# INTRODUCTION:
-> At the beginning of the twentieth century, Emilio Veratti found another subcellular structure in muscle cells, which he claimed to be distinct from the Golgi apparatus. Veratti’s work failed to convince his contemporaries.
-> About 50 years later, the electron microscope finally made the unambiguous identification of a network (reticulum) inside the cytoplasm of eukaryotic cells (endoplasmic).
-> In 1954, the endoplasmic reticulum (ER) entered the canon of cell biology as a distinct organelle separated from the cytosol by virtue of a membrane and forming a complex tubular or sheet-like network.
-> Its specialised derivative in muscle cells: the sarcoplasmic reticulum (SR).
# ORIGIN AND FUNCTIONS :
-> The ER serves as an anticipation of the extracellular world, to prepare molecules to exert their function in a different environment.-> Both the periplasm and the ER serve to groom proteins that are destined to be part of the cell membranes or to be secreted; these proteins enter the ER or periplasm first.
-> The nuclear envelope is continuous with the endoplasmic reticulum and proves that the ER is present in eukaryotic cells from "inception".z
-> Ribosomes that are translating proteins destined for the secretory pathway dock onto the cytosolic face of the ER membrane. These ribosomes give a ‘rough’ appearance to the ER, typical of protein secreting cells, therefore are referred to as 'rough ER', while the portion devoid of ribosomes is 'smooth ER'.
-> ER is the most important site of (membrane) lipid synthesis in the eukaryotic cell.-> Other organelles depend on membrane lipid supply from the ER, either through vesicular transport or through lipid transfer.
-> In other organelles, the membrane lipids that are derived from the ER are often further derivatised or asymmetrically distributed between membrane leaflets according to need.
-> In most eukaryotic cells, the ER is also the major store of intracellular ions and Ca2+.
-> Most ER-resident proteins in fact are low-affinity/high-capacity calcium-binding proteins.
# MORPHOLOGY:
-> ER is the largest and most extensive organelle in eukaryotic cells, occupying ~10% of cell volume (more in secretory cells).
-> It forms a continuous membrane system that includes:1) Rough ER 2) Smooth ER 3) Nuclear envelope
-> ER exists as a network of tubules and sheet-like cisternae extending throughout the cytoplasm.
#ER tubules:-> They are highly curved and branched.-> Their diameter is about 30–100 nm. -> They also form polygonal networks.-> They are highly curved structures.
#ER sheets:-> These are flat structures, often near the nucleus.-> Their width is about 50–100 nm.-> They are relatively flat.
# KEY PROTEINS SHAPING ER STRUCTURE:
- Reticulons & wedge-shaped proteins → induce curvature (tubules, sheet edges, nuclear pores).
- CLIMP-63 →maintains ER sheet thickness via luminal dimerisation.
- Kinectin & p180 → stabilise flat ER sheets.
-> ER is highly dynamic:- Constant fission and fusion.- Regulated by GTPases (Atlastins, Rab proteins).- Moves along the cytoskeleton using motor proteins.
*Muscle cells → specialised smooth ER (sarcoplasmic reticulum).*Liver cells → abundant smooth ER (detoxification, metabolism).
*Secretory cells (plasma cells, acinar cells) → expanded rough ER.
#MEMBRANE CONTACT SITES WITH OTHER ORGANELLES:
-> Subdomains of the ER make close contact with virtually all other cellular structures, foremost with the nucleus, as the nuclear envelope is continuous with the ER.
-> Contacts of the ER with other organelles are maintained by various tethering complexes that form a bridge between the apposing membranes.
-> Likewise, protein-mediated tethering of ER to mitochondria sustains close contacts, which are referred to as mitochondria-associated membranes (MAM) for signalling.
-> The ER establishes tether-mediated contacts also with the Golgi, lysosomes, and endosomes. Their main function seems to be the transfer from the ER of specific lipids that are in shortage in the target membrane.
-> The ER is the source of membrane lipids for the peroxisome.
-> The ER hosts an impressive repertoire of molecular chaperones and foldases.
-> Once client proteins are correctly folded, they are sorted to ER exit sites. Some clients team up with dedicated cargo receptors to promote this sorting process. Notable examples include the lectin ER–Golgi intermediate compartment (ERGIC-53).
-> To maintain homeostasis, the ER folding capacity or efficiency must be adjusted to the folding load, which may vary during differentiation or due to changing environmental cues.
The endoplasmic reticulum is far more than a passive membrane network; it is a highly dynamic and adaptive organelle that integrates protein synthesis, lipid metabolism, calcium signalling, and stress responses.
# INTRODUCTION:
-> At the beginning of the twentieth century, Emilio Veratti found another subcellular structure in muscle cells, which he claimed to be distinct from the Golgi apparatus. Veratti’s work failed to convince his contemporaries.
-> About 50 years later, the electron microscope finally made the unambiguous identification of a network (reticulum) inside the cytoplasm of eukaryotic cells (endoplasmic).
-> Both the periplasm and the ER serve to groom proteins that are destined to be part of the cell membranes or to be secreted; these proteins enter the ER or periplasm first.
-> The nuclear envelope is continuous with the endoplasmic reticulum and proves that the ER is present in eukaryotic cells from "inception".z
-> Ribosomes that are translating proteins destined for the secretory pathway dock onto the cytosolic face of the ER membrane. These ribosomes give a ‘rough’ appearance to the ER, typical of protein secreting cells, therefore are referred to as 'rough ER', while the portion devoid of ribosomes is 'smooth ER'.
-> Other organelles depend on membrane lipid supply from the ER, either through vesicular transport or through lipid transfer.
-> In other organelles, the membrane lipids that are derived from the ER are often further derivatised or asymmetrically distributed between membrane leaflets according to need.
*Liver cells → abundant smooth ER (detoxification, metabolism).
*Secretory cells (plasma cells, acinar cells) → expanded rough ER.
#MEMBRANE CONTACT SITES WITH OTHER ORGANELLES:
-> Subdomains of the ER make close contact with virtually all other cellular structures, foremost with the nucleus, as the nuclear envelope is continuous with the ER.
-> Contacts of the ER with other organelles are maintained by various tethering complexes that form a bridge between the apposing membranes.
-> Likewise, protein-mediated tethering of ER to mitochondria sustains close contacts, which are referred to as mitochondria-associated membranes (MAM) for signalling.
-> The ER establishes tether-mediated contacts also with the Golgi, lysosomes, and endosomes. Their main function seems to be the transfer from the ER of specific lipids that are in shortage in the target membrane.
-> The ER is the source of membrane lipids for the peroxisome.
-> The ER hosts an impressive repertoire of molecular chaperones and foldases.
The endoplasmic reticulum is far more than a passive membrane network; it is a highly dynamic and adaptive organelle that integrates protein synthesis, lipid metabolism, calcium signalling, and stress responses.
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