The most widely regarded function of ubiquitin is its ability to mark proteins that need to be degraded by the proteasome. It is for this reason that it is occasionally referred to as the molecular “kiss of death” for proteins.
However, the way in which ubiquitination occurs and, more specifically, the way the protein molecules link together can affect its function. This is because the formation of the chain can affect the structure of the protein, exposing or concealing specific sections that are relevant to its function. The various types of ubiquitination are broadly categorized as monoubiquitination and polyubiquitination.
Ubiquitin can be thought of as a “tag” that bonds to substrate proteins that should be degraded by the proteasome. It acts as a signal and, once bound, the protein is usually directed towards apoptosis.
This is not always the case, however, as ubiquitin proteins have a wide spectrum of functionality and play a role in various different body areas. As a result, there are many cellular processes that involve ubiquitin proteins, including:
- Apoptosis (cell death)
- Cell division and multiplication
- Degeneration of neurons and muscular cells
- DNA transcription and repair
- Immune and inflammatory response
- Neural network morphogenesis
- Organelle biogenesis
- Processing of antigens
- Receptor modulation
- Ribosome biogenesis
- Stress response pathway
- Viral infection
The effect on DNA transcription and repair can occur due to the ubiquitination of histones in the DNA. One or several ubiquitin molecules may bind to the histone and, in doing so, change the structure of the chromatin so that the enzymes can access the histone differently. Depending on the specific characteristics of this reaction, this can result in the activation of inhibition of DNA transcription and repair.
Monoubiquitination refers to when a single ubiquitin protein binds to one protein substrate. This type of ubiquitination is thought to be associated with certain cellular processes, including receptor modulation, endocytosis and viral infections.
When this process occurs to several proteins, forming separate pairs, it can be called multi-monoubiquitination. This often serves to mark receptor proteins on the membrane surface to be displaced inside the cell and cause a signal of events. The localization of the cells changes, which often leads to the destruction of the protein in lysosomes.
Polyubiquitination is the process that involves the formation of a chain of ubiquitin molecules on a single lysine of a substrate protein. This usually follows monoubiquitination, but can result in various alternative outcomes.
This type of ubiquitination is responsible for many of the cellular processes and function of ubiquitin, including the immune and inflammatory response and DNA translation and repair.
Similarly to monoubiquitination, ubiquitin chain can also affect the function of receptor proteins on the membrane of cells and lead to cell death in that manner.