Graduate Cell Biology · Secretory Pathway
The Endoplasmic Reticulum
The largest membrane system in the eukaryotic cell — where one-third of the proteome folds, where calcium is stored, where lipids are synthesised, and where chronic stress tips the cell toward apoptosis.
About This Course
The endoplasmic reticulum is a continuous network of tubules and sheets with a total surface area that can exceed the plasma membrane by an order of magnitude. It is the first environment encountered by the ~30% of cellular proteins destined for membranes or secretion. Three defining features separate its luminal interior from the cytosol: an oxidising redox potential that supports disulphide-bond formation, a high Ca2+store four orders of magnitude above cytosol, and a population of chaperones totalling ~100 mg/mL. The consequence is a compartment that is simultaneously a folding factory, a calcium battery, a lipid-synthesis organ, and a stress sensor.
Seven modules trace the ER from its architectural principles (Palade’s 1956 electron micrographs through Rapoport’s 2006 tubule-geometry theory), through Blobel’s signal hypothesis and cotranslational translocation, through the oxidative-folding machinery, through ERAD and the UPR, into lipid metabolism and Ca2+ signalling, and finally into the diseases — from cystic fibrosis to diabetes to hereditary spastic paraplegia — that trace to ER dysfunction.
Key Equations
Folding First-Passage
\( P_{\mathrm{fold}}(t) \approx 1 - e^{-t/\tau_f},\ \tau_f \sim \tau_0 e^{\beta \Delta G^{\ddagger}} \)
Chaperone Competition
\( v_{\mathrm{fold}} = k\,U\,\dfrac{C}{C+K_{\mathrm{m}}} \)
UPR Two-State System
\( \dot U = k_s - k_f\,U\,\dfrac{C}{C+K} - k_d U,\quad \dot C = g(U) - \gamma C \)
SERCA Ca²⁺ Gradient
\( \dfrac{[\mathrm{Ca^{2+}}]_{\mathrm{ER}}}{[\mathrm{Ca^{2+}}]_{\mathrm{cyto}}} \sim 10^4 \)
Ero1 Stoichiometry
\( \mathrm{Ero1 + PDI_{red}} \to \mathrm{PDI_{ox}} + \tfrac{1}{2}\mathrm{H_2O_2} \)
ERAD Flux
\( J_{\mathrm{ERAD}} = k_{\mathrm{retro}}\,[\mathrm{substrate}] \)
Seven Modules
M0
Architecture & Discovery
Palade 1956 EM, rough/smooth ER, sheets/tubules/junctions, reticulons/DP1, atlastin, three-way junction topology, ER-cortical tethers.
M1
Cotranslational Translocation
Signal peptide (Blobel), SRP / SR cycle, Sec61 heterotrimer, lateral gate, membrane topology (positive-inside rule), post-translational: GET/TRC40, EMC.
M2
Folding & Chaperones
Oxidising redox, BiP/Hsp70 cycle, calnexin/calreticulin cycle, PDI & Ero1, N-glycosylation, UGGT proof-reading, folding funnel kinetics.
M3
ERAD & Quality Control
Hrd1/Sel1L/Derlin retrotranslocation, Cdc48/p97 ATPase, ubiquitin-proteasome coupling, ERAD-L/M/C substrate classes, ER-phagy (FAM134B).
M4
Unfolded Protein Response
IRE1 RNase & XBP1 splicing, PERK / eIF2α / ATF4 / CHOP, ATF6 Golgi cleavage, UPR ODE dynamics, UPR-apoptosis bifurcation, ISRIB.
M5
Lipids, Ca²⁺ & Contact Sites
ER as lipid factory (PC, PE, PI, cholesterol), SERCA/IP3R/RyR, STIM-Orai SOCE, MAMs, ER-PM junctions, lipid transfer (VAP/OSBP/STARD3).
M6
ER in Disease
Neurodegeneration (HSP, CMT-via-REEP1), α1-antitrypsin, diabetes (β-cell UPR failure), cancer (tumour UPR addiction), cystic fibrosis F508del.
Cross-Links
Organelles (unified),Cell Physiology,Mitochondria,Stem Cells,Biochemistry,Molecular Biology.