Endocytosis - Principal Components of Endocytic Pathway

Principal Components of Endocytic Pathway

The endocytic pathway of mammalian cells consists of distinct membrane compartments, which internalize molecules from the plasma membrane and recycle them back to the surface (as in early endosomes and recycling endosomes), or sort them to degradation (as in late endosomes and lysosomes). The principal components of the endocytic pathway are:

• Early endosomes are the first station of the endocytic pathway. Early endosomes are often located in the periphery of the cell, and receive most types of vesicles coming from the cell surface. They have a characteristic tubulo-vesicular structure (vesicles up to 1 µm in diameter with connected tubules of approx. 50 nm diameter) and a mildly acid pH. They are principally sorting organelles where many ligands dissociate from their receptors in the acid pH of the lumen, and from which many of the receptors recycle to the cell surface (via tubules). It is also the site of sorting into transcytotic pathway to late components (via vesicular component, which can form multivesicular bodies (MVB) or endosomal carrier vesicles (ECVs)).
• Late endosomes receive internalized material en route to lysosomes, usually from early endosomes in the endocytic pathway, from trans-Golgi network (TGN) in the biosynthetic pathway, and from phagosomes in the phagocytic pathway. Late endosomes often contain many membrane vesicles or membrane lamellae and proteins characteristic of lysosomes, including lysosomal membrane glycoproteins and acid hydrolases. They are acidic (approx. pH 5.5), and are part of the trafficking itinerary of mannose-6-phosphate receptors. Late endosomes are thought to mediate a final set of sorting events prior to delivery of material to lysosomes.
• Lysosomes are the last compartment of the endocytic pathway. They are acidic (approx. pH 4.8) and by EM usually appear as large vacuoles (1-2 µm in diameter) containing electron dense material. They have a high content of lysosomal membrane proteins and active lysosomal hydrolases, but no mannose-6-phosphate receptor. They are generally regarded as the principal hydrolytic compartment of the cell. Lysosomes chief function in itself is to break down cellular waste products, fats, carbohydrates, proteins, and other macromolecules into simple compounds. These are then returned to the cytoplasm as new cell-building materials. To accomplish the tasks the lysosomes use some 40 different types of hydrolytic enzymes, all of which are manufactured in the endoplasmic reticulum and modified in the Golgi Apparatus.

It was recently found that an Eisosome serves as a portal of endocytosis in yeast.