Final answer:
The pinch-off of clathrin/AP coated vesicles is facilitated by dynamin which uses GTP hydrolysis to sever the vesicle from the plasma membrane, following assembly of clathrin and AP1 that promote vesicle budding and internalization.
Step-by-step explanation:
The pinch-off of clathrin/AP coated vesicles is primarily caused by a protein called dynamin, which aids the invagination and is crucial for the vesicle scission from the plasma membrane. The process is energy-dependent, utilizing GTP hydrolysis for the final severing of the vesicle.
In receptor-mediated endocytosis, substances that need to be internalized bind to cell membrane receptors, leading to the clustering and formation of a coated pit. Clathrin, along with adaptor protein 1 (AP1), assembles on the cytoplasmic side of the plasma membrane, providing a scaffold that induces curvature and facilitates the budding process. Dynamin wraps around the neck of the budding vesicle, and upon GTP hydrolysis, it constricts, leading to the vesicle's release into the cytoplasm.
After vesicle internalization, the clathrin coat is shed, and the now uncoated vesicle can fuse with an endosome. This further processes the vesicular content and recycles the disengaged clathrin to the plasma membrane.