Final answer:
The E3 ubiquitin ligase is the subunit of the ubiquitin ligase complex that binds to the target protein during ubiquitin-mediated proteolysis, determining its specificity for degradation.
Step-by-step explanation:
During ubiquitin-mediated proteolysis, the subunit of the ubiquitin ligase complex that binds to the target protein is the E3 ubiquitin ligase. The E3 enzyme is responsible for the specificity of protein degradation, determining which protein will be marked for destruction. The ubiquitination process involves several steps, beginning with the activation of ubiquitin and its conjugation to the target protein, facilitated by a series of enzymes: the E1 activating enzyme, the E2 conjugating enzyme, and the E3 ligase. Once the target protein is poly-ubiquitinated, it is delivered to the 26S proteasome, where the unwanted protein is unfolded and degraded into short peptide fragments. The proteolytic activity within the interior of the 20S core proteasome digests the target protein, and the resulting fragments are released into the cytoplasm where they are broken down into amino acids.