Breaking down unneeded proteins is accomplished by the orderly action of several multiprotein complexes. At the heart of this process is a multiprotein complex called the proteasome. These machines of destruction consist of a tunnel-like core with a cap at either or both ends. The core is formed by four stacked rings surrounding a central channel that acts as a degradation chamber. The caps recognize and bind to proteins targeted by the cell for destruction, then use chemical energy to unfold the proteins and inject them into the central core, where they are broken into pieces. This is a fundamental kind of machine that has been highly conserved during evolution. Some form of it is found in organisms ranging from simple bacteria to humans.
Credit or Source: Office of Biological and Environmental Research of the U.S. Department of Energy Office of Science. science.energy.gov/ber/
Genomes to Life Program Roadmap, April 2001, DOE/SC-0036, U.S. Department of Energy Office of Science. (website)