Figure 13–34 The three-dimensional structure of a small N-linked oligosaccharide. The structure was determined by x-ray crystallographic analysis of a glycoprotein. This oligosaccharide contains only 6 sugars, whereas there are 14 sugars in the n-linked oligosaccharide that is initially transferred to proteins in the er (see figure 12–47).
(a) a backbone model showing all atoms except hydrogens; only the asparagine of the protein is shown. (b) a space-filling model, with the asparagine and sugars indicated using the same color scheme as in (a). (b, courtesy of richard feldmann.)
Figure 13–35 Two possible models explaining the organization of the Golgi apparatus and how proteins move through it.
It is likely that the transport through the golgi apparatus in the forward direction (red arrows) involves elements of both models.
(a) according to the cisternal maturation model, each golgi cisterna matures as it migrates outward through the stack. at each stage, the golgi resident proteins that are carried forward in a maturing cisterna are moved backward to an earlier compartment in copI-coated vesicles. when a newly formed cisterna moves to a medial position, for example, “leftover” cis golgi enzymes would be extracted and transported retrogradely to a new cis cisterna behind. likewise, the medial enzymes would be received by retrograde transport from the cisternae just ahead. In this way, a cis cisterna would mature to a medial and then trans cisterna as it moves outward. (b) In the vesicle transport model, golgi cisternae are static compartments, which contain a characteristic complement of resident enzymes. The passing of molecules from cis to trans through the golgi is accomplished by forward-moving transport vesicles, which bud from one cisterna and fuse with the next in a cis-to-trans direction.
retrieves escaped ER and Golgi proteins and returns them to upstream compartments. Directional flow could be achieved because forward-moving cargo molecules are selectively packaged into forward-moving vesicles. Although both forwardand backward-moving vesicles would likely be COPI-coated, the coats may contain different adaptor proteins that confer selectivity on the packaging of cargo molecules. Alternatively, transport vesicles shuttling between Golgi cisternae might not be directional at all, transporting cargo randomly back and forth; directional flow would then occur because of the continual input to the cis cisterna and output from the trans cisterna.
The vesicle transport model is supported by experiments that show that cargo molecules are present in small COPI-coated vesicles and that these vesicles can deliver them to Golgi cisternae over large distances. In addition, when experimentally aggregated membrane proteins are introduced into Golgi cisternae, they can be observed staying in place, while soluble cargo, even if present as large aggregates, traverses the Golgi at normal rates.
It is likely that aspects of both models are true. A stable core of long-lasting cisternae might exist in the center of each Golgi cisterna, while regions at the rim may undergo continuous maturation, perhaps utilizing Rab cascades that change their identity. As matured pieces of the cisternae are formed, they might break off and fuse with downstream cisternae by homotypic fusion mechanisms, taking large cargo molecules with them. In addition, small COPI-coated vesicles might transport small cargo in the forward direction and retrieve escaped Golgi enzymes and return them to their appropriate upstream cisternae. Golgi golgi Matrix proteins help organize the stack
The unique architecture of the Golgi apparatus depends on both the microtubule cytoskeleton, as already mentioned, and cytoplasmic Golgi matrix proteins, which form a scaffold between adjacent cisternae and give the Golgi stack its structural integrity. Some of the matrix proteins, called golgins, form long tethers composed of stiff coiled-coil domains with interspersed hinge regions. Golgins form a forest
Figure 13–36 A model of golgin function.