Receptor-mediated endocytosis is a process in which receptors are used for importing material from extra-cellular matrix into the cells. This BiologyWise post elaborates more on this cellular process critical for the growth and development of cells.
Did You Know?
If endocytosis, an important cellular process is not taking place properly, it may cause chronic ailments like leukemia and Alzheimer’s disease.
Endocytosis is an important process used by body cells for their survival. It is essentially a cellular process in which cells ingest nutrients in the molecular form. This is how cells eat for their survival. It is a mechanism of nutrient absorption, which is critical for cellular growth.
In most cases, the material that needs to be absorbed by cells is available in the form of large molecules that are simply unable to penetrate the cell membrane. This is where endocytosis comes into picture, wherein the cell simply swallows the substance (molecule) and enters the cytoplasm. Thus, it is through endocytosis that the substances can move inside the cell.
Depending upon how endocytosis is carried out, it is classified into 3 main types, viz. phagocytosis, pinocytosis, and receptor-mediated. Here we will be discussing receptor-mediated endocytosis.
Receptor-mediated Endocytosis (RME)
This type of endocytosis makes use of receptors (attached to the cell membrane) to engulf molecules. It is also referred to as clathrin-dependent endocytosis, since clathrin (a type of protein) is crucial for the proper execution of this cellular process. In this technique, specific molecules that get bound to the receptors can only be engulfed by the cell.
- Whenever a receptor (molecule) that is embedded within the plasma membrane, detects a molecule (that it can bind to) outside the cell, it immediately hooks onto it. The molecule (ligand) attached to the receptor then travels all the way to the clathrin-coated pit.
- A clathrin-coated pit is a special area located on the plasma membrane that initiates the uptake of molecules present in the extracellular region. The pit shows a distinct polygonal lattice of clathrin chains on its inner surface. When the receptor-molecule reaches the clathrin-coated pit, it is observed that the pit folds inwards and then that part of the membrane detaches itself to form a closed coated vesicle.
- These vesicles act as tools to move molecules inside the cells. Simply put, they transport molecules within the cells. If the receptor-bound molecule is a pathogen, opsonization mechanism is activated, meaning the molecule is tagged as a pathogen for subsequent destruction. After opsonization, the protein coat of clathrin is shaken off to allow the vesicle to merge with an early endosome.
- An early endosome is an organelle, a sorting compartment that helps separate the molecule from its receptor. Once the vesicle fuses with the endosome, multiple compartments are formed within the endosome and at the same time the molecule detaches itself from the receptor.
- Chemical changes occur within the endosome to form a late endosome. The late endosome splits into two, in which one endosome contains the molecule, while the other contains the receptor.
- The molecule-containing endosome then combines with a lysosome, which is essentially a membrane-bound cell organelle that stores digestive enzymes. The main job of the lysosome (also referred as the stomach of the cell) is to promote breakdown of the molecule, which can either be a protein or fat. The lysosome can also ingest pathogens including bacteria and viruses, in turn helping to clear cellular debris.
- The lysosome is essentially involved in digesting the material present in the endosome. The receptor in the other endosome is then recycled and sent back to the cell surface.
Body cells use the mechanism of receptor-mediated endocytosis to absorb cholesterol, growth factor EGF, and the iron transport protein transferrin from the bloodstream.