Fig. 7-32
After translation on ribosomes in the cytosolic compartment all proteins are processed either in the cytosol or in the ER/Golgi system. The initial stages of protein processing involving folding. Remember that folding of proteins takes place through interaction with chaperone proteins (see pp 139-40 and 232, 468-9) proteins that are not properly folded are destroyed. In the cytosol compartment they are tagged with ubiquitin and destroyed by proteasomes.
Fig. 7-32 |
Proteasomes degrade unwanted proteins in the cytosol compartment. (A) EM of proteasomes (B) Computer enhanced image (C) Atrist's conception of how it works. The red ball attached to the protein to be degraded (green) represents the ubiquitin peptide tht is attached to the unwanted protein to tag it for destruction. |
Other forms of processing involve covalent modification (phosphorylation, methylation, acetylation) and cleavage of the initial protein product to produce a smaller active protein. Cleavage occurs commonly in the case of digestive enzymes.
Proteins targeted to the ER will end up as membrane proteins or as soluble proteins destined for vesicles (e.g. lysosomal proteins) or secretion.
Most proteins are modified in the Endoplasmic Reticulum and in the Golgi Apparatus by addition of polysaccharides. This process is called glycosylation. The enzymes that carry out these reactions are located in the lumen of the E R. and not in the cytosol. This means that the proteins being glycosylated are bound for secretion or are membrane proteins.
Thought question: In the case of membrane proteins, what part of the protein would be glycosylated. The inside (cytosolic) part or the outside part?
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Figure 14-22. Protein glycosylation in the ER. When polypeptide chains enter the endoplasmic reticulum they are immediately glycosylated by the addition of an oligosaccharide chain that is transferred as a single unit from a phospholipid called dolichol to as asparagine residue in the protein |
Note in the figure above (14-22) that the oligosaccharides are added as an intact pre-fabricated unit consisting of 14 linked sugar residues transferred from a phospholipid anchor in the membrane.
The glycolipid units are initially synthesized in the cytosol and embedded in the cytosolic face of the membrane. they are then flipped to the external (lumen) leaflet of the membrane.
Control of protein exit from the ER.
Some proteins are retained in the ER (for example, the enzymes that make the oligosaccharides that are added to proteins) These proteins carry an ER retention signal at their carboxyl ends. See Table 14-3. Even if they get out of the ER into vesicles they are brought back kicking and screaming.
On the other hand, the exit of proteins from the ER is highly controlled. Proteins that are normally exported from the E R must be properly folded. Abnormally proteins are retained by chaperone molecules and degraded if they do not cooperate and fold correctly. Many multi-polypeptide proteins, such as antibodies, are assembled in the E R. If these proteins are not properly assembled (via formation of disulfide bridges), the proteins, like those that are not properly folded are degraded.
Cells make lots of mistakes in the assembly of proteins. They just do not let them be seen in public.
Proteins that get out of the ER are usually transferred to the Golgi apparatus by COP-coated vesicles.
Golgi Complex Introduction
Here are some images of Golgi apparatus . See if you can identify the cis and trans faces of golgi, the trans Golgi network and transport vesicles in these pictures.
 Fig 14-24 |
Study Figures 14-24 and 14-17 in text for basic structure of the organelle. The Golgi complex consists of stack of flattened sacs (cisternae) with expanded or swollen ends. The Golgi complex has two functionally and structurally different faces. The behaviour of the Golgi depends on the presence of other organelles, e.g. cytoskeleton, for support and movement. |
The flow of material in the Golgi apparatus is from cis to trans. Transport vesicles from the E R containing membrane and soluble proteins fuse with the cis Golgi network. See Figure 14-24A. Proteins then move via vesicles from one cisterna to the next. As the proteins move along they are sorted. Vesicles leave the Golgi for a number of destinations. These include