Lecture 21 – Coated Vesicles, Endocytosis, Begin Nucleus
Endocytosis – Transport of materials into a cell through a membrane modification
• Types of Endocytosis
◦ Pinocytosis - “Drinking” one can see vesicles forming that are taking in extracellular liquid.
• In looking at the vesicles involved in endocytosis, there was noticed a network of fibers around
• This network, or “coating” was found to be composed of a single protein, called clathrin, which
forms a trimer, called a triskelion. Many triskelions come together to form a regular lattice, or
• The clathrin binds to a membrane receptor protein, which in turn binds to the plasma
membrane. Because Clathrin is curved, it also curves the plasma membrane, creating a greater
and greater curve (eventually a sphere) as more and more triskelions join the lattice-work.
• To finally separate the forming vesicle from the plasma membrane, a protein called dynamin
forms a ring around the neck of the vesicle, and when GTP is hydrolyzed, the dynamin
contracts, pulling the vesicle off of the membrane.
• To determine if GTP was truly necessary for correct dynamin formation, GTPγS was
substituted, but it caused dynamin to form a spiral, and only elongated the vesicle neck, instead
of severing it.
Vesicle Movement in Cell
• Vesicle loses Clathrin Coat, forms a sorting component (early peripheral endosome), lowers its
pH with a Proton Pump, divides receptors from proteins (in late endosome) using a lysozome to
lower pH, which causes receptors to release proteins.
• Starts in Nucleus
◦ Contains a mixture of DNAand Proteins:
◦ Histones, Polyme