Cells must dispose of unneeded proteins but spare others, requiring precise discrimination. Disposal usually involves ubiquitin tagging for delivery to proteasomes, the site of destruction. Coffino recognized alternate ways to tag proteins for destruction. He and his colleagues discovered that ornithine decarboxylase (ODC) contains a 37–amino acid region responsible for ODC’s degradation. Removing this so-called degron stabilizes ODC, while adding it to other proteins destabilizes them in a broad variety of organisms.
ODC’s degron couples two properties required for protein disposal: It attaches to proteasomes, and it provides an unstructured region big enough to thread into the proteasome region, an ATPase ring, that actively reels in the target, thrusting it toward the site of destruction.
This understanding made it possible to design novel target substrates to systematically test hypotheses about the proteasome’s function. For example, Coffino’s work demonstrated that substrate mechanical stability determined how long it takes to unwind, dismantle, and destroy a protein. Other experiments showed that a viral sequence, which frustrates immune system attack, impairs the grip of the proteasome ATPase ring on substrates. Follow-up work found that similar polypeptides frustrated bacterial ATP-dependent proteases, indicating that functional properties have been conserved for protease translocation machines across a broad swath of biology.
At Rockefeller, Coffino is continuing his studies of ATPase motors, augmenting biochemistry with single-molecule analysis to better understand mechanisms by which these motors impel their substrates, and applying these insights to fibrolamellar hepatocellular carcinoma, a pediatric liver cancer.