Proteins are molecular work horses in the cell that perform specific tasks, but it is essential that the timing of protein activities is exquisitely controlled. When proteins have fulfilled their tasks, degradation of these proteins will end processes that are unneeded or detrimental. To control timing, a label – called “ubiquitin” – is attached to unwanted proteins, marking the protein for degradation. Although complex molecular machineries were known to attach ubiquitin, how these machines carry out the labeling process was unknown. Researchers at MPIB, in collaboration with the University of Nevada Las Vegas, have revealed these mechanisms and published the results in the journal Nature.