The Golgi complex is the site of the modification, completion, and export of secretory proteins and glycoproteins. This organelle, first described by the Italian cytologist Camillo Golgi in 1898, has a characteristic structure composed of five to eight flattened, disk-shaped, membrane-defined cisternae arranged in a stack. Secretory proteins and glycoproteins, cell membrane proteins and glycoproteins, lysosomal proteins, and some glycolipids all pass through the Golgi structure at some point in their maturation. In plant cells, much of the cell wall material passes through the Golgi as well.
The Golgi apparatus itself is structurally polarized, consisting of a “cis” face near the transitional region of the RER, a medial segment, and a “trans” face near the cell membrane. These faces are biochemically distinct, and the enzymatic content of each segment is markedly different. The cis face membranes are generally thinner than the others.
As the secretory proteins move through the Golgi, a number of chemical modifications may transpire. Important among these is the modification of carbohydrate groups. As described above, many secretory proteins are glycosylated in the ER. In the Golgi, specific enzymes modify the oligosaccharide chains of the glycoproteins by removing certain mannose residues and adding other sugars, such as galactose and sialic acid. These enzymes are known collectively as glycosidases and glycosyltransferases. Some secretory proteins will cease to be transported if their carbohydrate groups are modified incorrectly or not permitted to form. In some cases the carbohydrate groups are necessary for the stability or activity of the protein or for targeting the molecule for a specific destination.
Also within the Golgi or secretory vesicles are proteases that cut many secretory proteins at specific amino acid positions. This often results in activation of the secretory protein, an example being the conversion of inactive proinsulin to active insulin by removing a series of amino acids.
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