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6. The Golgi Apparatus.pdf

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McGill University
Anatomy & Cell Biology
ANAT 262
John Presley

The Golgi Apparatus: The Golgi is the main sorting unit of the cell; everything that comes into or goes out of the cell passes through the Golgi at some point for modifications The Golgi uses COP-I vesicles and CCVs (clathrin coated vesicles) o CCVs are used for transport to endosomes and to the PM; their formation is very similar to COP-II vesicle formation Clathrin is also found on the PM for endocytosis, which we wont look at right now The Golgi can be separated into three components, cis, trans, and medial The cis Golgi is closer to the ER while the trans Golgi is closer to the plasma membrane (in terms of the secretory pathway) The Golgi as a whole is made up of stacks called cisternae which form a stack of pancake shape o The cis Golgi is the first cisterna o The trans Golgi is the last one o The medial Golgi are all of the cisternae in between The trans-Golgi network, TGN, comes after the Golgi The Golgi can vary in size, anywhere from 3 to 20 stacks, depending on the amount of secretion in the cell Clathrin coated vesicles also involve 2 layers, inner and outer The main difference between CCVs and COP-II is that CCV inner coat proteins are interchangeable, which means that the CCVs themselves are very versatile The Golgi has a bunch of different functions, the most notable being: Sequential modifications of N-linked oligosaccharides o Specificity between species usually starting with high mannose trees from the ER O-linked glycosylation, only in the Golgi o Proteoglycan glycosylation involves O-linked glycosylation to form GAGs Proteolytic modifications of proteins (late Golgi) o The trans Golgi lumen is slightly acidic, which activate proteolytic enzymes Sorting of proteins to various destinations (late Golgi) Production of secretory granules (late Golgi) o Such as, for example synaptic vesicles or zymogenic granules In the ER, N-linked glycosylation produces high mannose trees on asparginine residues which make their way to the Golgi In the Golgi, the mannoses are removed in the cis Golgi, and sequentially, as the protein moves through the Golgi, more and more different sugars are added on o The most common are NANA and GlcNAcs in humans, but other animals have different modifications Certain enzymes for specific glycosylation reactions are located in specific layers of the Golgi o Therefore, these reactions are sequential, only happening in certain parts of the Golgi o If you stain for a specific enzyme in EM, you will see that is can have a very exclusive location in either the cis, medial or trans Golgi One notable enzyme is the mannosidases, which remove mannoses; they are found in the cis Golgi because they need to react first to remove the mannose tree before other modifications can occur One other phenomenon as the pH of the Golgi gets lower as you go from cis to medial to trans o This may be important in the activity of specific enzymes O-linked glycosylation occurs uniquely in the Golgi, where N-acetylglucosamine is often linked to serine of threonine located before glycines Serine and tyrosine contain OH groups whose oxygen is the target of O-linked glycosylation In proteoglycan formation, other sugars may also be added, and the sugar linkages can be huge, over 100 or more repeating disaccharide units o These are different from N-linked modifications; theyre huge, than they can have a lot of variation o Proteoglycans can be further modified by sulfation Secreted proteoglycans can form a large portion of the extracellular matrix; they are useful in mucous formation and in cartilage Proteolytic enzymes are present in later parts of the Golgi, activated by the acidic lumen, and they can cleave certain polypeptide chains Some proteins, such as digestive enzymes lysosomes, are not active until certain sequences are cleaved off These modifications are often done in the Golgi, and then the not active proteins are sent off wherever they are needed The Golgi can target many different pieces of specialized membrane on target organelles o Vesicles will use kinesins to move out of the Golgi to their destination o The three places the Golgi can send to are the PM, endosomes, or back to the ER Sorting and packing into secretory granules are two things we wont look at. The Golgi, as we said before, is made of a cis and trans face, with a medial region in the middle Polar -- cis (entry) face and trans (exit) face Stacks consisting of layered cisternae (like a stack of pancakes) Golgi enzymes, most notably those that modify N-linked oligosaccharides, are localized sharply to particular regions of the Golgi apparatus Stacks are arranged in long ribbons with disorganized areas between stacks The trans face looks like its peeling off, a phenomenon what we will talk about soon in the cisternal maturation theory In the 1970s, it was known that things entered the Golgi at the cis face, and left at the trans face, but not much more There are multiple theories on how proteins actually move through the Golgi stacks: o The Golgi could be connected in such a way that proteins diffuse through connections between cisternae Or the cis cisterna could gradually mature to become the trans cisterna Or each cisterna could be a separate compartment with vesicles required to transport cargo Cisternal maturation was favoured by electron microscopists since they saw trans cisternae that appeared to be peeling off and disassembling o However, cisternal maturation could not explain how the different Golgi enzymes kept segregated in separate cisternae Except for retrograde trafficking to the ER, all proteins and other components exit from the Golgi from the trans face Golgu endosome/PM transport are transported by CCVs from the trans-Golgi face Multiple pathways exist, with both coated vesicles (clathrin) and tubular transport intermediates The cell surface destination can have different fates: Regulated secretion (such as secretion granules) Constitutive secretion, which are on all the time; the proteins are packages and sent as soon a
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