the proteasome
D Ubiquitin directly unfolds the protein and feeds it into the proteasome

Explanation

The primary mechanism by which ubiquitin marks proteins for degradation in the proteasome pathway involves the attachment of ubiquitin to the protein through an isopeptide bond, typically between the C-terminal glycine of ubiquitin and a lysine residue on the target protein.

Source

Alberts et al., Molecular Biology of the Cell
Protein Metabolism Biochemistry Protein Degradation Medium NURSING Confidence: high