The Golgi apparatus, vesicular transport and Lysosomes


Chapter 10, pages 408-410; 419-428

In the previous discussion we examined the role of the endoplasmic reticulum in the sorting of proteins in cells. The endoplasmic reticulum is just the first in a series of stops made by proteins that are destined for lysosomes or the plasma membrane or for secretion to the exterior of the cell. As discussed earlier, vesicles that bud off the ER move to the Golgi complex before they are sent on to their final destinations.

The Golgi apparatus serves as a receiving station where proteins from the ER are further processed before being sent on to other cellular compartments. In contrast to the movement of proteins out of the ER, which appears to be more or less non-specific and driven by bulk flow, movement of proteins from the Golgi complex to, say, the lysosomes, is driven by regulated processes that help ensure that lysosomal proteins end up in the lysosome, rather than, say, in the plasma membrane. We will now examine how these processes occur.


What do the Golgi complexes look like?
The Golgi complexes are composed of membrane-bounded sacs and vesicles that form flattened stacks in the cytoplasm. The side of the stack that is closest to the ER is called the cis face of the Golgi, while the side away from the ER is called the trans face of the Golgi.





What are the functions of the Golgi apparatus?
The Golgi apparatus has several important functions. They include:

1. Processing and sorting of glycoproteins

2. Synthesis of glycolipids and sphingomyelin, a major component of membranes, especially in the brain.

3. In plant cells, Golgi complexes are involved in the synthesis of cell-wall polysaccharides like hemicellulose and pectin.

We will mainly focus on the first of these functions.







What happens to proteins that come from the ER to the Golgi complex?
Proteins that arrive from the ER are taken up by the cis Golgi and passed through the stack. As they travel through the Golgi, they are further modified (recall that proteins have already been modified by the addition of various molecules such as sugars or lipids in the ER). These modifications help mark them for their final destinations. They are then passed from the trans Golgi into vesicles that travel to their targets and deliver the proteins to the desired region of the cell.







What kinds of modification do proteins undergo in the Golgi complex?
A major form of modification that proteins undergo in the Golgi complex is the alteration of glycoproteins (that is, proteins that have had sugars added to them in the ER). Many proteins that are glycosylated in the ER have an oligosaccharide added on to them. These proteins are modified by the clipping off of some of the sugars in the oligosaccharide and the addition of others in the Golgi complex.






Do all glycoproteins get modified in the same way in the Golgi?
No, different glycoproteins get modified to different extents, depending on the protein and where it is supposed to go. Plasma membrane proteins and secreted proteins get modified differently from proteins that are targeted to the lysosome.






What is special about the modification of proteins targeted to the lysosome?
Proteins that are to be sent to the lysosome receive special treatment in the Golgi complex. When these proteins enter the cis Golgi, they are phosphorylated on the specific mannose sugars attached to them. This results in the formation of mannose-6- phosphates on the lysosomal proteins.






How come the mannose sugars on the other proteins don't get phosphorylated?
The enzyme that adds phosphate to the mannose can recognize proteins that are destined for the lysosome.






How does the enzyme recognize which proteins are intended for the lysosome?
Proteins that are to be sent to the lysosome have a common feature that is recognized by the phosphorylating enzyme. This feature is dependent on the 3-D conformation (shape) in a particular part of the proteins, and is called the "signal patch".






How is a signal patch different from a signal sequence?
A signal sequence is simply a stretch of amino acids that targets a protein to a particular location in the cell. A signal patch is a three dimensional feature of a protein that serves the same function as a signal sequence.






What is the significance of phosphorylation of the mannose sugars on lysosomal proteins?
In glycoproteins going to the plasma membrane or which are going to be secreted, the first step in processing in the Golgi is removal of three mannose sugars attached to them.
By contrast, in proteins destined for the lysosome, these mannose sugars have phosphate groups attached to them. This serves to distinguish the lysosomal proteins from the other proteins.






How does the presence of mannose-6-phosphate target proteins to the lysosome?
The mannose-6-phosphate is recognized by a special receptor protein in the trans Golgi that then binds the lysosomal proteins and sends them to the lysosome.






What is the fate of proteins once they have been modified in the Golgi complex?
After modification in the Golgi complex, proteins must be sent on to their final destinations or, if they are Golgi complex related proteins, be retained in the Golgi.






How is the movement of proteins out of the Golgi complex controlled?
Some proteins leave the Golgi are simply enclosed in vesicles that bud off the trans face of the Golgi and move to the surface of the cell, where they are either incorporated into the plasma membrane or sent out of the cell. Proteins that are targeted to the lysosome have a special mechanism that sends only proteins with the mannose phosphate tag to the lysosome (details below).






Is the secretion of proteins from the cell regulated at all?
While some proteins may be continuously secreted via the constitutive secretory pathway, the secretion of others is regulated in response to signals from the environment.
An example is the secretion of digestive enzymes that are packaged in the trans Golgi in special regulated vesicles. The proteins in these vesicles stay put till a signal (food in the stomach) indicates that the vesicles should fuse with the plasma membrane, releasing their contents to the exterior of the cell.








How do vesicles form in the first place?
The formation of vesicles is triggered by the assembly of a lattice or network of special proteins called clathrins on the cytosolic side of the Golgi membranes. These proteins start vesicle formation by forming a net over a part of the membrane of the trans Golgi and pulling the mouth shut, like a drawstring bag.






How does a given vesicle contain just the proteins going to, say the lysosome?
A vesicle destined for the lysosome is assembled in the following way:
Proteins to be sent to the lysosome have phosphorylated mannose attached to them. This is recognized by the mannose phosphate receptor in the membrane of the trans-Golgi network. The receptor is a protein that spans the membrane, one part being inside the lumen of the Golgi, where it binds the lysosomal protein and the other end in the cytosol. The cytosolic side of the receptors is recognized and bound by proteins called adaptins. The clathrin network is formed by the binding of clathrin to the adaptins.






How does a vesicle, once formed, deliver its contents to the correct destination?
Transport vesicles in cells display on their surfaces molecular markers that must be recognized by receptor molecules on the target organelle before the vesicle can fuse with the target and deliver the contents.






What kinds of molecular markers are there on the vesicle surface?
Special proteins called SNAREs are found on the surfaces of both transport vesicles (v-SNAREs) and of the target organelles (t-SNAREs). The docking of the v-SNARE with the t-SNAREs signals that the vesicle has reached its correct destination and can now fuse with the target organelle's membrane to deliver the contents.

(Just in case you wondered, SNARE stands for soluble N-ethylmaleimide-sensitive factor attachment protein receptor).






How many different kinds of SNAREs are there?
Each organelle and each type of transport vesicle is believed to have unique SNAREs. This accounts for the high specificity of vesicular transport.






What are lysosomes?
Lysosomes are membrane bounded organelles that contain a large number of digestive enzymes that can break down all kinds of cellular molecules (remember protein degradation in lysosomes?)






What is the function of lysosomes?
Lysosomes function as the cell's wrecking yard and recycling centers. They can break down carbohydrate, lipids, nucleic acids and proteins. In fact, they can digest materials taken up from outside the cell, like bacteria or dead cells, as well as aging organelles within the cell. The breakdown products can then be reused by the cell.






What is special about the enzymes in the lysosomes?
Enzymes in the lysosome are distinguished by the fact that they are acid hydrolases, which means that they are enzymes that break down or hydrolyze molecules under acidic conditions. The pH inside the lysosome is low, meaning that the environment is acidic, which is suitable for the optimal activity of these enzymes.






Is there an advantage to having the lysosomal enzymes be acid hydrolases?
The acidic environment of the lysosome is ideal for the functioning of the acid hydrolases in it. However, if a lysosome membrane was accidentally damaged, and the contents of the lysosome leaked into the cell, the acid hydrolases would no longer work well. This is because the environment of the cytosol is close to neutral, rather than being acidic. This is a protective mechanism that guards the cell against accidental release of the digestive enzymes, which would otherwise destroy the cell.

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Copyright © 2008 Indira Rajagopal