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McGill University
Anatomy & Cell Biology
ANAT 262
John Presley

Most of the membrane is recycled back to the surface, and what is inside of the early endosome goes onto the multi-vesicular body (late endosome) and then to the lysosome... - clathrin is a major way of budding at the cell surface - excess membranes containing lipids and receptors are recycled back to the surface (recycling of receptors is really efficient) Clathrin is made of a heavy chain and a light chain, adaptor proteins bind to the cargo and to the clathrin - in the picture below receptors are blue, ligands are red, adaptors are light green - main adaptor protein on the surface is AP2 which does not require binding to Arf - a number of “alternate adaptors” can work like AP2 this is an LDL particle, it is mostly made of cholesterol esters surrounded by a phospholipid monolayer (size varies) - bigger LDL is better - then this will bind to a LDL receptor and get exocytosed - research on families with people with high levels of LDL in their blood stream (die of heart disease at early ages) had a number of problems - LDL receptors may malfunc- tion and not go into clathrin pits so the LDL binds right onto the cell surface - 1960 - calthrin was discovered by EM LDL is taken up by AP2 APs are made up of four domains, specific to type and tissue - can being receptor tails via tyrosine and dileucine motifs (LDL receptor does not have either of these but can still react with AP2) - binds clathrin through ear - AP2 is on the surface - AP1, AP2, and AP4 are on the Golgi - if you give people statins (cholesterol lowering drugs) you can make their medical problems much more manageable - mutant flies that were heated (at high temperature the dynamin would not function properly) became paralyzed and fell to the bottom of the cage - their clathrin pits could not pinch off their vesicles (so they produced more and more dynamin) Receptor-Mediated Endocytosis Pathways: 1) Pathway 1: LDL, insulin, prolactin...the receptor is recycled back to the cell sur- face and the ligand degraded in the lysosomes - one of the simplest pathways - LDL binds to the LDL receptor at neutral pH, goes to the clathrin pit, and then to the early endosome...then the LDL will pop off the LDL receptor (receptor is recycled back to the cell surface) and the ligand will stay inside the early endosome - LDL can be degraded by proteases, but not the receptors - if these are activated for too long it is not too bad (may have low level of glucose in the blood and too much in the cell)... - hormones bind to the cell surface, are endocytosed and then get degraded - example: if you have insulin floating around the cells will want to send signals to activate other processes (such as glucose transport) but then the insulin needs to be degraded to stop the signal (but insulin receptors are kept) 2) Pathway 2: EGF...the EGF receptor accumulates in coated pits only after its binding to EGF and after its dimerization. The EGF receptor is not recycled back but degraded in the lysosomes (down regulation) - functions for many growth factor receptors (many show the same behavior) - if these are activated for too long it could create a cancer cell...may induce mitosis growth (need tight regulation) - even if you leave EGF around you will not continue to send the signal at a high level because the receptors will be degraded - EGF is endocytosed and both the EGF and the receptor are degraded (receptors stay inside internal vesicles and are degraded by the lysosome) 2) Pathway 3: Diferric transferrin. Both the receptor and the ligand are recycled back to the cells surface - transferrin receptor is found on the surface of cells and it can bind transferrin if the transferrin is loaded with two iron molecules - transferrin receptor binds to AP2 and ends up in the early endosome - in the early endosome transferrin will stay on the transferrin receptor but the iron will be stripped from the receptor (now transferrin will stay on the receptor with or without iron) - differic transferrin is transferrin with iron - apotransferrin (trasnferrin without the iron) only binds to the receptor at acid pH - at the cell surface (neutral pH) the transferrin will come off the receptor (unless there is a lot of iron present for it to bind to) - this process takes iron from outside of the cell and brings it inside 2) Pathway 4: IgA and IgG. Transcystosis...this process couples endocytosis and exocytosis (also known as “trans-epithelial vesicular transport”) - one kind of transcytosis - protein molecule is endocytosed from one side of the cell and secreted from the other side - IgA has a chain called the J chain which takes two antibodies and links them back to back (the secretory component can bind to IgA receptors on the surface of cells) - IgA is produced by lymphocytes and plasma cells but it ends up being secreted from various epithelia (so if you have a cold it can get to the cold viruses) - IgA receptor at the cell surface (of epithelial cells) is endocyto
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