Editing Proteolysis (section)

====Lysosome and proteasome====
[[File:Proteaosome 1fnt side.png|thumb|upright=0.6|Structure of a proteasome. Its active sites are inside the tube (blue) where proteins are degraded.]]

The intracellular degradation of protein may be achieved in two ways—proteolysis in [[lysosome]], or a [[ubiquitin]]-dependent process that targets unwanted proteins to [[proteasome]]. The [[autophagy]]-lysosomal pathway is normally a non-selective process, but it may become selective upon starvation whereby proteins with peptide sequence KFERQ or similar are selectively broken down. The lysosome contains a large number of proteases such as [[cathepsins]].
 
The ubiquitin-mediated process is selective. Proteins marked for degradation are covalently linked to ubiquitin. Many molecules of ubiquitin may be linked in tandem to a protein destined for degradation. The polyubiquinated protein is targeted to an ATP-dependent protease complex, the proteasome. The ubiquitin is released and reused, while the targeted protein is degraded.