Protein folding and quality control in the endoplasmic reticulum

Abstract

The endoplasmic reticulum (ER) is a highly versatile protein factory that is equipped with chaperones and folding enzymes essential for protein folding. ER quality control guided by these chaperones is essential for life. Whereas correctly folded proteins are exported from the ER, misfolded proteins are retained and selectively degraded. At least two main chaperone classes, BiP and calnexin/calreticulin, are active in ER quality control. Folding factors usually are found in complexes. Recent work emphasises more than ever that chaperones act in concert with co-factors and with each other. IntroductionThe endoplasmic reticulum (ER) has many functions, including lipid donation to other organelles (reviewed by van Meer and Sprong in this issue), Ca 2þ homeostasis [1], biogenesis of organelles [2], protein folding, quality control (QC) [3,4] and protein degradation. Although the native conformation of a protein lies encoded in its primary amino acid sequence, the ER greatly enhances protein folding efficiency [5]. The ER is highly specialised for folding: approximately one-third of all proteins in a eukaryotic cell are translocated into the ER [6]; the ER has unique oxidizing potential that supports disulphide bond formation during protein folding [7 ]; and the ER lumen is very crowded, with a protein concentration of >100 mg/ml. In this gel-like protein matrix, chaperones and folding enzymes are abundant, greatly outnumbering the newly synthesised substrates [8]. These folding factors in general prevent aggregation and thereby allow more efficient folding of a large variety of proteins. In this review, we highlight the latest advances in understanding how these chaperones and folding enzymes cooperate in assisting protein folding and mediating quality control. Co-translational and post-translational foldingMammalian secretory and membrane proteins are synthesised and translocated into the ER by the ribosome/sec61 translation/translocation machinery, of which various enlightening X-ray structures have recently been determined [9,10 ]. During translation/translocation newly synthesised proteins immediately start to fold. Combining these processes allows sequential folding which may greatly enhance folding efficiency, especially of multidomain proteins [11]. The immunoglobulin molecule with its heavy and light chains undergoes extensive folding and assembly already during synthesis [12]. Another example is the ribosome-bound nascent chain of the Semliki Forest virus capsid protease domain, which was shown to be folded and autoproteolytically active immediately after translocon exit, indicating that folding occurs co-translationally but after translocation [13 ]. Other proteins, on the other hand, need extensive post-translational folding to acquire their proper native conformation. Envelope glycoprotein gp160 of HIV-1, for example, is synthesised within approximately five minutes, but resides for hours in the ER with no apparent degradation [14]. The LDL receptor (LDL-R) also folds after synthesis: it collapses in...

Monitoring chaperone engagement of substrates in the endoplasmic reticulum of live cells

“…Furthermore, under no condition was there significant evidence for either a matrix of chaperones or for extremely large folding assemblies containing Crt (10,13,33). Rather, the data support a more dynamic situation in which folding complexes remain relatively small and͞or assemble and disassemble (34) over short time frames.…”

“…Although this approach has resulted in numerous insights into the substrate specificities, affinities, and functional cycles of individual chaperones, it has not clarified their organization and dynamics in vivo. The organizational state of any ER chaperone in its native environment under either quiescent or substrate-engaged conditions continues to be a matter for speculation (13). In this study, we have analyzed the dynamics of a functional ER chaperone in live cells by using fluorescent fusion proteins combined with photobleaching methods.…”

Interplay between redox and protein homeostasis

“…The folding process needs to be assisted by several chaperones and glycosylation enzymes. 26 Secreted and ER-resident proteins are enriched in disulfide bonds, which are critical for their conformational stability. The oxidizing environment of the ER is a prerequisite for the formation of disulfide bonds and is maintained by a network of redox enzymes such as oxidases and protein disulfide isomerases (PDIs).…”

Structural Similarity between a Starch-hydrolyzing Enzyme and an N-Glycan-Hydrolyzing Enzyme: Exohydrolases Cleaving α-1,X-Glucosidic Linkages to Produce β-Glucose

“…For example, starch, a polysaccharide consisting of glucose residues, functions as an energy storage material, while in the eukaryotic N-glycans, processing of glucose residues is important for the calnexin/calreticulin binding cycle involved in protein folding and quality control (1)(2)(3). This multi-functionality of carbohydrates can be attributed to wide variety of glycosidic linkages.…”