All intracellular proteins and many extracellular proteins are continually “turning over;” ie,
they are being hydrolyzed to their constituent amino acids and replaced by new synthesis.
Although the continual destruction of cell proteins might seem wasteful, this process serves
several important homeostatic functions. Individual proteins in the nucleus and cytosol, as
well as in the endoplasmic reticulum (ER) and mitochondria, are degraded at widely
differing rates that vary from minutes for some regulatory enzymes to days or weeks for …