Lecture 26 - Molecular and Cell Biology

Regulation of translation:
Important in both prokaryotes and eukaryotes (although regulation via transcription even more important)

Example for one mechanism:
binding of repressor protein to specific sequence in particular mRNA
---------- translation inhibited

specific example for regulation via repressor protein (Fig. 7.15):
ferritin (protein that stores iron in cell)

• much iron: much ferritin needed
  
• little iron: little ferritin needed

• much iron: repressor binds iron, ferritin mRNA is translated, much ferritin
  
• little iron: repressor cannot bind iron, instead binds to specific RNA sequence in ferritin mRNA called iron response element (IRE), ferritin mRNA is not translated, little ferritin

very important in eukaryotes because of many compartments

remember:

• polypeptides synthesized at ribosomes
  
• ribosomes located in the cytoplasm (whole cell except nucleus, plasma membrane, cell wall or extracellular matrix)
  
• but ribosomes are more specifically located in the cytosol (cytoplasm except all membrane surrounded compartments)
  

How do they get there?
And how does the cell know in which compartment they belong?
(Remember: protein composition very different in different compartments)

proteins are either synthesized on free ribosomes in the cytosol
or they are synthesized on the membrane-bound ribosomes of the rough ER (Fig. 9.3)

the membrane-bound ribosomes of the rough ER synthesize proteins that are localized in:

• ER
  
• Golgi
  
• Lysosomes
  
• Plasma membrane
  
• cell exterior

• Cytosol
  
• Nucleus
  
• Peroxisomes
  
• Mitochondria
  
• Chloroplasts

How is the cell able to differ which protein should be translated from membrane-bound ribosomes and which from free ribosomes?

Proteins are targeted to the ER by a signal sequence in their amino terminus!

Experimental evidence for signal sequence in secreted proteins:
-- in vitro translation of mRNAs on free ribosomes yielded slightly larger proteins.
-- If rough ER was added to test tube: cleavage to normal size and location in ER

Mechanism of targeting proteins to ER: (Fig. 9.7)

Signal sequence (~20 AA) usually at amino terminus of polypeptide
------ first part to emerge from ribosome

• 1. while emerging, signal sequence is specifically recognized and bound by signal recognition particle (SRP) which consists, next to polypeptides, of a small cytosolic or scRNA. Binding inhibits further translation

• 2. complex binds to SRP receptor on ER membrane
  ----- targeting

• 3.
  • receptor releases free SRP
    
  • ribosome binds to protein translocation complex localized in ER membrane
    
  • signal sequence is inserted into membrane channel of protein translocation complex

• 4. translation is resumed with polypeptide growing through channel,
  during translocation (growth through channel) signal sequence is cleaved by signal peptidase

• 5. complete polypeptide released into ER lumen

Proteins localized in membranes of ER, Golgi, lysosome and plasma membrane

• same transport route like water-soluble proteins, but travel as membrane components (Fig. 9.9, note the orientation that is preserved during transport: protein part exposed to lumen will end up exposed to cell exterior)
  
• inserted into ER membrane while synthesized
  

protein folding and processing in ER

translocation into lumen as unfolded polypeptide chain

folding: (like for proteins synthesized in cytosol) assisted by molecular chaperones
especially from the HSP70 family (HSP70 means heat shock protein of about 70 kD, protein family is always present but shows higher expression during stress that leads to increased protein denaturation like elevated temperatures)

formation of disulfide bonds: facilitated by protein disulfide isomerase (disulfide bonds possible because oxidizing athmosphere in ER, different from cytosol where athmoshere is reducing and therefore reduced group -SH instead of oxidized group -S-S-)

protein glycosylation: means addition of sugar residues. Specifically, a complex oligosaccharide is added to an asparagine side chain (N-linked glycosylation, because at -NH2 group of asparagine) in consensus sequence Asn-X-Ser/Thr (X stands for any amino acid residue).

Comments or questions should be directed to Cheryl.Davis@wku.edu
Last Modified: August 16, 1997
All contents copyright (C) 1997.
Western Kentucky University.