Because chains of sugars have limited flexibility, even a small N-linked oligosaccharide protruding from the surface of a glycoprotein (Figure 13–34) can limit the approach of other macromolecules to the protein surface. In this way, for example, the presence of oligosaccharides tends to make a glycoprotein more resistant to digestion by proteolytic enzymes. It may be that the oligosaccharides on cell-surface proteins originally provided an ancestral cell with a protective coat; compared to the rigid bacterial cell wall, such a sugar coat has the advantage that it leaves the cell with the freedom to change shape and move.
The sugar chains have since become modified to serve other purposes as well. The mucus coat of lung and intestinal cells, for example, protects against many pathogens. The recognition of sugar chains by lectins in the extracellular space is important in many developmental processes and in cell–cell recognition: selectins, for example, are transmembrane lectins that function in cell–cell adhesion during blood cell migration, as discussed in Chapter 19. The presence of oligosaccharides may modify a protein’s antigenic and functional properties, making glycosylation an important factor in the production of proteins for pharmaceutical purposes.
Glycosylation can also have important regulatory roles. Signaling through the cell-surface signaling receptor Notch, for example, is an important factor in determining the cell’s fate in development (discussed in Chapter 21). Notch is a transmembrane protein that is O-glycosylated by addition of a single fucose to some serines, threonines, and hydroxylysines. Some cell types express an additional glycosyl transferase that adds an N-acetylglucosamine to each of these fucoses in the Golgi apparatus. This addition changes the specificity of Notch for the cell-surface signal proteins that activate it.
Transport Through the golgi apparatus May occur by cisternal Maturation
It is still uncertain how the Golgi apparatus achieves and maintains its polarized structure and how molecules move from one cisterna to another, and it is likely that more than one mechanism is involved in each case. One hypothesis, called the cisternal maturation model, views the Golgi cisternae as dynamic structures that mature from early to late by acquiring and then losing specific Golgi-resident proteins. According to this view, new cis cisternae continually form as vesicular tubular clusters arrive from the ER and progressively mature to become a medial cisterna and then a trans cisterna (Figure 13–35A). A cisterna therefore moves through the Golgi stack with cargo in its lumen. Retrograde transport of the Golgi enzymes by budding COPI-coated vesicles explains their characteristic distribution. As we discuss later, when a cisterna finally moves forward to become part of the trans Golgi network, various types of coated vesicles bud off it until this network disappears, to be replaced by a maturing cisterna just behind. At the same time, other transport vesicles are continually retrieving membrane from post-Golgi compartments and returning it to the trans Golgi network.
The cisternal maturation model is supported by studies using Golgi enzymes from different cisternae that were fluorescently labeled with different colors. Such studies performed in yeast cells where Golgi cisternae are not stacked reveal that individual Golgi cisternae change their color, thereby demonstrating that they change their complement of resident enzymes as they mature, even though they are not stacked. In further support of the model, electron microscopic observations found that large structures such as procollagen rods in fibroblasts and scales in certain algae move progressively through the Golgi stack.
An alternative view holds that Golgi cisternae are long-lived structures that retain their characteristic set of Golgi-resident proteins firmly in place, and cargo proteins are transported from one cisterna to the next by transport vesicles (Figure 13–35B). According to this vesicle transport model, retrograde flow of vesicles