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#ER

2 public questions tagged with this topic.

Basics of Cell Biology

Which of the following statements about endoplasmic reticulum is false?

Endoplasmic reticulum is continuous reticulum of sheets and tubules emanating from outer nuclear membrane, enclosing single lumen bounded by phospholipid bilayer containing translocon Sec61 for cotranslational import. Rough ER domain densely studded with 80S ribosomes docking via SRP receptor synthesizes secretory and membrane proteins that fold aided by chaperone BiP, protein disulfide isomerase PDI forming disulfides, and undergo initial N-glycosylation by oligosaccharyltransferase. Smooth ER abundant in hepatocytes and steroidogenic cells lacks ribosomes, houses lipid synthesis enzymes including HMG-CoA reductase for cholesterol, phosphatidylcholine synthesis, plus cytochrome P450 family for drug metabolism and calcium pump SERCA storing calcium released via IP3 receptors for signaling. ER exists exclusively in eukaryotes where endomembrane system compartmentalizes; prokaryotes lack internal organelles exporting proteins directly across plasma membrane. Classification as simple single-membrane vesicle like lysosome understates its network complexity, paired cisternae forming double membrane appearance around lumen, contact sites with mitochondria and plasma membrane for lipid exchange. Moreover claim of prokaryotic exclusivity reverses reality.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 12: Endoplasmic Reticulum Organization.

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Protein Sorting and Trafficking

Which of the following recognizes the signal sequence of nascent proteins targeting the ER?

Early sorting of nascent secretory and membrane proteins is performed by signal recognition particle, conserved ribonucleoprotein containing 7SL RNA scaffold and six protein subunits. When hydrophobic signal sequence of about eight to twelve non-polar residues emerges from ribosomal exit tunnel, methionine-rich M domain of SRP54 forms flexible hydrophobic groove whose abundance of sulfur-containing methionine side chains allows plastic accommodation of diverse signal sequences via induced fit. Binding is communicated through 7SL RNA to Alu domain comprised of SRP9 and SRP14 heterodimer that docks at elongation factor binding site, temporarily pausing translation to prevent premature folding and aggregation in cytosol. SRP-ribosome-nascent chain complex then diffuses to ER where GTP-dependent interaction with heterodimeric SRP receptor made of SRα and SRβ GTPases delivers complex to Sec61 channel for hand-off. Ran-GTP controls nuclear import via importins, KDEL receptor retrieves escaped ER chaperones via COPI, Rab GTPases govern vesicle tethering and fusion specificity, none directly scan ribosome exit tunnel for hydrophobic nascent signals at this early checkpoint of protein sorting to ER lumen.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 13: SRP Recognizes Signal Sequences.

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