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Lecture 9

Lecture #9 notes

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University of Toronto St. George
Darrel Desveaux

Tuesday, February 3, 2009 - We continue with vesicle transport, were moving away from the Golgi, the trans Golgi network, that is basically the major branching point in vesicle transport and how do vesicles now make it to the outside or the plasma membrane and release their contents into the outside of the cell (extracellular space)? - Just to recap, this slide is what weve been looking at in terms of vesicle transport. We looked at vesicle transport last lecture from the trans Golgi network to the lysosome and how theres a signal that is added on in the cis Golgi onto proteins that are to be directed to the lysosome. These are for example, lysosomal hydrolases that need to end up in the lysosome and provide the functions of the lysosome in terms of degrading macromolecules. - Just to recap, the trans Golgi network is a major branching point for proteins that are to be sorted to different compartments in vesicles. Proteins that move through the Golgi will end up in the trans Golgi network and the vesicles that bud off the trans Golgi network can have different destinations. - We looked at the lysosome, going through either the late endosome or early endosome, these will eventually mature or become the lysosome. - But vesicles can also bud off the TGN and make it to the plasma membrane, this is called exocytosis. There are two major forms of exocytosis that can occur. One is regulated and one is constitutive and basically what that means is that when the vesicle buds off, it either goes to the plasma membrane and fuses automatically, and that is the constitutive version or the vesicle can sit around in the cytoplasm and wait for a signal in order to fuse to the plasma membrane. That is called the regulated secretory pathway. - Wel l lo o kat th ose two examples so basically, proteins that are going through the secretion pathway enter the ER, go through the Golgi, there are two different ways they can go through the Golgi that we looked at and then theyre sorted out at the major branching point which is the trans Golgi network to either the lysosome via the early endosomes or to the plasma membrane in a process called exocytosis. - Well now look at a small animation that goes through the process till where we are. Animation - The sig nal seq uenc e of a membrane protein directs the ribosome to the rough ER. The growing polypeptide chain enters the lumen of the ER through a protein import channel in the membrane. There the signal sequence is cleaved, sugars are added and the protein folds. Some enzymes or structural proteins remain in the ER. Most proteins that are soluble in the ER are transported in vesicles to the cis Golgi. - In the cisternal maturat io n mo del, the cis Golgi matures to be come the medial Golgi, a new cis Golgi is formed by fusion of the ER vesicles. The maturation process includes retrograde vesicular transport of resident Golgi proteins. - Proteins dest n ied for se rcet i n after mo d ific ti n in the Golgi are transported from the trans Golgi to the plasma membrane. When the membranes fuse, the proteins are released into the extracellular space. - Soluble proteins on the outside of a cell can reenter by endocytosis and are sorted to lysosomes for degradation. Other proteins, including lysosomal enzymes sort directly to lysosomes from the trans Golgi.
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