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Mystery Rays from Outer Space
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Intracellular proteins have to be degraded, more or less at the same rate as new proteins are produced (or the cell would eventually burst). On the other hand, you can’t go about degrading proteins willy-nilly. There are vast and complex systems for identifying proteins that should be destroyed, tagging them, and then moving them into a controlled destruction chamber.
The most important of these systems is the ubiquitin-proteasome degradation pathway. Proteins that are destined for destruction are tagged with a chain of ubiquitin molecules.1 There are multiple steps in this pathway, in which ubiquitin is prepared for tagging, target proteins are identified, and ubiquitin is transferred from the activating components to the targeted protein.
Target proteins are destroyed when a chain of ubiquitin molecules (head to tail) are attached to them. An unanswered question has been how this works. Is the ubiquitin chain formed first, and then transferred to the target en bloc? Or are single ubiquitin transferred one at a time, sequentially, first to the target protein and then to the previously-attached ubiquitins? The problem has been that the process …