Endocytosis: Import system of the cell

Endocytosis is the import system of the cell where the macromolecules and particles from the surrounding medium is taken inside the cell. Cells internalize the materials by using plasma membrane components and deliver them to an internal compartment called endosomes.  From this endosomes they can be recycled to same or different regions of the plasma membrane or can be delivered to lysosomes for degradation.

There are three types of endosomes in the cell,

Early endosomes

Endosomes which are found just beneath the plasma membrane which consists of the components obtained directly from outside of the cell like carbohydrates, fats etc. The components may go to the Golgi apparatus or may be transferred to the lysosomes via late endosomes. In early endosomes, mild digestion may start, hence many hydrolases are synthesized and delivered there as proenzymes called Zymogens which contain extra inhibitory domains at their N terminus that keep the hydrolases inactive until these domains are proteolytically removed. 

Late endosomes

Endosomes which are found close to Golgi apparatus and near the nucleus. These are actually intermediate between early endosomes and lysosomes.

Recycling endosomes

Endosomes which is found near the early endosome will be containing certain membrane proteins or receptor. It will fuse with the plasma membrane when a signaling molecule activates the signaling cascade which inturn activates the endosome to bind to the cell membrane. After finishing their function, they will get back to the cell as vesicle. Best example for this is the GLUT4 proteins which is found inside the cell bounded to the vesicle. 

Both the late and early endosomes differ in their protein compositions and transition from early to late endosomes is accompanied by the release of Rab5 and the binding of Rab7. 

The acidity inside the late endosomes is higher compared to that of the earlier one. This gradient of acidic environments has a crucial role in the function of these organelles.  Endosomes has a vascular H+ ATPase over the membrane which is responsible for making the vesicle acidic by bringing in more amount of H+ ion. The pH basically in early endosomes is 6.

Coated vesicles: They are having a distinctive cage of proteins covering their cytosolic surface. Now before the vesicles fuse with a target membrane, they discard their coat as it is required for the two cytosolic membrane surfaces to interact directly and fuse.

The coat has two functions,

1) It concentrates specific membrane proteins in a specialized patch which then gives rise to the vesicle membrane.

2) The coat molds the forming vesicle.

These proteins are assembles into a curved basketlike lattice that deforms the membrane patch and thereby shapes the vesicle. 

Three well characterized types of coated vesicles distinguished by their coat proteins:

Clathrin coated à mediate transport from the endosomal-Golgi compartments and from the plasma membrane. 

COPII coated and COPI coated àmediate transport from the ER and Golgi cistern 

Clathrin is a coat protein whose subunits consists of three large and three small polypeptide chains  that together form a three legged structure called triskelion. These triskelions assemble together into a basketlike convex framework of hexagons and pentagons to form coated pits on the cytosolic surface of membranes.

Adapter protein which is a coat component in Clathrin coated vesicles form a discrete second layer of the coated positioned between the Clathrin cage and the membrane. They bind the Clathrin coat to the membrane and trap various transmembrane proteins which include the transmembrane receptors that capture soluble cargo molecules inside the vesicle so called cargo receptors.

Dynamins: These are the cytoplasmic proteins which helps in regulation of pinch off and uncoating of coated vesicles.  The protein consists of PIP2 binding doman which tethers the protein to the membrane and a GTPase domain, which regulates the rate at which the vesicles pinch off from the membrane. 

There are three types of Endocytosis: 

• Phagocytosis
• Pinocytosis
• Receptor Mediated Endocytosis

Phagocytosis: large particles are ingested via large vesicles called phagosome, which is about > 250 nm in diameter. Phagosome end up in lysosomes and the products of the subsequent digestive processes pass into cytosol to be used as food. Phagocytosis required other than nutrition is carried out by specialized cells called professional phagocytes. In mammals two main classes of professional phagocytes are macrophages and neutrophils. Any indigestible particle inside the phagocytes forms residual bodies which will be expelled out of the cell through Exocytosis.

It’s a triggered process which required the activation of receptors that transmit signals to the cell interior and initiate the response. Eg: Antibodies triggers phagocytosis by exposing the tail Fc region to the phagocytic cells.

Localized actin polymerization initiated by Rho family GTPases and their activating Rho-GEFs shapes the pseudopods. An active Rho GTPase switches on the kinase activity of local PI kinases and initial actin polymerization occurs in response to an accumulation of PIP2. To seal off the phagosome and complete its engulfment, actin is depolymerized at its base as PIP2 is subject to a PIP3 kinase which converts it into PIP3 is required for closure of the phagosome and may also contribute to reshaping the actin network to help drive the invagination of the forming phagosome.

Don’t eat me signal: the cell surface protein binds to the inhibitory receptor of the phagocytic cell and recruits tyrosine phosphatase proteins which antagonize the intracellular signals required for the phagocytosis.

Eat me signal: Phosphatidyl serine present inside the cell membrane will get exposed to the extracellular region by the enzyme Flipase. The exposure will lead to the initiation of eat me signal and macrophage will phagocytose the whole cell.   

Pinocytosis: Fluid and solutes are ingested via small pinocytic vesicles which are about 100 nm in diameter. This process is a constitutive process where it occurs continuously regardless of the needs of the cell. The pinocytic vesicles are formed from the coated pits of the plasma membrane. The pits are mostly coated by Clathrin and it will be pinched off from the membrane by dynamins.

Not all the pinocytic vesicle is coated with Clathrin, but there are other vesicles which are not understood very well and they are caveolae. They are recognized by their ability to transport molecules across endothelial cells which form the inner lining of blood vessels. They are thought to form from membrane microdomains or lipid rafts. The major structural proteins in caveolae are caveolins which are integral membrane proteins that each insert a hydrophobic loop into the membrane from the cytosolic side but do not extend across the membrane.   The caveolins don’t not dissociate like Clathrin do because these proteins are integral proteins hence they stick with the vesicle surface even after they are pinched off. Instead they are delivered to the target compartments where they are maintained as discrete membrane domains. 

Receptor mediated endocytosis:

The macromolecules bind to complementary transmembrane receptor proteins, accumulate in coated pits and then enter the cell as receptor macromolecules complexes in clathrin coated vesicles. This process is called receptor mediated endocytosis. This happens basically because the ligands are selectively captured by receptors, receptor mediated endocytosis provides a selective concentrating mechanism that increases the efficacy of internalization of particular ligands more than a hundred fold. The LDL receptor goes to the early endosome and gives off the cholestrol molecules. Then retrives back to the plasma membrane without any coating.