Premed · Premed · Cell Biology
Lecture 10: The Endoplasmic Reticulum and Protein Synthesis
Cell Biology
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the structure and functions of the rough and smooth ER
- Explain the signal hypothesis and co-translational translocation into the ER
- Describe protein folding and quality control in the ER lumen
- Explain N-linked glycosylation and its role in protein folding
- Describe the unfolded protein response (UPR) and ER-associated degradation (ERAD)
Lecture Content
I. Structure and Organization of the ER
The endoplasmic reticulum is the largest membrane-bound organelle in most eukaryotic cells, forming a continuous and extensive network of tubules and flattened cisternae. Its lumen (cisternal space) is topologically equivalent to the extracellular space, a fact with important implications for protein processing and sorting.
The rough ER (RER) is characterized by ribosomes studding its cytoplasmic face, which gives it a rough appearance in electron micrographs. It consists predominantly of flattened, often stacked cisternae and is particularly abundant in cells that produce large quantities of secretory proteins, such as pancreatic acinar cells and antibody-secreting plasma cells. The rough ER is the site of synthesis for secretory proteins, membrane proteins, and lysosomal enzymes.
The smooth ER (SER) lacks ribosomes and forms a more tubular network. It serves diverse functions depending on the cell type: lipid and steroid hormone synthesis (abundant in adrenal cortex and gonad cells), drug detoxification by cytochrome P450 enzymes (prominent in hepatocytes), calcium storage and release, and glycogen metabolism. The sarcoplasmic reticulum (SR) is a specialized form of smooth ER found in muscle cells that stores the Ca2+ required for contraction.
The ER membrane is continuous with the outer nuclear membrane. The characteristic tubular shape of ER tubules is maintained by reticulon proteins that induce membrane curvature and atlastin GTPases that mediate membrane fusion.
II. The Signal Hypothesis and Co-translational Translocation
The signal hypothesis, for which Gunter Blobel received the Nobel Prize in 1999, explains how proteins destined for the secretory pathway are directed to the ER. These proteins carry an N-terminal signal sequence, typically 16 to 30 amino acids long, whose essential feature is a hydrophobic core of 8 to 12 hydrophobic residues. This signal sequence is cleaved by signal peptidase once the protein reaches the ER lumen.
Co-translational translocation proceeds through a series of coordinated steps. Translation begins on a free ribosome in the cytoplasm. As the signal sequence emerges from the ribosome, it is recognized and bound by the Signal Recognition Particle (SRP), a ribonucleoprotein complex composed of six proteins and 7SL RNA. SRP binding pauses translation through elongation arrest. The SRP-ribosome complex then docks at the SRP receptor (composed of SR-alpha and SR-beta, both GTPases) on the ER membrane. The ribosome is transferred to the Sec61 translocon, a heterotrimeric protein-conducting channel (alpha, beta, gamma subunits) that forms an aqueous pore through the ER membrane. GTP hydrolysis releases SRP and the SRP receptor, and translation resumes with the growing polypeptide being threaded through the translocon into the ER lumen. Signal peptidase cleaves the signal sequence, and when translation is complete, the ribosome detaches and the translocon closes.
In some cases, particularly in yeast and for certain ER proteins, post-translational translocation occurs. Here, the protein is fully translated in the cytoplasm and then threaded through the Sec61 channel with the help of BiP, an Hsp70 family chaperone in the ER lumen that acts as a molecular ratchet to prevent backsliding.
<image>Co-translational translocation of a secretory protein into the ER. Step-by-step diagram showing: (1) Ribosome translating mRNA with emerging signal sequence. (2) SRP binds the signal sequence, pausing translation. (3) SRP-ribosome complex docks to SRP receptor on the ER membrane. (4) Ribosome is transferred to the Sec61 translocon channel. (5) SRP and SRP receptor dissociate (GTP hydrolysis shown). (6) Translation resumes; growing polypeptide is threaded through the translocon into the ER lumen. (7) Signal peptidase cleaves the signal sequence. (8) Completed protein released into the ER lumen, ribosome detaches. Each step labeled with arrows showing the sequential process.</image>
III. Insertion of Transmembrane Proteins
Transmembrane proteins are inserted into the ER membrane during translation through a process that involves additional topogenic sequences beyond the signal sequence. Stop-transfer anchor sequences are hydrophobic segments that halt translocation, leaving the transmembrane domain embedded in the lipid bilayer. A single-pass type I membrane protein uses an N-terminal signal sequence followed by a stop-transfer anchor, resulting in the N-terminus facing the lumen and the C-terminus in the cytoplasm. A single-pass type II protein uses an internal signal-anchor sequence (which is not cleaved), placing the N-terminus in the cytoplasm and the C-terminus in the lumen.
Multi-pass transmembrane proteins use alternating signal-anchor and stop-transfer sequences to thread multiple segments through the membrane. Each pair of topogenic signals inserts one transmembrane helix, producing proteins with multiple membrane-spanning alpha-helices such as GPCRs with their seven transmembrane domains.
GPI-anchored proteins are initially inserted into the ER lumen via a signal sequence. A GPI (glycosylphosphatidylinositol) anchor is then attached to the C-terminus in the ER, with the C-terminal peptide cleaved off, leaving the protein attached to the outer leaflet of the membrane.
IV. Protein Folding and Quality Control in the ER
The ER lumen provides a unique chemical environment for protein folding, notably an oxidizing milieu that supports disulfide bond formation (in contrast to the reducing environment of the cytoplasm).
Chaperone-assisted folding is critical for ensuring that newly synthesized proteins reach their correct three-dimensional structures. BiP (GRP78), an Hsp70 family member, binds to hydrophobic patches on unfolded or partially folded proteins, preventing aggregation and assisting folding through ATP-dependent bind-release cycles. Calnexin and calreticulin are lectin chaperones that bind monoglucosylated N-linked glycans on glycoproteins, retaining incompletely folded glycoproteins in the ER. Calnexin is membrane-bound while calreticulin is soluble in the lumen.
Disulfide bond formation is catalyzed by protein disulfide isomerase (PDI), which oxidizes pairs of cysteine residues to form stabilizing S-S bonds. PDI also possesses isomerase activity, allowing it to rearrange incorrectly formed disulfide bonds. Ero1 regenerates the oxidized form of PDI, using molecular oxygen as the terminal electron acceptor.
N-linked glycosylation begins with the pre-assembly of a core oligosaccharide (Glc3Man9GlcNAc2) on the lipid carrier dolichol phosphate on the cytoplasmic face of the ER. This oligosaccharide is flipped to the luminal face, where oligosaccharyltransferase (OST) transfers it to asparagine residues within the consensus sequence Asn-X-Ser/Thr (where X is any amino acid except proline). Glucosidases I and II then sequentially trim glucose residues. The resulting monoglucosylated protein enters the calnexin/calreticulin cycle: after glucosidase II removes the last glucose, the protein is released from calnexin. If it is properly folded, it exits the ER. If it remains misfolded, the enzyme UGGT (UDP-glucose:glycoprotein glucosyltransferase), which functions as a folding sensor, adds a glucose back, returning the protein to the calnexin cycle for another folding attempt.
<image>The calnexin/calreticulin cycle for protein quality control. Panel A: Oligosaccharyltransferase (OST) attaches the core oligosaccharide to an Asn residue of the nascent polypeptide in the ER lumen. Glucosidases I and II trim two glucose residues. Panel B: Monoglucosylated glycoprotein binds calnexin (membrane-bound) or calreticulin (soluble). Glucosidase II removes the last glucose, releasing the protein. Panel C: Decision point — if properly folded, the protein exits the ER via COPII vesicles. If misfolded, UGGT adds a glucose back, and the protein re-enters the calnexin cycle. After repeated failures, the protein is targeted for ERAD. Circular arrows emphasize the cyclic nature of quality control.</image>
V. ER-Associated Degradation (ERAD)
Proteins that fail to fold properly after multiple attempts are eliminated through ER-associated degradation. The ERAD pathway involves recognition of terminally misfolded proteins by ERAD lectins (such as EDEM and OS-9) and chaperones. Mannose trimming by ER mannosidase I marks proteins for degradation. The misfolded protein is then retrotranslocated (dislocated) through a protein channel formed by the Hrd1/Derlin complex back into the cytoplasm, where it is ubiquitinated by ER-associated E3 ubiquitin ligases (Hrd1, gp78). The AAA-ATPase p97/VCP extracts the ubiquitinated protein from the membrane, after which it is deglycosylated and delivered to the 26S proteasome for degradation.
ERAD has significant clinical relevance. The delta-F508 mutation in CFTR, the most common cause of cystic fibrosis, produces a protein that misfolds in the ER and is degraded by ERAD before it can reach the plasma membrane. Pharmacological chaperones such as ivacaftor and lumacaftor can partially rescue the folding of this mutant protein. In alpha-1 antitrypsin deficiency (Z allele), misfolded protein accumulates in the hepatocyte ER, causing liver damage.
VI. The Unfolded Protein Response (UPR)
When misfolded proteins accumulate in the ER beyond the capacity of normal quality control mechanisms, the cell activates the unfolded protein response, a coordinated signaling program that operates through three distinct branches, each initiated by a different ER transmembrane sensor.
IRE1 possesses both kinase and endoribonuclease activity. Under ER stress, IRE1 splices XBP1 mRNA to produce the transcription factor XBP1s, which upregulates genes encoding ER chaperones, ERAD components, and lipid synthesis enzymes. IRE1 also carries out RIDD (Regulated IRE1-Dependent Decay), degrading ER-localized mRNAs to reduce the protein load entering the ER.
PERK phosphorylates the translation initiation factor eIF2-alpha, which globally reduces translation and thereby decreases the influx of new proteins into the ER. However, certain mRNAs, including that for ATF4, are selectively translated under these conditions. ATF4 upregulates genes involved in amino acid metabolism and antioxidant defense, but also induces CHOP, a pro-apoptotic transcription factor activated when ER stress is prolonged.
ATF6 responds to ER stress by translocating to the Golgi, where it is cleaved by the S1P and S2P proteases. The released cytoplasmic domain (ATF6f) acts as a transcription factor that upregulates chaperones (including BiP) and ERAD components.
When ER stress is mild or transient, the UPR restores homeostasis through its adaptive response. However, if stress is severe or prolonged, the accumulation of pro-apoptotic factors such as CHOP triggers apoptosis. Dysregulation of the UPR has been implicated in diabetes, neurodegeneration, and cancer.
