Break it down!
The proteins that dispose of proteins
Cells make proteins, but they must also dispose of the unwanted, damaged or misfolded ones. In eukaryotic organisms, some protein degradation happens in special bags of enzymes called lysosomes inside the cells, and some in a big protein complex that floats free in the cell, the proteasome. The proteasome stops proteases attacking everything in the cell by containing them in a protein assembly.
The proteins destined for destruction are tagged with a small protein named ubiquitin, which is found in all eukaryotic cells and has an identical sequence in all animal species. Enzymes recognise proteins that are no longer required and tag them.
There is also a more drastic process of self-digestion: programmed cell death, or apoptosis. This involves the activation of a whole set of enzymes, mainly proteases, that degrade large molecules and destroy the structure inside a cell. Apoptosis is triggered by various kinds of cell damage, as cells can be sacrificed for the greater good of the organism. It also occurs in development when there is large-scale cell loss (eg when the webbing between a growing fetus’s fingers disappears).