the lysosomal membrane. Chaperone-mediated direct transport resembles the process of protein import to various other cellular organelles: hsc73 binds to the protein and assists in its transport through the lysosomal membrane into the lumen, where it is finally degraded. Chaperone-mediated autophagy is responsible for the degradation of approximately 30% of cytoplasmic proteins in organs such as the liver and kidney.
In addition to the lysosomal pathway of protein degradation, cells have an ability to destroy proteins without involvement of lysosomes. Such process occurs within large cytoplasmic or nuclear protein complexes called proteasomes. They represent
FIGURE 2.23 • Electron micrograph of autophagosomes in a hepatocyte. This electron micrograph shows several autophagosomes containing degenerating mitochondria. Note the surrounding lysosomes that have been stained with acid phosphatase.
12,600. (Courtesy of Dr. William A. Dunn, Jr.)
ATP-dependent protease complexes that destroy proteins that have been specifically tagged for this pathway. Proteasomemediated degradation is used by cells to destroy abnormal proteins that are misfolded, denaturated, or contain abnormal amino acids. This pathway also degrades normal short-lived regulatory proteins that need to be rapidly inactivated and degraded such as mitotic cyclins that regulate cell-cycle progression, transcriptional factors, tumor suppressors, or tumor promoters.
Proteins destined for proteasome-mediated degradation need to be recognized and specifically tagged by the polyubiquitin chain.
Degradation of a protein in the proteasome-mediated pathway involves two successive steps:  Polyubiquitination, in which proteins targeted for destruction are repeatedly tagged by covalent attachments of a small (8.5-kilodalton) protein called ubiquitin. The tagging reaction is catalyzed by three ubiquitin ligases called ubiquitin-activating enzymes E1, E2, and E3. In a cascade of enzymatic reactions, the targeted protein is first marked by a single ubiquitin molecule. This creates a signal for consecutive attachment of several other ubiquitin molecules, resulting in a linear chain of ubiquitin conjugates. A protein target for destruction within the proteasome must be labeled with at least four ubiquitin molecules in the form of a polyubiquitin chain that serves as a degradation signal for proteasome complex.
 Degradation of the tagged protein by the 26S proteasome complex. Each proteasome consists of a hollow cylinder, shaped like a barrel, containing a 20S core particle (CP) that facilitates the multicatalytic protease activity in which polyubiquitinated proteins are degraded into small polypeptides and amino acids. On both ends of the CP cylinder are two 19S regulatory particles (RPs); one RP that forms the lid of the barrel recognizes polyubiquitin tags, unfolds the protein, and regulates its entry into the destruction chamber. The RP on the opposite side (on the base) of the barrel releases short peptides and amino acids after degradation of the protein is completed. Free ubiquitin molecules are released by de-ubiquitinating (DUB) enzymes and recycled. (Fig. 2.24).
Two groups of pathologic conditions are associated with the malfunction of proteasome-mediated degradation. The frst group of diseases results from a loss of proteasome function because of mutations in the system of ubiquitinactivating enzymes. This leads to a decrease in protein degradation and their subsequent accumulation in the cell cytoplasm (e.g., in Angelman syndrome and Alzheimer’s disease). The second group of diseases results from an accelerated degradation of proteins by overexpressed proteins involved in this system (e.g., infections with human papilloma virus). The recent discovery of specifc proteasome inhibitors holds promise for treatment of cancers and certain viral infections.