Further components like the ERdj proteins are omitted for simplicity. extracellular proteins, multiple pathways to their formation have evolved. However, the advantages that disulfide bonds impart to these proteins come at a high cost to the cell. Very recent reports have shed light on how the cell can deal with or even exploit the side reactions of disulfide bond formation to maintain homeostasis of the ER and its folding machinery. Introduction The extracellular environment where cell surface and secreted proteins must function is generally quite different than that of the cytosol, particularly in terms of redox potential and certain ion concentrations. Thus a major obstacle for the eukaryotic cell Monomethyl auristatin F (MMAF) is to not only fold and assemble complex hetero-oligomeric proteins from extended polypeptide chains, but to do so under cellular conditions where the outcome can be monitored and the protein Monomethyl auristatin F (MMAF) will function in its requisite location. The solution to this challenge was the development of the endoplasmic reticulum (ER), a vast reticular organelle present in all nucleated eukaryotic cells. The ER is similar to the extracellular environment in terms of its ion concentrations and Rabbit polyclonal to STK6 oxidizing capabilities; but in contrast provides dedicated folding, oxidation, and quality control machineries. The biosynthesis of secretory proteins is closely aided and monitored by a vast array of resident ER proteins that comprise the quality control apparatus of ER, which allows only properly matured proteins to transit to the Golgi [1;2] Some quality control measures exist in post-ER compartments, but they become much more limited [3;4]. Thus, the ER must set and uphold high standards for protein quality control that can function under the complicating conditions of high concentrations of unfolded polypeptide chains, significantly differing clients, and changing metabolic needs. Protein folding, post-translational modification and quality control in the ER In addition to the free energy-driven conformational folding of the newly synthesized polypeptide chains (hydrogen bond formation and burying hydrophobic residues), a number of additional reactions take place in the ER that can influence the folding and the final state of a protein, including N-linked glycosylation (reviewed in Chapter XX of this issue), oligomerization, and formation of disulfide bonds (Figure 1). Their facilitation by resident ER chaperones and enzymes aids folding by restricting off pathway possibilities and accelerating slow folding reactions like disulfide bridge formation or peptidyl-prolyl isomerization [2]. While regulating steps on the pathway to the biologically active state, they also provide means to detect proteins that are not completely or properly matured (Figure 1). Open in a separate window Figure 1 Folding reactions occurring in the ER and features recognized by ER quality control componentsFolding and oligomerization take place in the ER and features that occur on nonnative structures are identified by various chaperones and folding enzymes, which serve to prevent incompletely folded or assembled proteins from moving further along the secretory pathway. (A) Exposed hydrophobic regions (yellow) that will eventually be buried upon folding or oligomerization are often recognized by the Hsp70 chaperone BiP (red). (B) The lectins calreticulin Monomethyl auristatin F (MMAF) (blue) Monomethyl auristatin F (MMAF) and calnexin bind to sugar moieties possessing one terminal glucose residue (grey hexagon), which can be found on incompletely folded glycoproteins. (C) PDIs (green) form mixed disulfide bonds with free thiol groups to catalyze disulfide bond formation, reduction, or isomerization. Disulfide bond formation in protein folding and oligomerization Due to their covalent nature, disulfide bonds can have profound effects on the folding pathway and the stability of a protein, thus increasing its suitability for existence in the extracellular milieu. Disulfide bonds can stabilize a protein by reducing the entropy of the unfolded state [5]. Furthermore, they can facilitate the path to the native state if they link parts of a protein that must come into contact early during a folding reaction and can make unfolding less likely if they occur in particularly labile parts of a protein [5]. During the folding of secretory pathway proteins, native intra- and inter-molecular disulfide bonds can form co-translationally, well before more C-terminal regions of the protein have entered the ER [6]. This can occur when the bonds form between adjacent cysteines [7;8] or when the protein is composed of.