Do you know that cells mark certain proteins for “death”? I placed death in quotations because proteins are not alive, so they don’t die. Rather, they are degraded or broken into smaller units. However, “marked for death” is certainly a more catchy title than “marked for degradation”. Don’t you agree?
Anyway, our cells are constantly synthesizing proteins, exporting them, as well as degrading them. Since proteins form structures as well as act as enzymes and information molecules in cells, their amounts have to be constantly regulated. There should not be too much or too little of them inside the cell. Getting rid of excess proteins is thus a carefully controlled process.
The first step in the process is to mark the potential candidate for degradation. This is done by attaching ubiquitin molecules at several points in the protein where the lysine residues are found. Ubiquitination is the mark of “death”. Once marked, then large complexes of protein enzymes called proteasomes attack and break the protein into small pieces. Ubiquitin is thus the marker and proteasomes are the executioner in the “death” of a protein molecule. After the process, ubiquitin detaches and is recycled for future use.
So, if you are a protein molecule, beware of ubiquitin!