35 public questions tagged with this topic.
The rough ER is involved in protein synthesis, while the smooth ER is responsible for lipid synthesis. This follows from NCERT principle where the relation explains the outcome clearly for students in simple steps.
Ref: NCERT Biology Textbook for Class XI and XII (Botany section), Chapter: Morphology and Anatomy of Flowering Plants, Topic: Plant structure and tissue systems.
The smooth endoplasmic reticulum (SER) lacks ribosomes, distinguishing it from the rough endoplasmic reticulum. This follows from NCERT principle where relation explains outcome clearly for students.
Ref: NCERT Biology Textbook for Class XI and XII (Botany section), Chapter: Biology - Botany portion covering relevant concept, Topic: Plant structure, physiology and applications.
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 incl
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 12: Endoplasmic Reticulum Organization.
Secretory proteins undergo co-translational modification ensuring solubility, folding, and functional diversity needed for extracellular environment. N-glycosylation begins in rough ER lumen where oligosaccharyltransferase complex scans nascent polypeptide emerging from Sec61 translocon, transferring preassembled 14-sugar oligosaccharide Glc3Man9GlcNAc2 from dolichol phosphate lipid anchor to asparagine in consensus sequon Asn-X-Ser/Thr, followed by trimming by glucosidase I and II and binding to lectin chaperones calnexin-calreticulin that monitor folding, with UGGT reglucosylating misfolded
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 12: Protein Glycosylation in ER and Golgi.
Teaching analogy depicts Golgi as postman because secretory pathway requires centralized sorting distribution center that receives products from ER, completes addressing via covalent modifications, then dispatches parcels to correct cellular addresses. Proteins exiting rough ER in COPII vesicles contain transient signals; upon arrival cis Golgi they undergo processing removing signal peptides, trimming N-glycans, adding terminal sugars that create affinity for lectins. Trans Golgi network acts as sorting station with lipid microdomains and adaptors selecting mannose-six-phosphate tagged lysoso
Ref: Farquhar Annu Rev Cell Biol; Golgi postman receives ER modifies sorts to PM lysosome secretion.
Golgi polarity defined by cis entry face adjacent to ER exit sites and trans exit face oriented toward plasma membrane. Newly synthesized cargo packaged into COPII coated vesicles decorated with Sec23/24 cargo selection and Sar1 GTPase buds from ER after BiP quality control, fuses via TRAPP tethering to ER-Golgi intermediate compartment then to cis Golgi network marked by Rab1 and cis golgin GM130 and GRASP65. Here resident enzymes mannosidase I and N-acetylglucosamine transferase I begin processing, and retrieval of ER proteins via KDEL receptor and COPI retrograde occurs. Medial cisternae co
Ref: Lippincott-S Cell Bio; COPII ER exit to cis Golgi CGN receives cargo, Rab1, VSV-G trafficking.
Endoplasmic reticulum lumen is specialized folding compartment with oxidizing redox potential, high calcium millimolar and abundant chaperones distinguishing it from reducing cytosol. Newly translocated polypeptides entering via Sec61 encounter BiP also called GRP78, luminal Hsp70 family member binding transiently to exposed hydrophobic segments in ATP-regulated cycle where ATP binding opens substrate binding domain, hydrolysis closes it trapping unfolded chain preventing aggregation and promoting correct disassembly of aggregates. Cooperation with co-chaperone ERdj proteins stimulates ATPase,
Ref: Kozlov Trends Biochem; ER lumen BiP GRP78 Hsp70 folding quality control UPR sensor.
Maintaining blood glucose between meals requires hepatic glucose output via glycogen breakdown and gluconeogenesis. Both pathways converge on glucose-six-phosphate cytosolic that cannot exit cell due to charge; final dephosphorylation occurs inside endoplasmic reticulum lumen by glucose-six-phosphatase complex composed of transporter T1 importing G6P, catalytic subunit facing lumen hydrolyzing to glucose and Pi, transporters T2 and T3 exporting Pi and glucose. Active site luminal protects against futile cycling by cytosolic hexokinase. Enzyme enriched in smooth ER of hepatocytes and kidney cor
Ref: Foster J Biol Chem; glucose-6-phosphatase ER lumen T1 translocase gluconeogenesis von Gierke.
Cell fractionation pioneered by Claude and Palade showed that mechanical homogenization shears endoplasmic reticulum tubular network into small vesicles that reseal spontaneously due to energetically unfavorable exposed hydrophobic edges. These vesicles called microsomes sediment at one hundred thousand times g, one hundred to three hundred nanometers diameter retaining orientation original membrane. Rough microsomes bear ribosomes and contain Sec61 translocon, oligosaccharyltransferase, BiP, permitting in vitro translation translocation assays that defined signal peptide cleavage and core gly
Ref: Palade Science 1975; microsomes ER fragments 100k g in vitro translation translocation Nobel work.
Membrane lipid synthesis predominantly occurs in endoplasmic reticulum where fatty acid activation, glycerol-three-phosphate acyltransferase and phospholipid synthases reside on cytosolic face of ER membranes. Phosphatidylcholine formed via Kennedy pathway using choline, phosphatidylethanolamine, phosphatidylserine interconverted by decarboxylase at mitochondria associated membranes, phosphatidylinositol synthesized from CDP-DAG. These lipids transfer to growing membranes via vesicular transport and lipid transfer proteins at contact sites. Golgi then converts PC to sphingomyelin and synthesiz
Ref: van Meer Nat Rev Mol Cell Biol; phospholipid synthesis Kennedy pathway ER cytosolic leaflet enzymes.
Targeting of secretory proteins to endoplasmic reticulum uses signal hypothesis where N-terminal fifteen to thirty amino acid hydrophobic sequence emerges from ribosome exit tunnel exposing binding site for signal recognition particle, a three hundred kDa ribonucleoprotein comprising seven SL RNA and six protein subunits including SRP54 that contains methionine-rich pocket accommodating hydrophobic signals via induced fit. Binding transiently arrests elongation, complex diffuses to ER membrane where heterodimeric SRP receptor composed of SR alpha and SR beta, both GTPases, docks particle. GTP
Ref: Walter Nature 1981 SRP discovery; SRP54 binds signal peptide, GTP-dependent delivery to Sec61.
Smooth ER forms anastomosing tubules without ribosomes enriched in enzymes for lipid and xenobiotic metabolism. In hepatocytes it contains cytochrome P450 monooxygenases, NADPH-cytochrome P450 reductase, UDP-glucuronosyltransferases and sulfotransferases that hydroxylate, reduce and conjugate lipophilic drugs, environmental pollutants and endogenous steroids to make them water soluble for biliary or renal excretion. Chronic exposure to phenobarbital induces proliferation of smooth ER increasing detoxification capacity, observed by proliferation of membranes in electron micrographs. Additionall
Ref: Alberts Ch 12; smooth ER CYP450 detoxification, lipid synthesis, SERCA calcium storage, SAR.
