Proteasome
Proteasome is a highly organized protease complex located in the cytosol as well as
the eukaryotic cell nucleus comprising a catalytic 20S core particle (CP) and two 19S
regulatory particles (RP), which together form the 26S structure. The 26S
proteasome is responsible for the degradation of most ubiquitylated proteins
through a multistep process involving recognition of the polyubiquitin chain,
unfolding of the substrate, and translocation of the substrate into the active site in
the cavity of the CP.
In eukaryotic cells, the ubiquitin–proteasome system (UPS) controls almost all basic
cellular processes –such as progression through the cell cycle, signal transduction,
cell death, immune responses, metabolism, protein quality control, and development
–by degrading short-lived regulatory or structurally aberrant proteins. Proteins
destined for degradation are modified by a small degradation label called ubiquitin
(Ub) modified by a series of ubiquitination synergies. Repeated rounds of ubiquitin
conjugation lead to the formation of a polyubiquitin chain on the target protein. The
polyubiquitin chains with four or more ubiquitin are targeted by the 26S proteasome
and the target protein is degraded into an oligopeptide. Concomitantly, the
polyubiquitin chains are disassembled by deubiquitylating enzymes, which cleave the
ubiquitin molecules from the strand to allow ubiquitin recirculation.
