Lysosomes contain several different proteases, with acid pH optima, called cathepsins . Lysosomes degrade proteins taken in by endocytosis . In a process called autophagy , a membrane may enclose part of the cytoplasm. Fusion of the resulting vacuole with a lysosome may result in degradation of enclosed cytoplasmic macromolecules. This mechanism does not allow selective degradation of individual proteins.
Most protein degradation involves the proteasome, which carries out selective protein degradation.
Proteasomes are widely distributed in the nucleus and cytosol of eukaryotic cells. The proteasome is a large protein complex, with a 20S sedimentation coefficient. It contains 2 copies each of 14 different polypeptides. 7 a -type proteins form the a rings at the ends of the cylindrical structure. 7 b -type proteins form the central b rings.