Protein degradation and the lysosomal system

This chapter defines the group of intracellular proteases, assisting components, and intracellular compartments that participate in complete degradation of proteins into their constitutive amino acids. Two major proteolytic systems are responsible for most of the intracellular protein degradation: the ubiquitin-proteasome system (UPS) and the lysosomes. The chapter briefly reviews the characteristics of the UPS. The proteasome is a multicatalytic complex with a proteolytic core, known as the 20S proteasome, which in high eukaryotes results from the association of 28 subunits in four rings stacked as a cylinder-like structure. Malfunctioning of the UPS severely impairs cell viability. Lysosomes are single-membrane organelles containing a wide variety of hydrolases, including proteases, lipases, glycosidases, and nucleotidases, which make them able to degrade all kinds of macromolecules. Macroautophagy is quantitatively the most important form of autophagy and also the best characterized as a result of the identification of the autophagy-related genes.