Eukaryotic cells contain several organelles. Each organelle, in turn, requires specific proteins, only a few of which are synthesized within the organelles themselves.
Proteins synthesized in the cytoplasm - These include proteins destined for the cytoplasm and those to be incorporated into mitochondria, chloroplasts, or nuclei. Newly synthesized proteins targeted to mitochondria (the chloroplast mechanism is probably similar) contain specific signal sequences (i.e., specific amino acid sequences) at their N-terminal ends. The signal sequences probably aid in membrane insertions, but they also signal that these polypeptides will interact with a particular class of chaperonins (see here). These chaperones are members of the "heat shock" HSP70 family, which act to insure that the newly synthesized protein remains unfolded and is delivered to a receptor site on the organelle membrane. The unfolded protein then passes through gates in the inner and outer membranes which discriminate between proteins destined for the lumen, the membrane, or the matrix. If it passes into the matrix, the protein may be taken up by intra-organelle chaperonins for final folding. The N-terminal targeting sequence is also cleaved off during this transport (Figure 28.38).
Proteins Synthesized on the rough endoplasmic reticulum - Proteins destined for cellular membranes, lysosomes, or extracellular transport use a special distribution system involving the rough endoplasmic reticulum (RER) and the Golgi complex. The RER is a network of membrane-enclosed spaces within the cytoplasm, which is heavily coated on the outer, cytosolic surface with polyribosomes, giving the membrane its rough appearance. The Golgi complex resembles the RER in that it is a stack of thin, membrane-bound sacs, but the Golgi sacs are not interconnected, nor do they carry polyribosomes on their surfaces. The Golgi complex acts as a "switching center" for proteins with various destinations.
Proteins to be directed to their destinations via the Golgi complex are synthesized by polyribosomes associated with the RER as follows:
1. Synthesis begins (Figure 28.38) when an N-terminal hydrophobic signal sequence is synthesized by ribosomes on cytoplasmic mRNA.
2. Signal recognition particles (SRPs), containing several proteins and a small (7SL) RNA, recognize the signal sequences of the appropriate nascent proteins and bind to them as they are being extruded from the ribosomes. The SRP temporarily halts translation, so that no more than the N-terminal signal sequence extends from the ribosome and recognizes a docking protein in the RER membrane.
3. The docking protein binds the ribosome to the RER, and the signal sequence is inserted into the RER membrane.
4. The SRP is released (Figure 28.38, step 4), allowing translation to resume.
5. The protein being synthesized is actually pulled through the membrane by an ATP-dependent process.
6. Before translation is complete, signal sequences are cleaved from some proteins by an RER-associated protease. These proteins are released into the lumen of the RER and further transported. Proteins that will remain in the endoplasmic reticulum have resistant signal peptides and thereby remain anchored to the RER membrane.
Role of the Golgi complex - In the lumen of the RER, proteins undergo the first stages of glycosylation. Vesicles carrying these proteins bud off the RER and move to the Golgi complex (Figure 28.40) where the carbohydrate moieties of glycoproteins are completed (see here). The membrane sacs of the Golgi complex are a multilayer arena for sorting modified proteins. Vesicles from the RER enter at the cis face of the Golgi complex (that closest to the RER) and fuse with the Golgi membrane. Proteins are then passed, again via vesicles, to the intermediate layers. Finally, vesicles bud off from the trans face of the Golgi complex (that furthest from the RER) to form lysosomes, peroxisomes, or glyoxysomes or to travel to the plasma membrane.
Vesicle targeting - Vesicle transport of proteins from the Golgi complex requires high specificity in targeting, for transport of vesicles to the wrong destinations would cause cellular chaos. Each kind of protein carbon packed in a vesicle is marked by specific vesicle membrane proteins. In some cases, target membranes contain complementary proteins (called SNARES), which interact with the vesicle and cause membrane fusion and accurate delivery of the cargo proteins (Figure 28.41).
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