Endoplasmic reticulum

The endoplasmic reticulum is a membrane-enclosed network of tubules and flattened sacs in a typical eukaryotic biological cell. Its continuous boundary encloses the ER lumen, separate from the cytosol, and connects with the outer nuclear envelope. Rough endoplasmic reticulum and smooth endoplasmic reticulum are regions of this network.^[ref]

The sarcoplasmic reticulum is the specialized endoplasmic reticulum of muscle cells that stores and releases calcium. Pump uptake, channel-mediated release and passive leak exchange calcium with the cytoplasm; these internal transfers cancel in the appropriately volume-weighted total-calcium balance. They can nevertheless change cytoplasmic calcium rapidly and control contraction.

endoplasmic reticulum stress arises when ER demands exceed its processing or homeostatic capacity. It can alter folding, degradation and transport, confounding interpretation of a supposedly selective secretion defect.

The endoplasmic reticulum lumen is the aqueous space enclosed by the ER membrane. A soluble lumenal cargo cannot directly contact a cytoplasmic coat across an intact membrane.

An endoplasmic reticulum exit site is a region organizing export machinery and cargo departure from the ER. Local coat assembly, cargo concentration and membrane supply can be coordinated there.

The ER lumen is the internal aqueous compartment of the endoplasmic reticulum. A ribosome attached to its membrane sits on the cytosol side, not inside the ER lumen. This distinction separates the machinery making a protein from the compartment receiving it.^[ref]

Rough endoplasmic reticulum bears attached ribosomes on its cytosol surface. In a common mammalian route, protein targeting delivers a nascent chain into the ER lumen during biological translation. Other proteins enter the membrane. Free and attached ribosomes share the same basic machinery; targeting depends on their cargo.^[ref]

An ER signal sequence is polypeptide chain targeting information recognized by machinery delivering a protein to the endoplasmic reticulum. Many soluble cargoes have a removable N-terminus signal; membrane insertion can use internal signals. An ER signal sequence alone does not specify the final destination.^[ref]

For an original counting example, stipulate an unbranched precursor polypeptide chain of $180$ amino acid residues whose first $20$ amino acid residues form a cleaved ER signal sequence. With no further processing, the remaining chain has $180-20=160$ amino acid residues and $159$ peptide bonds. The initial chain had $179$ peptide bonds; the detached signal has $19$, so $159+19=178$ remain after one bond is cut. The check counts adjacent positions: an unbranched chain of $n$ amino acid residues has $n-1$ peptide bonds. These stipulated lengths are not a natural sequence, a measured size or evidence that a particular sequence functions as an ER signal sequence.

Smooth endoplasmic reticulum lacks attached ribosomes. Specialized regions support lipid metabolism and processing of foreign compounds; the endoplasmic reticulum also stores and releases calcium ions. Regional specialization does not split the network into two separate organelles, nor make every smooth region perform every function.^[ref]

Secretory protein routing connects entry into the endoplasmic reticulum, protein folding and sorting, transfer to the Golgi apparatus, and delivery onward. In the conventional soluble-cargo route, transport vesicles carry lumenal contents between compartments; exocytosis releases cargo outside the cell membrane. Resident endoplasmic reticulum proteins can be retained or retrieved, and defective cargo can be held back. Many cytosol proteins never enter this route.^[ref]

The useful geometric rule is preservation of membrane sides: during budding and fusion, a lumen-facing region of a membrane protein stays lumen-facing, then faces the exterior after exocytosis; its cytosol-facing region stays cytosolic.^[ref]

Work through the schematic using two distinguishable labels A and B on a stipulated membrane protein. Start with A facing the ER lumen and B facing the cytosol. At each budding or fusion step, carry the surrounding membrane together with the labels; do not turn the protein over. The resulting pair is A facing the Golgi apparatus interior and B facing the cytosol, then A facing a transport vesicle interior and B facing the cytosol, and finally A facing the exterior and B facing the cytosol. This is a stepwise argument about the depicted operations, not a claim that the compartments have identical chemical contents. Likewise, once soluble cargo is inside the ER lumen, this schematic reaches the exterior with zero further crossings through a lipid bilayer: enclosed spaces join when membranes fuse. Delivery therefore need not mean repeatedly pushing a folded protein through a membrane.

In 1967, James D. Jamieson and George E. Palade incubated guinea pig pancreas slices, following a three-minute pulse of tritium-labeled leucine with excess unlabeled leucine. This pulse-chase analysis marked newly made proteins while reducing subsequent incorporation. Fixed, Epon-embedded thin sections were mounted on coated grids, covered with Ilford L-4 photographic emulsion, developed and stained for electron microscopy. Autoradiography located photographic grains relative to pancreatic acinar cell structures.^[ref]

Pulse-end samples provided a temporal reference: label lay mainly over rough endoplasmic reticulum; later samples shifted toward peripheral Golgi apparatus elements, condensing vacuoles and secretory granules. As a contamination check, isolated granule pellets were also examined by autoradiography: early pellet labeling chiefly marked condensing vacuoles. Grids from the compared pellet samples were coated together to control photographic processing. This supported transport of a previously labeled cohort rather than separate synthesis at each location. Fixed samples did not track individual proteins; localization was finite, and labeling was not absolutely exclusive to secretory cargo. The result concerns this preparation, not every protein in every biological cell.^[ref]

A microsome is a closed lipid bilayer compartment produced during disruption of a biological tissue; rough endoplasmic reticulum-derived microsomes provide a sealed compartment for protein targeting experiments.