Endoplasmic reticulum

\( \newcommand\) • • • • • • The endoplasmic reticulum is an organelle that is responsible for the synthesis of lipids and the modification of proteins. Rough ER Rough Endoplasmic Reticulum: This transmission electron micrograph shows the rough endoplasmic reticulum and other organelles in a pancreatic cell. The rough endoplasmic reticulum (RER) is so named because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewed through an electron microscope. Ribosomes transfer their newly synthesized proteins into the lumen of the RER where they undergo structural modifications, such as folding or the acquisition of side chains. These modified proteins will be incorporated into cellular membranes—the membrane of the ER or those of other organelles —or secreted from the cell (such as protein hormones, enzymes ). The RER also makes phospholipids for cellular membranes. If the phospholipids or modified proteins are not destined to stay in the RER, they will reach their destinations via transport vesicles that bud from the RER’s membrane. Since the RER is engaged in modifying proteins (such as enzymes, for example) that will be secreted from the cell, the RER is abundant in cells that secrete proteins. This is the case with cells of the liver, for example.

Quick look Rough ER (RER) is involved in some protein production, protein folding, quality control and despatch. It is called ‘rough’ because it is studded with ribosomes Smooth E R (SER) is associated with the production and metabolism of fats and steroid hormones. It is ‘smooth’ because it is not studded with ribosomes and is associated with smooth slippery fats. To view a micrograph of ER interpreted using the Gridpoint cross-hairs device, CELLS NEED THE ROUGH AND THE SMOOTH Think of a cell as a “multitude of membranes” we said in an earlier section. This statement certainly applies to the endoplasmic reticulum an organelle found in eukaryotic cells. About 50% of the total membrane surface in an animal cell is provided by endoplasmic reticulum (ER). The organelle called ‘endoplasmic reticulum’ occurs in both plants and animals and is a very important manufacturing site for lipids (fats) and many proteins. Many of these products are made for and exported to other organelles. This is an electron microscope image showing part of the rough endoplasmic reticulum in a plant root cell from maize. The dark spots are ribosomes. (courtesy of Chris Hawes, The Research School of Biology & Molecular Sciences, Oxford Brookes University, Oxford, UK) There are two types of endoplasmic reticulum: rough endoplasmic reticulum (rough ER) and smooth endoplasmic reticulum (smooth ER). Both types are present in plant and animal cells. The two types of ER often appear as if separate, but they ...

Rough Endoplasmic Reticulum Definition The rough endoplasmic reticulum (rough ER) is a part of the endomembrane system of the cell and a subset of the endoplasmic reticulum (ER). This organelle is primarily concerned with the synthesis, folding and modification of proteins, especially those that need to be delivered to different organelles within the cell, or secreted from the cell. The rough ER is also involved in the response of the cell to unfolded proteins and plays a role in the induction of apoptosis, due to its close interaction with mitochondria. The rough ER is characterized by the presence of membrane-bound ribosomes that give it a distinctive appearance under the microscope. These ribosomes look like studs and distinguish the organelle from the smooth sections of the ER. Some proteins are also synthesized by strings of ribosomes, called polysomes. The rough ER can be identified by its morphology as well – it often consists of convoluted, flattened sac-like structures that originate near the nucleus. The lumen of the rough ER is contiguous with the perinuclear space and the membranes of the rough ER are associated with the outer nuclear membrane. Structure of the Rough Endoplasmic Reticulum The ER can be morphologically divided into two structures–cisternae and sheets. The rough endoplasmic reticulum is largely made of sheets – a two-dimensional array of flattened sacs that extend across the cytoplasm. In addition to ribosomes, these membranes contain an importan...

\( \newcommand\). Protein folding is optimized in the ER. Proteins such as calnexin can temporarily bind to nascent polypeptides, preventing them from forming secondary structures from incomplete information, releasing the protein for folding once the entire polypeptide has been translated. This mechanism simply makes finding the thermodynamically optimal conformation easier by preventing the formation of some potential suboptimal conformations. These chaperone proteins bind to the new proteins as they enter the lumen through the translocon and in addition to simply preventing incorrect bonds that would have to be broken, they also prevent premature interaction of multiple polypeptides with one another. This can be a problem because prior to the proper folding that would normally hide such domains within the protein, the immature polypeptides may have interaction domains exposed, leading to indiscriminate binding, and potentially precipitation of insoluble protein aggregates. Chaperones Chaperone proteins can also be found in prokaryotes, archaea, and in the cytoplasm of eukaryotes. These are somewhat similar to each other, and function somewhat differently than the types of folding proteins found in the ER lumen. They are referred to generally as chaperonins, and the best characterized is the GroEL/ GroES complex in E. coli. As the structure in Figure \(\PageIndex\). GroEL/GroES complex. The two heptameric rings of GroEL are shown in green and blue/purple. The GroES hepta...