Endoplasmic reticulum

The endoplasmic reticulum or ER (endoplasmic means "within the cytoplasm", reticulum means "little net") is an organelle found in all eukaryotic cells. The ER modifies proteins, makes macromolecules, and transfers substances throughout the cell. Prokaryotic organisms do not have organelles and thus do not have an ER. ER's base structure and composition is similar to the plasma membrane, though it is an extension of the nuclear membrane. The ER is the site of the translation and folding of and transport of proteins that are to become part of the cell membrane (e.g., transmembrane receptors and other integral membrane proteins) as well as proteins that are to be secreted or "exocytosed" from the cell (e.g., digestive enzymes).

Structure

The ER consists of an extensive membrane network of tubes and cisternae (sac-like structures). The membrane encloses a space, the cisternal space (or internal lumen) from the cytosol. This space is acting as a gateway. Parts of the ER membrane are continuous with the outer membrane of the nuclear envelope, and the cisternal space of the ER is continuous with the space in between the two layers of the nuclear envelope.

Parts of the ER are covered with ribosomes (which assemble amino acids into proteins based on instructions from the nucleus). Their rough appearance under electron microscopy led to their being called rough ER (RER), other parts are free of ribosomes and are called smooth ER (SER). The ribosomes on the surface of the rough ER insert the freshly produced proteins directly into the ER, which processes them and then passes them on to the Golgi apparatus (Fig. 1). Rough and smooth ER differ not only in appearance, but also in function.

Rough ER

The coarse ER manufactures and transports proteins destined for membranes and secretion. It synthesizes membrane, organellar, and excreted proteins. Minutes after proteins are synthesized most of them leave to the Golgi apparatus within vesicles. The rough ER also modifies, folds, and controls the quality of proteins.

Smooth ER

The smooth ER has functions in several metabolic processes. It takes part in the synthesis of various lipids (e.g., for building membranes such as phospholipids), fatty acids and steroids (e.g., hormones), and also plays an important role in carbohydrate metabolism, detoxification of the cell (enzymes in the smooth ER detoxify chemicals), and calcium storage. It also is a large transporter of nutrient found in each cell.

Functions

The endoplasmic reticulum serves many general functions, including the facilitation of protein folding, and the transport of proteins. Correct folding of newly made proteins is made possible by several ER proteins including: PDI, Hsc70 family , calnexin , calreticulin, and the peptidylpropyl isomerase family . Only properly folded proteins are transported from the RER to the Golgi complex.

Transport of proteins

Secretory proteins are moved across the ER membrane. Proteins that are transported by the ER and from there throughout the cell are marked with an address tag that are called a signal sequence. Gnter Blobel was awarded the 1999 Nobel Prize in Physiology or Medicine for his discovery of these signal sequences in 1975. The N-terminus (one end) of a polypeptide chain (e.g., a protein) contains a few amino acids that work as an address tag, which are removed when the polypeptide reaches its destination. Proteins that are destined for places outside the ER are packed into transport vesicles and moved along the cytoskeleton towards their destination. The ER is also part of a protein sorting pathway.

Other functions

• Insertion of proteins into the ER membrane. Integral proteins need to be inserted into the ER membrane after they are synthesized. Insertion into the ER membrane requires the correct topogenic sequences.
• Glycosylation. Glycosylation involves the attachment of oligosaccharides.
• Disulfide bond formation and rearrangement. Disulfide bonds stabilize the tertiary and quaternary structure of many proteins.
• Sarcoplasmic reticulum. The endoplasmic reticulum found in muscle fibers is called sarcoplasmic reticulum.