The ubiquitin-proteasome pathway is an intracellular pathway that allows a cell to digest proteins that are old, and therefore not likely to function well, or proteins that were malformed as they were created. Without the ubiquitin-proteasome pathway tracking proteins that aren't functioning with as much efficacy as newer proteins, a cell would end up wasting precious biological resources, which would have a negative effect on the entire organism, whether that organism is unicellular or multi-cellular. This pathway is ubiquitous — as is the appropriately named "ubiquitin" — and can be found in organisms ranging from unicellular archaebacteria to human beings.
Ubiquitination is a process that, as its name would indicate, happens to most cellular protein molecules. As cellular proteins cycle through metabolic processes or other biological cycles, they are often ubiquitinated with use and cyclic repetition. This is a biological indicator for how old or how used a certain protein is. By marking a protein with ubiquitin chains that indicate how many times a protein has been cycled, a cell has a way of knowing which proteins are breaking down and should be digested, making way for new proteins to be created by the recycled amino acid constituents that result from that breakdown.
For some molecules, ubiquitination can change the function of a protein, much in the same way phosphorylation can. A single instance of ubiquitination doesn't tag a protein for digestion through the proteasome. Poly-ubiquitination, however, marks a protein for degradation in the ubiquitin-proteasome pathway by shunting that poly-ubiquitinated molecule into the proteasome.
The ubiquitin-proteasome pathway is driven by adenosine triphosphate (ATP), a common intracellular source of energy. Enzymes associated with ubiquitination, like E1, E2, E3 and E4, facilitate the ubiquitination of proteins, regardless of whether that protein is tagged only once or becomes poly-ubiquitinated. There are many different variants of specific enzymes within these E1-E4 enzyme families. When the poly-ubiquitinated protein is finally routed to the proteasome, ATP must also be provided for the proteasome to have enough activation energy to break bonds of the protein marked for degradation.
The ubiquitin-proteasome pathway can thus be thought of as a intracellular protein recycler. It breaks apart old or misfolded proteins into smaller units — amino acids or small groups of amino acids — that can then be used by the cell to create new proteins. While poly-ubiquitination is the most common cause of proteasome-driven protein degradation, the proteasome can also break down dysfunctional or misfolded proteins in a ubiquitin-independent manner. Since the proteasome plays a helpful role in the function of the immune system, some intracellular molecules can also be degraded by the proteasome due to antigen presentation as a consequence of intracellular immune system responses.