The protein ubiquitination pathway plays a major role in the degradation of short-lived or regulatory proteins involved in a variety of cellular processes, including cell cycle, cell proliferation, apoptosis, DNA repair, transcription regulation, cell surface receptors and ion channels regulation, and antigen presentation.The degradation of proteins via the protein ubiquitination pathway involves two successive steps:
1. conjugation of multiple ubiquitin moieties (Ub) to the target protein. 2. degradation of the polyubiquitinated protein by the 26S proteasome complex.
The first step consists of a highly organized cascade of enzymatic reactions, which require three types of enzymes, ubiquitin-activating enzyme (E1), ubiquitin-conjugating enzyme (E2) and ubiquitin-protein ligase (E3). First, Ub is activated by an ATP-dependent reaction and conjugated to a cysteine residue of E1. Next, Ub is transferred to a similar cysteine residue of E2 and finally, Ub is transferred directly or through the participation of E3 to the target protein. The ubiquitin-protein ligases (E3) are grouped in two main families: E3-HECT and E3-RING. In reactions mediated by E3-HECT, Ub is first transferred from the E2 enzyme to the E3-HECT and then to the target protein. In E3-RING-mediated reactions, E3-RING functions as an adaptor protein and Ub is transferred directly from the E2 enzyme to the target protein. Cycles of this first step link additional Ub to lysine residues within Ub added previously. The polyubiquitin chain is recognized by the downstream 26S proteasome complex leading to the second step of the protein ubiquitination pathway: degradation of the polyubiquitinated proteins.
The proteasome is a large mutlicatalytic complex, localized in the nucleus, the endoplasmic reticulum, and the cytoplasm of the cells. It consists of two parts, the 20S core particle (CP) and the 19S regulatory particle (RP or PA700). The 20S core is a cylinder composed of four stacked rings, which carries the catalytic activity. Each extremity of the 20S core is capped by a 19S RP, which is involved in the recognition, binding and unfolding of the polyubiquitinated target proteins. After protein degradation, Ub is released and recycled owing to the activity of deubiquitinating enzymes (DUB). DUBs can also perform editing by removing ubiquitin chain from mistakenly tagged proteins and therefore inhibiting their proteolysis.
For proteasomal degradation, ubiquitin chains are linked through K48 in ubiquitin, but alternate linkages have been shown to be involved in DNA repair, endocytosis of cell surface receptors, and cell apoptosis.
Upon interferon-γ induction, an alternative proteasome activator (PA28), made up of different but closely related subunits, associates with the 20S core to form the immunoproteasome, which has been implicated in the processing of MHC class I antigen.