Proteasomes recycle amino acids by degrading damaged, misfolded or unneeded proteins into reusable components for new protein synthesis and quality control.
Proteins selected for degradation are commonly marked with a chain of ubiquitin molecules.
The proteasome recognizes the tag, removes and recycles ubiquitin, unfolds the target protein and threads it into a proteolytic core.
The protein is cut into short peptides, which can be hydrolysed to amino acids for new synthesis or metabolism.
Selective degradation removes damaged or misfolded proteins and can rapidly lower the abundance of regulatory proteins. Protein concentration therefore depends on both synthesis and breakdown.