Exam 3 BIO 362 Creighton University (Parts 3 and 4)Questions and Answers with complete solution

1) What is desensitization? 2) What kind of cell receptors can this happen to? - Answer-1) When a cell decreases the amount of receptors on its surface to decrease the amount of response that the cell has to a ligand/stimulus 2) Can happen to any cell surface signalling receptor How is the endocytic pathway mediated? - Answer-Clathrin based endocytosis Describe the endocytic pathway in a stepwise fashion - Answer-1) Plasma membrane breaks off into free floating vesicles in the cytoplasm 2) The vesicles will fuse with the early endosome 3) Endosome will go through a series of maturation steps to become a late endosome 4) The late endosome will then fuse with a lysosome to become an endolysosome What are the 2 different pathways that an early endosome can go through? - Answer-1) Go through the endosomal systems and be degrades by lysosome (transported to to the lysosome) 2) Sorted into recycling endosome and sent back to the plasma membrane Describe the steps of how the cell surface receptors get to the early endosome - Answer-1) Cell surface receptors bind to a signal and recruit AP2 2) Clathrin will bind to form a curving membrane 3) Dynamin will pinch off the vesicles to become a free floating vesicle in the cytoplasm 4) Clathrin coat will then fall off 5) Rabs and PIPs will attach to the correct motor protein and vesicles will get trafficked to the early endosome What happens to a LDL cholesterol receptor and it's ligand once it reaches the early endosome - Answer- 1) The receptor and the ligand will get packaged into a vesicles and get sent to the early endosome2) The LDL and the receptor gets packages into a intralumenal vesicle (inside of the early endosome) 3) The packaging vesicles will remove the receptor and the receptor itself will travel to another part of the early endosome and get packaged and sent back to the plasma membrane of the cell to be reused 4) The early endosome will still contain the ligand and then it will go through maturation steps to become a late endosome 5) The late endosome will then fuse with a lysosome and then the packaging vesicle will be degraded 6) Once the vesicle is degraded then the ligand (LDL cholesterol in this case) can be released into the cell Describe the process of how a receptor gets degraded like in desensitization (in a stepwise fashion) - Answer-1) A ubiquitin tag is added to a lysine amino acid when the receptor is in the early endosome 2) The early endosome will then become a late endosome and the ubiquitin tag is still one the receptor 3) The ubiquitin tag will then get removed and the receptor and the ligand will then get internalized and become a multivesicular body (vesicles inside of vesicles) 4) This multivesicular body will then fuse with a lysosome to become a endolysosome and the receptor and the ligand will then be degraded Why do the outer membrane proteins in the endosome not get degraded? - Answer-If it did then it would destroy the proteins of the endosome itself (e.g. proton pump), instead it will only destroy the inside layer of membrane Describe the formation of the multivesicular bodies in a stepwise fashion - Answer-1) ESCRT-0 will bind to the cargo with the ubiquitin tag on it and the PI(3)P 2) This will recruit ESCRT-I and ESCRT-II (these will remove the ubiquitin tags 3) This will then recruit ESCRT-III (this is a curved protein and will bend the membrane of the endosome and they will eventually come together to make a circular vesicle) which will essentially for the vesicle 4) The vesicle will then break off and go to the lysosome 1) What does the cytosolic sides of the cell membrane come once the multivesicular body is formed? 2) How about the lumenal sides of the endosome - Answer-1) It then becomes the inside of the multivesicular body 2) It will remain the lumenal side of the endosomeWhat happens to receptors when... 1) There is a ubiquitin tag added to it? 2) There is NO ubiquitin tag added to it? - Answer-1) Will be sent to the lysosome for degradation 2) Will be sent to the recycling endosome so it can be sent back to the plasma membrane so it can be used again True or False? Recycling endosomes only function is to send receptors back to the plasma membrane of a cell - AnswerFALSE! It can also be used as a temporary storage for membrane receptors What is the purpose for recycling endosomes to serve as a temporary storage center? - Answer-It is used for when cells need a quick response to changing environment by sending protein channels to the surface (e.g. insulin and it's insulin channel). The cell cannot wait for the ER to produce the protein channel and have it sent to the cell surface, it would take too long.