What is Mastocytosis?

Endocytosis or endocytosis is an important way for cells to obtain macromolecules and particulate matter from the outside of the cell. It is a common physiological phenomenon in eukaryotic cells. Extracellular material is wrapped by the plasma membrane, the plasma membrane invades and forms membrane-coated vesicles. The vesicles and the plasma membrane detach from the human cell and produce a series of physiological activities and functions in the cell. Endocytosis is closely related to a variety of life activities, such as immune response, neurotransmitter transport, cell signal transduction, and cell and tissue metabolic balance. With the deepening of its research in recent years, we have a deeper understanding of the function and mechanism of endocytosis.

According to the size of the internalization vesicles formed by endocytosis, endocytosis can be divided into

Endocytosis receptor-mediated endocytosis

In simple terms, the process of endocytosis by receptor-mediated endocytosis is divided into three steps: Macromolecules form ligand-receptor complexes with cell surface receptors. The ligand-receptor complex is concentrated in the coated pits. Coated pits are areas where the plasma membrane is sunken inward, which is equivalent to a molecular filter. Receptors, clathrins, and adaptors are concentrated here, and these specific regions account for about 2% of the plasma membrane area. The cytoplasmic end of the receptor has a sequence consisting of 4 amino acid residues (Tyr-XX-country), X is any amino acid, and West is a large hydrophobic amino acid, such as Phe, Leu, Met, etc. Recognize this sequence. After the receptor binds to the ligand, internalization is initiated, and clathrins begin to assemble. Coated pits that accumulate ligand-receptor complexes continue to invade into the cell, and finally, under the action of related proteins, coated vesicles are formed.

Coat vesicles are composed of clathrin and adapter proteins. Clathrins are also known as clathrins or clathrins, and their protein molecules are connected to form a scaffold at the coat. When the coat pit becomes a coat vesicle, the clathrin is removed and returned to the lower part of the plasma membrane. Adaptin. It has basically the same function as clathrin, but it exists in different types and can bind different receptors. The adaptor protein is interposed between the clathrin and the ligand and receptor complexes, and plays a connecting role. The process of clathrin forming the vesicles will be described in detail in Chapter 8.

After the cytosolic vesicles enter the cell, they first fuse with the early endocytosis and form three different destinations through three pathways: Fusion with lysosomes, the plasma membrane and ligand-receptor complexes are degraded, such as EGF Receptors, called receptor down-regulation; In the early stage of endocytosis, the ligand is separated from the receptor, and the receptor vesicles are evacuated to the original cell surface through the efflux to replenish the lost plasma membrane. Such as LDL receptors; the ligand-receptor complex does not decompose, reaches the other side of the cell in the vesicle, fuses with the plasma membrane, the ligand is separated from the receptor, and is secreted outside the cell, which is transcellular transport ( transcytosis), for example, antibodies from mother mice enter the breast milk through epithelial cells, and intestinal epithelial cells from suckling mice capture antibodies into the body, which is accomplished by transcellular transport.

Absorption of endocytic cholesterol and endocytosis of transferrin

Cholesterol absorption by animal cells is a well-known receptor-mediated endocytosis. Cholesterol is one of the constituents of membrane lipids. Cells absorb most of the cholesterol required by receptor-mediated endocytosis. Cholesterol is synthesized mainly in liver cells, and then forms low-density lipoproteins (U) L with phospholipids and proteins, and is released into the blood. The LDL particles have a molecular weight of 3 × 103 kDa, and the core contains approximately 1,500 cholesterol molecules that are esterified into long-chain fatty acids. The larger Apo-B protein (ligand) is organized as LDL particles. When the cell needs cholesterol for membrane synthesis, the cell synthesizes the LDL transmembrane receptor protein and inserts it into the plasma membrane. After the receptor binds to the LDL particle receptor, a coat vesicle is formed; the coat vesicle that enters the cytoplasm immediately removes the clathrin coat and becomes a smooth vesicle, which fuses with the early endosome and has a low pH in endosome The receptor is separated from the IDL particles; the LDL is then sent to the lysosome via the advanced endosome. In lysosomes, cholesterol esters in LDL particles are hydrolyzed to free cholesterol and used.

Transferrin is a glycoprotein in the blood that specifically transports iron to cells, with a molecular weight of 80kDa. Each transferrin molecule can bind 2 Fe3 +. There is a transferrin receptor on the cell surface. After the transferrin and the receptor specifically bind, iron-bound transferrin is delivered to the early endosomes through receptor-mediated endocytosis, and then in the acidic environment of the endosome In the process, transferrin releases the iron bound to it and becomes apotransferrin. A transferrin still binds to the receptor and returns to the plasma membrane in the form of a complex. In a neutral environment outside the cell, the transferrin leaves the receptor and participates in a new round of carrier transport.