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Proteins are synthesised by ribosomes. ER (Endoplasmic Reticulum) with attached ribosomes is called RER (Rough ER). The proteins that are synthesised are packaged into vesicles formed by the budding of RER and are then transported to the Golgi apparatus by specialised protein trafficking mechanism. The vesicles are of various types like COPI coated, COPII coated, clathrin coated etc. Each serve a distinct function. Once they reach the cis side cisternae of the Golgi, the vesicles are joined with the Golgi and the proteins get transported to the trans side where they again get packed in vesicles that pinch off from the trans side of the Golgi. This time each protein have an address tag based on their destination. Also, several other modifications like glycosylations are done on the proteins which make them active in function. All these are done by several enzymes that are present in the Golgi cisternae.
These enzymes which are present in the Golgi cisternae are called Resident Golgi enzymes. These enzymes are also proteins synthesised in the RER and transported to the Golgi via specially coated vesicles. The difference from others is that they are tagged specifically for the Golgi apparatus and hence are not trafficked out of the Golgi apparatus. The cisternal maturation model hypothesis of the Golgi explains that these resident enzymes are carried along with the proteins that they modify, in the vesicles. Once on the trans side, COPI coated vesicles carry them back to the cis side of the Golgi body. Thus the resident enzymes are not lost from the apparatus.
Some of the resident glycosylation enzymes of the golgi vesicles are mannosidase I and II, GlcNac transferase, Gal transferase and NANA transferase.
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