Endocytose

Endocytosis, also spelled endocytose in some contexts, is a cellular process by which eukaryotic cells internalize extracellular material by invagination and scission of the plasma membrane, forming vesicles that transport contents into the cell. The process is energy-dependent and requires coordination among membrane curvature, coat proteins, cytoskeletal elements, and small GTPases. Endocytosis plays a central role in nutrient uptake, regulation of receptor signaling, and immune defense, among other functions.

Major pathways include phagocytosis, pinocytosis, and receptor-mediated endocytosis. Phagocytosis involves the uptake of large particles by specialized cells such as macrophages and neutrophils, producing phagosomes that fuse with lysosomes for degradation. Pinocytosis, or fluid-phase endocytosis, engulfs extracellular fluid and solutes; macropinocytosis forms large vesicles and is often used by antigen-presenting cells. Receptor-mediated endocytosis is highly specific and relies on receptors to concentrate ligands at the plasma membrane; most commonly this pathway is clathrin-dependent, with adaptor protein complexes (such as AP2) linking receptors to clathrin coats and dynamin driving vesicle scission. Some forms are clathrin-independent and may involve caveolae, which use caveolin proteins to form flask-shaped invaginations.

After vesicle formation, the endocytic cargo is trafficked to early endosomes (Rab5), where sorting occurs. Some receptors are recycled back to the plasma membrane (Rab11-dependent recycling), while others are directed to late endosomes (Rab7) and lysosomes for degradation. This trafficking regulates receptor abundance, nutrient uptake, and signal transduction and can generate secondary vesicles such as exosomes through multivesicular body pathways.

Functions include uptake of iron and cholesterol (e.g., transferrin and LDL receptors), regulation of signaling, antigen processing and presentation, and defense against pathogens. Many viruses and other microbes exploit endocytic routes to enter host cells.