Receptor-mediated Endocytosis

Receptor-mediated endocytosis is a process where specific molecules bind to cell-surface receptors, triggering the membrane to fold inward and form a vesicle. Clathrin proteins coat the budding vesicle, and accessory proteins pinch off the neck to release it into the cell. The vesicle then fuses with the early endosome for further processing.

Clathrin is a structural protein that binds to adaptor proteins on the inner membrane surface. It polymerizes to form a clathrin-coated pit that bends the membrane inward. The clathrin coat is rapidly released once the vesicle forms, exposing a naked vesicle ready to fuse with the early endosome.

The early endosome contains two domains with different internal pH levels. The low pH in the vacuolar domain causes endocytosed ligands to dissociate from their receptors. Receptors remain in the tubular domain due to its large surface area and are recycled back to the cell membrane.

LDL binds to receptors and enters the cell via clathrin-coated vesicles. In the early endosome, LDL dissociates from receptors due to low pH. The LDL remains in the endosome, which later fuses with a lysosome where digestive enzymes break it down into free cholesterol for cellular use.

Receptor-mediated endocytosis regulates signaling through sequestration, bringing receptors inside the cell to control signal intensity. Some receptors are stored in vesicles until needed, while others are degraded by proteolytic enzymes. Additionally, some signaling pathways require endocytosis to allow signal transduction into the cell.

Bacteria and viruses hijack host cell receptors to gain entry. The influenza virus binds to cell-surface receptors and uses clathrin-mediated endocytosis to enter. Bacillus anthracis produces anthrax toxin that binds to host receptors, undergoes endocytosis, and escapes the endosome to cause cellular damage.

Clathrin binds to the outer surface of the cell membrane and forms coated pits that bud inward. Caveolin, an alternative endocytic pathway, inserts itself directly into the lipid bilayer rather than coating the membrane surface. Both pathways internalize molecules, but they differ in their structural mechanisms and membrane interactions.