Endoplasmic Reticulum & Golgi Apparatus – Detailed Study Notes

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Lecture Context & Contact
• Course: Cell Biology – Lecture 4
• Topic: Endoplasmic Reticulum (ER) & Golgi Apparatus
• Instructor: Dr. Yalda Moayedi (email: ym2994@nyu.edu)

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High-Level Outline of Today’s Material

Ultrastructure review of eukaryotic cells & evolutionary origin of organelle membranes.
Mechanisms of protein and lipid synthesis in the ER.
Golgi apparatus – post-ER protein modifications & vesicular traffic.
ER participation in intracellular $\mathrm{Ca^{2+}}$ signaling.

Significance
– Frames the ER/Golgi as central hubs for biosynthesis, trafficking, and signaling, tying cell structure to dynamic function.

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Overview of Cell Organelles
• Each organelle possesses distinct:
– Membrane composition (lipids, proteins, curvature).
– Enzymatic repertoire.
– Functional specialization (energy, biosynthesis, degradation, signaling).
• Understanding compartmentalization explains how conflicting biochemical reactions coexist in one cytoplasm.

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Eukaryotic Cells Contain Multiple Membrane-Enclosed Organelles
• Key organelles visible in transmission electron micrograph (TEM) of hepatocyte: nucleus, rough ER, mitochondria, lysosomes, peroxisomes.
• Scale bar: $5\ \mu m$ emphasizes nano- to micro-architecture.
• Rough ER distinguished by ribosome dots; mitochondria by double membrane & cristae.

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Table 15-1 – Main Functions of Compartments

Compartment	Representative Functions	Connection to Lecture
Cytosol	Metabolism, protein synthesis, cytoskeleton	Provides substrate pools for ER/Golgi.
Nucleus	Genome housing, transcription/replication	mRNAs exit to cytosol for ER-bound translation.
ER	Lipid synthesis; protein synthesis/initial modification	Primary focus today.
Golgi	Modification, sorting, packaging	Secondary focus today.
Lysosomes	Intracellular degradation	Destination of some Golgi vesicles.
Endosomes	Sorting of endocytosed cargo	Interface w/ Golgi & plasma membrane.
Mitochondria & Chloroplasts	ATP generation; carbon fixation (plants)	Energetic support; independent protein import routes.
Peroxisomes	Detoxification, β-oxidation	Receive newly synthesized lipids from ER.

Pedagogical Note
– Organelles form an endomembrane system, i.e., functionally integrated network of trafficking pathways.

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Evolutionary Origin of ER & Endomembrane System

Ancestral Prokaryotic Scenario: DNA & ribosomes tethered to plasma membrane.
Invagination Hypothesis: Plasma membrane folded inward, surrounding DNA ⇒ proto-nucleus + internal sacs.
Detachment & specialization yielded: nucleus, ER, Golgi, endosomes, lysosomes, peroxisomes, plasma membrane.
Demonstrates co-evolution of transcriptional regulation (nucleus) with biosynthetic capacity (ER/Golgi).

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Morphological Types of ER
• Smooth ER (SER)
– Lacks ribosomes ⇒ appears “smooth” on TEM.
– Primary site of lipid & steroid synthesis, detoxification (hepatocytes), glycogen metabolism, and calcium storage.
• Rough ER (RER)
– Ribosome-studded membranes ⇒ “rough.”
– Site of co-translational protein synthesis for secretory pathway.

Example / Clinical Correlation
– Hepatocytes expand SER in response to phenobarbital due to up-regulated cytochrome P450 detox enzymes.

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Smooth ER – Mechanism of New Membrane Synthesis
• Phospholipid Biosynthesis occurs on cytosolic leaflet of SER via enzymes such as glycerol-3-phosphate acyltransferase.
• Asymmetry Correction:
– Scramblase flips newly made phospholipids randomly across bilayer ⇒ symmetric growth (crucial for expansion).
– Reaction schematic:
$\text{Phospholipid}<em>{cyto} \xrightarrow{\text{Scramblase}} \text{Phospholipid}</em>{lumen}$
• Lipids then delivered to other membranes via vesicular traffic or lipid-transfer proteins.

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Rough ER – Targeted (Co-Translational) Protein Synthesis

Ribosome initiates translation in cytosol.
Signal Peptide / ER Targeting Sequence emerges (≈ 15–30 hydrophobic AAs).
Signal Recognition Particle (SRP) binds peptide & pauses translation.
SRP–ribosome complex docks at SRP Receptor on ER membrane.
Ribosome engages Translocon (Sec61); translation resumes, threading polypeptide into lumen or membrane.
Energy: GTP hydrolysis by SRP/SRP-receptor provides unidirectionality.

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Soluble Protein Synthesis in RER
• Entire polypeptide translocated into ER lumen.
• Signal Peptidase cleaves the N-terminal signal peptide, which remains inserted in the bilayer & is later degraded.
• Protein folds with chaperones (e.g., BiP), undergoes N-glycosylation (see p.12).
• Quality control: mis-folded soluble proteins are retro-translocated & degraded via ER-associated degradation (ERAD).

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Membrane Protein Synthesis
• After signal peptide cleavage, hydrophobic Stop-Transfer Sequence (STS) halts translocation, anchoring protein.
• Orientation: N-terminus luminal, C-terminus cytosolic for type I proteins (classical single-pass).
• Multi-pass proteins contain alternating signal-anchor & STS sequences.
• Topology determines extracellular vs. cytoplasmic exposure of functional domains (e.g., GPCRs, channels).

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Post-Translational Modifications (PTMs) in the ER
• PTM Definition: Covalent addition/removal of chemical groups not directly encoded in genetic sequence.
• >200 PTM types exist; ER mainly supports glycosylation, disulfide bonding, hydroxylation, and GPI-anchor addition.
• N-Link Glycosylation
– Consensus sequon: $\text{Asn–X–Ser/Thr}$ .
– Dolichol phosphate lipid carrier transfers pre-assembled oligosaccharide (Glc3Man9GlcNAc2) en bloc.
– Functions: folding quality control (calnexin cycle), solubility, trafficking signals, eventual formation of glycolipids/–proteins forming the Glycocalyx (recall Lecture #2).

Biological Significance
– Glycosylation defects (e.g., Congenital Disorders of Glycosylation) highlight ER’s centrality to development & immunity.

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Golgi Apparatus – Principal Destination for ER Proteins
• Trafficking routes:
– Anterograde (ER → Golgi → PM/Lysosome) via COPII & clathrin coated vesicles.
– Retrograde (Golgi → ER) via COPI, retrieving ER-resident proteins (KDEL/HDEL retention signal).
• Intersects with Endocytosis (early → late endosome) and Exocytosis pathways ⇒ integrative trafficking hub.

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Golgi Structure
• Stacks of flattened membrane sacs—cisternae (≈ 3–20 per stack).
• Cis-Golgi (entry) faces ER; Trans-Golgi Network (TGN, exit) faces plasma membrane.
• Lateral connections allow cisternal maturation (each cisterna moves forward).

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Vesicular Traffic & Processing in Golgi

Incoming proteins at cis-Golgi assessed for folding; mis-folded returned to ER.
Sequential Enzymatic Zones
– cis: trimming of high-mannose N-glycans.
– medial: addition of N-acetyl-glucosamine.
– trans: galactose, sialic acid, sulfation.
Outbound Sorting (TGN):
– Lysosomal: Mannose-6-phosphate tagging.
– Constitutive secretion: continuous delivery of membrane proteins, ECM molecules.
– Regulated secretion: hormones, neurotransmitters stored in dense-core vesicles until stimulus triggers fusion.

Physiological Implication
– Dysregulation causes diseases: e.g., I-cell disease (defective GlcNAc-phosphotransferase → lysosomal enzyme missorting).

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ER in Calcium Signaling
• ER lumen stores millimolar $\mathrm{[Ca^{2+}]<em>{ER}}$ vs. nanomolar cytosolic $\mathrm{[Ca^{2+}]</em>{cyt}}$ .
• Specialized SER in muscle = Sarcoplasmic Reticulum (SR).
• SR houses SERCA pumps (ATP-driven $\mathrm{Ca^{2+}}$ uptake) & Ryanodine Receptors (RyR) / IP₃ Receptors (ligand-gated $\mathrm{Ca^{2+}}$ release).

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Excitation–Contraction (E-C) Coupling
• Action potential travels along Transverse (T-) tubules.
• Depolarization triggers:
$\text{Voltage-gated }DHP\;\text{receptor} \rightarrow \text{mechanical coupling} \rightarrow \text{RyR opening} \rightarrow \mathrm{Ca^{2+}}_{SR}\;\text{release}$
• $\mathrm{Ca^{2+}}$ binds troponin C → cross-bridge cycling → muscle contraction.
• Mitochondria localize near SR to capture $\mathrm{Ca^{2+}}$ for ATP production (metabolic coupling).

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Imaging Evidence
• Fluorescent live-cell image of cardiomyocyte:
– Blue: ER-targeted fluorescent protein ⇒ network geometry.
– Red: Mitochondrial membrane-potential dye.
• Reveals tight ER–mitochondria contacts (MAMs) facilitating $\mathrm{Ca^{2+}}$ transfer & lipid exchange.

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Broader Relevance of ER $\mathrm{Ca^{2+}}$ Stores
• Non-muscle cells (neurons, immune cells, hepatocytes) rely on ER $\mathrm{Ca^{2+}}$ for secretion, gene expression, apoptosis.
• Store-Operated Calcium Entry (SOCE): Depletion sensed by STIM1, opening Orai1 on plasma membrane, refilling ER.
• Clinical tie-ins: malignant hyperthermia (RyR mutation), neurodegeneration (ER stress & $\mathrm{Ca^{2+}}$ dys-homeostasis).

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Comprehensive Summary
• Smooth ER – lipid & steroid biosynthesis; initial steps for new membranes.
• Rough ER – co-translational synthesis of secretory & membrane proteins; quality control; PTMs (esp. N-glycosylation).
• Golgi Apparatus – polarized stack (cis→trans); further glycan remodeling; key sorting station producing secretory, lysosomal, and plasma-membrane-bound vesicles.
• Secretory Vesicles – undergo regulated (stimulus-dependent) or constitutive (default) exocytosis.
• ER/SR & $\mathrm{Ca^{2+}}$ Signaling – integral to excitation–contraction in muscle; broader roles in cellular signaling and apoptosis.
• Without ER integrity, lipid homeostasis, protein trafficking, and calcium-dependent physiology would fail, underscoring its centrality to eukaryotic life.