Question

What role does clathrin and adaptin play in the secretory and endocytic pathway? (define all 4 terminologies)

Answer 1

Please find below your explanation. If I summarize your answer that clathrin plays major role for secretary and endocytosis however adaptin plays role with clathrin.

Clathrin role in secretary: a second class of coat proteins that form a layer between the clathrin and the membrane

The secretory pathway in eukaryotic cells is used to send proteins and lipids to the plasma membrane and certain membrane-bound organelles and to release material outside the cell. There are two types of secretion: constitutive and regulated. Constitutive secretion is the default pathway and is used primarily to replenish material at the plasma membrane and certain membrane-bound organelles. Regulated secretion terminates in secretory vesicles that store secreted material until a signal triggers fusion with the plasma membrane. Both types of secretion use the same pathway but signal sequences divert proteins into the regulated pathway. Cells also retrieve material from the plasma membrane through

• Clathrin forms the coat around vesicles, and the vesicles accumulate lysosomal proteins before budding from the trans-Golgi network. These vesicles fuse with endosomes. The lumen of endosomes has a low pH causing the mannose 6-phosphate receptor to dissociate from lysosomal proteins. The mannose 6-phosphate receptor is returned to the trans-Golgi network and the vesicle containing the lysosomal proteins matures into a functional lysosome.
• Some proteins are sorted into secretory vesicles that store these proteins until the cell is signaled to release them. The mechanism of by which proteins are sorted into secretory vesicles as these proteins do not share a common sorting signal sequence.

Clathrin role in Endocytosis:

Cells not only release material to the external environment but they also take up material from outside the plasma membrane through endocytosis. There are several forms of endocytosis.

• Phagocytosis allows some cells (macrophages, neutrophils) to engulf and take up large particles such as microorganisms and dead cells. Phagocytosis involves the protrusion of the plasma membrane around the particle. Protrusion is driven by actin polymerization. The plasma membrane eventually surrounds the particle and fuses to completely enclose it and form a large endocytic vesicle.
• Pinocytosis forms much smaller vesicles (~ 100 nm) and allows cells to take up small amounts of extracellular fluid and portions of the plasma membrane. One form of pinocytosis is clathrin-mediated endocytosis that allows cells to take up specific proteins from the cell surface.

Clathrin-mediated endocytosis begins with the formation of pit in the plasma membrane. The pit is surrounded on the cytoplasmic side by adaptor proteins that link clathrin to the pit. The adaptors also interact with proteins in the plasma membrane that are targeted for endocytosis. The pit can accommodate ~ 1000 proteins. Polymerization of clathrin drives formation of a vesicle that eventually pinches off from the plasma membrane. The GTPase dynamin catalyzes the pinching off reaction. The clathrin-coated vesicles fuse with endosomes where the low pH dissociates ligands from receptors. Some proteins are then returned to the plasma membrane whereas others are targeted to the lysosome where they are degraded.

Adaptin role:

• Clathrin adaptor proteins, also known as adaptins, are proteins that mediate the formation of vesicles for intracellular trafficking and secretion. Adaptins are clustered subunits of adaptor protein (AP) complexes.There are several types of adaptin, each related to a different AP complex.
• Clathrin molecules seem to be specific for certain receptors. The way that specificity is achieved is through the use of a molecule called adaptin. The cytoplasmic side of individual receptors may bind to specific adaptin molecules, which are bound to clathrin. This chain of associations can give specificity to clathrin-coated pits, meaning that specific receptors can be endocytosed into one pit. In some cases, receptors are endocytosed whether they have bound a ligand or not. In other cases, the only time that the receptor takes on a conformation that is amenable to binding adaptin is after the receptor has bound its specific ligand.