Final answer:
The post-translational regulator that does not target proteins for degradation but modifies interaction abilities is SUMO, essential for various cellular processes without proteolysis.
Step-by-step explanation:
The post-translational regulators found in all eukaryotic cells, which are covalently attached to lysine residues in target proteins by a specific three-enzyme conjugation system, are crucial in regulating protein function. The correct regulator that fits the description provided – not targeting proteins for degradation but altering the protein's ability to interact with other proteins – is SUMO (Small Ubiquitin-like Modifier). Unlike ubiquitin, which typically mark proteins for destruction by the proteasome, SUMO modification can influence the subcellular localization, stability, or activity of proteins, allowing for various cellular responses without the irreversible process of proteolysis.