Question

Receptor mediated endocytosis definition

Answer 1

Endocytosis is a form of active transport in which a cell transports molecules (such as proteins) into the cell (endo- + cytosis) by engulfing them in an energy-using process. Endocytosis and its counterpart, exocytosis, are used by all cells because most chemical substances important to them are large polarmolecules that cannot pass through the hydrophobic plasma or cell membrane by passive means.

Endocytosis includes pinocytosis (cell drinking) and phagocytosis (cell eating).

History

The term was proposed by De Duve in 1963.[1]Phagocytosis was discovered by Élie Metchnikoff in 1882.[2]

Endocytosis pathways



Schematic drawing illustrating clathrin-mediated (left) and clathrin-independent endocytosis (right) of synaptic vesicle membranes.

Endocytosis pathways can be subdivided into four categories: namely, receptor-mediated endocytosis (also known as clathrin-mediated endocytosis), caveolae, macropinocytosis, and phagocytosis.[3]

Clathrin-mediated endocytosis is mediated by the production of small (approx. 100 nm in diameter) vesicles that have a morphologically characteristic coat made up of the cytosolic protein clathrin. Clathrin-coated vesicles (CCVs) are found in virtually all cells and form domains of the plasma membrane termed clathrin-coated pits. Coated pits can concentrate large extracellular molecules that have different receptorsresponsible for the receptor-mediated endocytosis of ligands, e.g. low density lipoprotein, transferrin, growth factors, antibodies and many others.[4]

Study by [5] in mammalian cells confirm a reduction in clathrin coat size in an increased tension environment. In addition, it suggests that the two apparently distinct clathrin assembly modes, namely coated pits and coated plaques, observed in experimental investigations might be a consequence of varied tensions in the plasma membrane.

Caveolae are the most common reported non-clathrin-coated plasma membrane buds, which exist on the surface of many, but not all cell types. They consist of the cholesterol-binding protein caveolin (Vip21) with a bilayer enriched in cholesterol and glycolipids. Caveolae are small (approx. 50 nm in diameter) flask-shape pits in the membrane that resemble the shape of a cave (hence the name caveolae). They can constitute up to a third of the plasma membrane area of the cells of some tissues, being especially abundant in smooth muscle, type I pneumocytes, fibroblasts, adipocytes, and endothelial cells.[6] Uptake of extracellular molecules is also believed to be specifically mediated via receptors in caveolae.

Potocytosis is a form of receptor-mediated endocytosis that uses caveolae vesicles to bring molecules of various sizes into the cell. Unlike most endocytosis that uses caveolae to deliver contents of vesicles to lysosomes or other organelles, material endocytosed via potocytosis is released into the cytosol.[7]

Macropinocytosis, which usually occurs from highly ruffled regions of the plasma membrane, is the invagination of the cell membrane to form a pocket, which then pinches off into the cell to form a vesicle (0.5–5 µm in diameter) filled with a large volume of extracellular fluid and molecules within it (equivalent to ~100 CCVs). The filling of the pocket occurs in a non-specific manner. The vesicle then travels into the cytosol and fuses with other vesicles such as endosomes and lysosomes.[8]

Phagocytosis is the process by which cells bind and internalize particulate matter larger than around 0.75 µm in diameter, such as small-sized dust particles, cell debris, micro-organismsand apoptotic cells. These processes involve the uptake of larger membrane areas than clathrin-mediated endocytosis and caveolae pathway.