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ER Proteins

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ER Proteins

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ER Proteins Background

The endoplasmic reticulum is a series of important biological macromolecules other than nucleic acids in the cell, such as proteins, lipids (such as triglycerides) and sugar synthesis bases. The slippery endoplasmic reticulum also has detoxification functions, such as the slippery endoplasmic reticulum in hepatocytes containing enzymes to remove fat-soluble waste and metabolically produced harmful substances.

Figure 1. Components of a typical animal cell:8 represents endoplasmic reticulum.

Classification

According to whether there is ribosome attachment on the outer surface of the endoplasmic reticulum membrane, the endoplasmic reticulum is usually divided into two types: rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).

Rough endoplasmic reticulum

The main morphological feature is that the cytoplasmic surface of the retina has ribosomal particles attached, and hence the name. The rough endoplasmic reticulum is mostly arranged in a squamous sac. It is mainly related to the synthesis, processing and transport of exogenous proteins and various membrane proteins. Therefore, in cells having a function of secreting peptide hormones or proteins, rough endoplasmic reticulum is developed, such as pancreatic cells, plasma cells, and the like. Relatively underdeveloped in undifferentiated or poorly differentiated cells, such as embryonic cells, tumor cells, etc.

Figure 2. Rough endoplasmic reticulum.

Smooth endoplasmic reticulum, SER

Under the electron microscope, it has a smooth small tube and a small bubble-like network structure, which is often connected with the rough endoplasmic reticulum. The smooth endoplasmic reticulum is a multifunctional organelle. In different cells, different developmental stages of different cells or different physiological periods, their morphological structure, quantity, intracellular spatial distribution and development degree are quite different, and often exhibit different functional characteristics. For example, Leydig cells, ovarian corpus luteum cells and adrenocortical cells have a large number of smooth endoplasmic reticulum, which is related to the function of their synthetic steroid hormones; the rich smooth surface endoplasmic reticulum in hepatocytes is related to its attenuating function; The smooth endoplasmic reticulum in the striated muscle is specialized into the sarcoplasmic reticulum, which regulates muscle contraction by storing and releasing Ca2+.

Figure 3. Slip surface endoplasmic reticulum.

Function

The endoplasmic reticulum is a cytoplasmic membrane system that is connected to the outer membrane of the cytoplasm and communicates with the outer membrane of the nuclear membrane, organically connecting various structures within the cell into a whole, effectively increasing the membrane area within the cell, The role of material transport. The main function of ER is to synthesize proteins and lipids. Both secreted and transmembrane proteins are synthesized in ER. ER-synthesized lipids not only meet their own needs, but also provide membrane cell structures such as Golgi apparatus, lysosome, endosome, plasma membrane, mitochondria, and chloroplast. There is no ribosome attachment on the smooth endoplasmic reticulum, which has a small proportion, but its function is more complex. It is related to lipid and sugar metabolism, participates in the synthesis of glycogen and lipids, the synthesis of steroid hormones, and has secretion. Features. The accumulation of CI has been found on the smooth endoplasmic reticulum of certain cells in gastric tissue, which indicates that it is related to the secretion of HCl. In small intestinal epithelial cells, it has been observed that it is involved in transporting fat. The smooth endoplasmic reticulum in cardiomyocytes and skeletal muscle cells is related to the role of conduction excitation, and in smooth muscle cells, it is related to the uptake and release of Ca2+. Ribosomes are attached to the rough endoplasmic reticulum, and their arrangement is more regular than that of the smooth endoplasmic reticulum. Their function is to synthesize protein macromolecules and transport them out of the cell or transport them to other parts within the cell. Cells with strong protein synthesis have a rough endoplasmic reticulum. In nerve cells, the development of the rough endoplasmic reticulum is related to memory. The function of smooth endoplasmic reticulum is related to the synthesis, detoxification, and assimilation of sugars and lipids, and it has the function of transporting proteins.

ERS-induced Apoptosis Program

ER's stress response (ERS), after the stress response is still unregulated, and a persistent imbalance appears, the cell will start the apoptosis program. It is generally believed that the apoptosis program is caused by calcium homeostasis, and another important function of the endoplasmic reticulum is to store calcium ions. Under the stimulation of certain signals of the ERS response, the IP3R channel on the endoplasmic reticulum membrane is opened, causing a large amount of calcium ions to flow out. The combination of high levels of calcium ions and calpain requires the hydrolysis of calpain zymogen subunits to be activated. Activated calpain It can cleave a variety of protein substrates, such as cleaving vinculin to destroy the stability of the cytoskeleton and make cells apoptotic. It can also cut Bcl-xL to change from anti-apoptosis to pro-apoptosis. Activated calpain can also be transferred to the endoplasmic reticulum, caspase-12 zymogen is cleaved at several cleavage sites to activate it, acting as an important hydrolase for apoptosis.

Reference:

1. Porter KR.; et al. A study of tissue culture cells by electron microscopy: methods and preliminary observations. The Journal of Experimental Medicine. 1945, 81 (3): 233–46.

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