Maintenance of homeostasis in the ERis important for normal cell function. Perturbation of the unique environment within the lumen of the ER (eg, by changes in ER Ca2+, alterations of redox status, exposure to various toxins or some viruses), can lead to reduced protein folding capacity and the accumulation of misfolded proteins. The accumulation of misfolded proteins in the ER is referred to as ER stress. The unfolded protein response (UPR) is a mechanism within cells which senses the levels of misfolded proteins and activates intracellular signaling mechanisms to restore ER homeostasis. The UPR is initiated by ER stress sensors, which are transmembrane proteins embedded in the ER membrane. Their activation causes three principal effects: (1) transient inhibition of translation so that the synthesis of new proteins is reduced, (2) induction of transcription to increase the expression of ER chaperones, and (3) increased synthesis of proteins involved in the degradation of misfolded ER proteins (discussed later). Thus, the UPR increases the ER folding capacity and prevents a buildup of unproductive and potentially toxic protein products, as well as promoting other responses to restore cellular homeostasis. However, if impairment of folding persists, cell death pathways (apoptosis) are activated. A more complete understanding of the UPR is likely to provide new approaches to treating dis eases in which ER stress and defective protein folding occur (Table1).
Table1. Some Conformational Diseases That Are Caused by Abnormalities in Intracellular Transport of Specific Proteins & Enzymes due to Mutations
Proteins that misfold in the ER are degraded by the ERAD pathway (Figure 1). This process selectively transports both luminal and membrane proteins back across the ER (retrograde translocation or dislocation) into the cytosol where they are degraded in proteasomes. Chaperones present in the lumen of the ER (eg, BiP) help to target misfolded proteins to proteasomes.
Fig1. Simplified scheme of the events in ERAD. An ER target protein which is misfolded either in the lumen or in the membrane undergoes retrograde transport into the cytosol via a translocon. Chaperone proteins (CP) target misfolded proteins for retrotranslocation. As the protein enters the cytosol, it is ubiquinated by ubiquitin ligases (UL), pulled out of the membrane by P97, an ATPase, and delivered to the proteasome. Inside the proteasome it is degraded to small peptides that may have several fates after exit.
The assembly of the retrotranslocation translocon comprising a number of proteins is initiated by recognition of misfolded proteins by chaperones and adaptor proteins. As ret rotranslocation occurs, misfolded proteins are polyubiquinated by ubiquitin ligases on the cytosolic side, and are then pulled from the membrane by P97 (also called Vcp), an ATPase and delivered to the proteasome for degradation (see Figure1).
Ubiquitin Is a Key Molecule in Protein Degradation
There are two major pathways of protein degradation in eukaryotes. One involves lysosomal proteases and does not require ATP, but the major pathway involves ubiquitin and is ATP-dependent. The ubiquitin pathway is particularly associated with disposal of misfolded proteins and regulatory enzymes that have short half-lives. Ubiquitin is known to be involved in diverse important physiologic processes including cell cycle regulation (degradation of cyclins), DNA repair, inflammation and the immune response, muscle wasting, viral infections, and many others. Ubiquitin is a small (76 amino acids), highly conserved protein that tags various proteins for degradation in proteasomes. The mechanism of attachment of ubiquitin to a target protein (eg, a misfolded form of cystic fibrosis transmembrane conductance regulator [CFTR], the protein involved in the causation of cystic fibrosis).
Ubiquitinated Proteins Are Degraded in Proteasomes
Polyubiquitinated target proteins enter proteasomes located in the cytosol. Proteasomes are protein complexes with a relatively large cylindrical structure and are composed of four rings with a hollow core containing the protease active sites, and one or two caps or regulatory particles that recognize the polyubiquinated substrates and initiate degradation. Target proteins are unfolded by ATPases present in the proteasome caps and pass into the core to be degraded to small peptides, which then exit the proteasome to be further degraded by cytosolic peptidases. Both normally and abnormally folded proteins are substrates for the proteasome, which can hydrolyze a wide variety of peptide bonds. Liberated ubiquitin molecules are recycled. The proteasome plays an important role in presenting small peptides produced by degradation of various viruses and other molecules to MHC class I molecules, a key step in anti gen presentation to T lymphocytes.
