Proteins with N-terminal Met, Ser, Ala, Thr, Val, or Gly have half lives greater than 20 hours. Proteins with N-terminal Phe, Leu, Asp, Lys, or Arg have half lives of 3 min or less. Regulatory proteins are degraded at particular phases of the cell cycle, or in response to internal or external signals. Some protein degradation occurs in lysosomes . 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.