Receptor-mediated endocytosis is when bulk amounts of specific molecules are imported into a cell after binding to cell surface receptors. The molecules bound to these receptors are taken into the cell through inward folding of the cell surface membrane, which is eventually pinched off into a vesicle within the cell. Structural proteins, such as clathrin, coat the budding vesicle.
Clathrin-Mediated Endocytosis of LDL
One well-characterized example of receptor-mediated endocytosis is the transport of low-density lipoproteins (LDL cholesterol) into the cell. LDL binds to transmembrane receptors on the cell membrane. Adapter proteins allow clathrin to attach to the inner surface of the membrane. These protein complexes bend the membrane inward, creating a clathrin-coated vesicle inside the cell. The neck of the endocytic vesicle is pinched off from the membrane by a complex of the protein dynamin and other accessory proteins.
The endocytic vesicle fuses with an early endosome, and the LDL dissociates from the receptor proteins due to a lower pH environment. Empty receptor proteins are separated into transport vesicles to be re-inserted into the outer cell membrane. LDL remains in the endosome, which binds with a lysosome. The lysosome provides digestive enzymes that break up LDL into free cholesterol that can be used by the cell.
Roles of Receptor-Mediated Endocytosis
Receptor-mediated endocytosis is also used to regulate cell signaling. Sequestration is one of the primary ways to regulate signal receptors, and the process involves bringing receptors inside the cell using endocytosis. Some receptors are stored within vesicles until needed again, and some are degraded by proteolytic enzymes. Other signaling pathways require receptor-mediated endocytosis to allow signal transduction (i.e., passing the signal into the cell). There are also alternative pathways for endocytosis, of which caveolin is the most studied. Unlike clathrin, which binds to the surface of the bilayer, caveolin inserts itself into the lipid bilayer.
Use of Receptor-Based Endocytic Pathways by Pathogens
Some bacteria and viruses can invade host cells by hijacking the host's native receptors. The influenza virus can invade host cells using clathrin-mediated and other endocytic pathways. The virus binds to receptors on the cell surface, gaining access to the host cell and later escaping from the endosome.
Some pathogens release toxins that attach to host receptors to trick the cell into taking them inside. The bacterium Bacillus anthracis produces the toxin known as anthrax; this toxin can bind to the host cell's receptors, undergo endocytosis, and then escape the late endosome to cause clinical necrosis symptoms.
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