Fine equilibrium between protein synthesis and protein degradation is essential for cell survival and function. After initial synthesis, membrane and secretory proteins are modified, folded, and assembled in the endoplasmic reticulum, whereas other proteins are synthesized and processed in the cytosol. Proteins are subject to different quality control systems to minimize aberrant production that could lead to cellular damage. The molecular chaperones help protein folding and stabilization, whereas the ubiquitin-proteasome system (UPS) and lysosomes degrade proteins. The UPS has been increasingly recognized as a major system involved in several biological processes such as cell proliferation, adaptation to stress and cell death.
