Naked Science Forum
Life Sciences => Cells, Microbes & Viruses => Topic started by: alastair84 on 15/05/2004 18:26:16
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I've decided to cut my losses and skip embryology in my revision and move on to subjects i'm better at.
2 past exam questions, they are TRUE/FALSE
1. A 2 base insertion into an exon results in the amino acid sequence, C-terminal to the insertion, being scrambled in the resulting protein.
2. Signal peptides are removed from the NH2-terminus of proteins as they pass through the Golgi apparatus.
1, I know a 2 base insertion will cause a frameshift. I know the exon is the coding region too. I even know that amino acids are made in the NH2 to COOH direction. But i don't know what it means by "C-terminal to the insertion."
2, I don't know much about signal peptide removal.
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1. TRUE C-terminal means from the insertion to the COOH group
2. FALSE A correct response is in the endoplasmatic reticulum
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ahh, excellent, thanks. I'll buy you a beer after these exams.
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This is correct.
The carboxy terminus of the protein can also refer to the nascent polypeptide chain, in the same way that 3' (3-prime) can be used to refer to a nascent strand of DNA or RNA. It merely means downstream and is intended to test your knowledge of the direction of protein synthesis.
The signal sequence is a hydrophobic motif at the amino terminus which interacts with an SRP (signal recognition particle) in the cytoplasm. The SRP / signal sequence complex locks on to a 'pore' on the rough endoplasmic reticulum (RER) and the nascent protein is 'fed' into the RER like a piece of cotton through the eye of a needle. A signal peptidase loitering on the inside of the RER cleaves off the signal sequence once the protein strand has been navigated through the pore.
The second question is incorrect because, as outlined above, this process takes place in the RER. The role of the golgi is glycosylation (adding sugars) to form glyopeptides, and assembly of proteoglycans.
Chris
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx
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Let`s complete the story and say that the receptor for the SRP is called Riboforin.
I`m looking forward to that beer. You can post it to me, in Belgrade.[:D][:D][:D]
Let`s clarify one more thing. The proteins intended to be integral membrane proteins, are thay stayed "trapped" in the RER membrane during the entry in the RER, or are thay incomposed later during processing ?
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The endoplasmic reticulum (ER) is a network of tubules, vesicles and sacs that are interconnected.
As we have already discussed, the signal sequence contains between 15 and 30 hydrophobic amino acids, the precise sequence of which from protein to protein.
Within the ER, the newly synthesized proteins are sequestered in sacs, called cisternae. Packaged into tiny vesicles, the proteins are then shipped to the golgi apparatus for further modification. The vesicles merge with the membrane-bound sacs that form the golgi and discharge their contents therein.
The golgi apparatus then applies glycosylation, the nature of which determines the ultimate fate of the protein within the cell. If a mannose-6-phosphate group is added then the protein is directed to the lysosome. Proteins not bearing this marker are directed to the cell surface as membrane or secretory proteins.
It is worth noting that not all proteins are synthesised on the rough endoplasmic reticulum. Some mRNAs, which give rise to peptides without a signal sequence, are translated by free ribosomes. These protein products are usually involved in cellular housekeeping including the enzymes involved in glycolysis, cytoskeletal elements (microtubules and microfilaments), nuclear histones and transcription factors, and mitochondrial proteins. Plant chloroplast proteins are also synthesised in this way.
As an aside, colleagues of mine were interested in looking at the question of protein targeting in the context of viral infections within cells. Viruses bud off from the cell surface acquiring their outer envelope from the cell membrane. The proteins that coat the virus envelope therefore have to be resident on the cell membrane before the virus leaves, but it was unclear how this was achieved.
They tackled the question by linking a jellyfish GFP (green fluorescent protein) gene to the viral proteins so that when the proteins were made in the RER they glowed green and could be followed as they were processed through the cell. Neat work.
Chris
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx
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Lehninger Principles of Biochemistry says that the membrane proteins are partly translocates and then left imbedded in the ER membrane, while soluble proteins are fully translocated in the lumen of the ER.
However, this is not how I`ve pictured it, so could I get a conformation of this, or denialation in case it is false?