All cells recycle. Here, we see actin filaments (red) direct unwanted (malformed, damaged, or toxic) proteins to proteasomes (green). In these barrel-shaped compartments, proteins are chopped up into their basic building blocks, called amino acids, and recycled to make new healthy proteins.
You might wonder what this has to do with disease. Defective recycling is associated with diseases like cancer, neurodegenerative disorders, and muscle wasting. In some cancers—like multiple myeloma—the proteasomes are hyperactive, working overtime to dispose of an accumulation of unfolded proteins. Blocking this disposal system with proteasome inhibitors like bortezomib (Velcade®) puts the cancer cells in such a state of stress that they undergo apoptosis—basically committing suicide.
Proteasome Regulation by ADP-Ribosylation. Cho-Park PF, Steller H. Cell. 2013 Apr 25;153(3):614-27.
NIH support: National Institute of General Medical Sciences