Confusion about signal recongition particle (TBR Bio 2)

[whoadude]

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(This is TBR Bio 2 Section 1 #8)
I'm hella confused about this stuff...
My basic question is, is a polypeptide fully translated before or after it is moved into the ER lumen? The signal peptide is bringing the polypeptide and the still-bound ribosome along with it. However, if the ribosome is still bound, doesn't that mean translation isn't finished? And if translation isn't finished until after the binding occurs and the protein is being translated into the lumen, doesn't that mean the protein is shorter than it would be if the whole process completed? The passage states that translation is arrested until the ribosome binds the SRP receptor.

(from http://goo.gl/04Gnbj)
In this picture you can see that the protein gets longer after the binding of the ribosome to the membrane occurs. However, TBR says the answer is B, longer in the absence of microsomes. Is the fact that the question states the hormone is "translated by a free ribosome in the presence and absence of microsomes" mean that complete translation of this protein occurs without the whole SRP stuff going on? In this case, the only purpose of the SRP stuff is to make the peptide hormone shorter by cleavage of the signal peptide?

 

[sillyjoe]

I personally thought this question was poorly written because when you say that a protein is "translated by a free ribosome" you tend to think that it is going to be destined for the nucleus, peroxisome, cytosol, mitochondria etc.

However. when they say it is a hormone in the context of translation (peptide hormone), you know it needs to be released via exocytosis and therefore through the RER pathway with the SRP and cleaving of the signal.

Therefore, I think it is ambiguous at best to say free ribosome and hormone in the same question because it leads to confusion as to what is intended which I believe was that an mRNA first binds to a free ribosome regardless of whether translation will occur in the cytosol or the RER so even though it is translated on a "free ribosome" in the absence and presence of a microsomes (RER), it only is translocated to the RER and thus shortened in a microsomal environment.

 

[Chrisz]

Tbr is correct. If the translated protein is not cut at the signal sequence, it will still be bound to the ER membrane. If there is no microsome, the protein would still be translated after the srp is attached to the srp receptor. In the presence, microsome, protein is cut at the signal peptide, which means it will be lost since signal peptide was originally part of the translated material. In the absence of microsome, it will be longer but not released. Translation always starts in the free ribosome. It will be halted, if the pro is not destined for cytoplasmic purpose, but will continue once bound to ER.

 

[sillyjoe]

Tbr is correct. If the translated protein is not cut at the signal sequence, it will still be bound to the ER membrane. If there is no microsome, the protein would still be translated after the srp is attached to the srp receptor. In the presence, microsome, protein is cut at the signal peptide, which means it will be lost since signal peptide was originally part of the translated material. In the absence of microsome, it will be longer but not released. Translation always starts in the free ribosome. It will be halted, if the pro is not destined for cytoplasmic purpose, but will continue once bound to ER.

"If there is no microsome, the protein would still be translated after the srp is attached to the srp receptor."

How is the SRP attaching to the SRP receptor with no microsome? The SRP binding to the ribosome causes a pause in translation until translocation to the microsome. There would be no translocation if there is no microsome and the translation would halt.

My understanding was that in the absence of the microsome it will translate without the SRP binding to it since it is under experimental conditions and not the cytosol where the SRP is present. Thus the protein in the absence of the microsome would be bigger due to the lack of peptide signal cleavage at the RER (microsome).

I still think this is a poorly worded question because they threw in "free ribosome" which tends to lead you to think it is a protein destined for nucleus, peroxisome, cytosol, mitochondria etc. as opposed to the RER --> golgi pathway. The fact that it is a hormone confirms that it is in fact going to default go through the RER pathway since it is released from the cell and thus would go through the SRP/translocation to RER. However, in cell biology when they use the term "free ribosome" they generally are referring to the non-RER protein synthesis pathway.

 

[Chrisz]

"If there is no microsome, the protein would still be translated after the srp is attached to the srp receptor."

How is the SRP attaching to the SRP receptor with no microsome? The SRP binding to the ribosome causes a pause in translation until translocation to the microsome. There would be no translocation if there is no microsome and the translation would halt.

My understanding was that in the absence of the microsome it will translate without the SRP binding to it since it is under experimental conditions and not the cytosol where the SRP is present. Thus the protein in the absence of the microsome would be bigger due to the lack of peptide signal cleavage at the RER (microsome).

I still think this is a poorly worded question because they threw in "free ribosome" which tends to lead you to think it is a protein destined for nucleus, peroxisome, cytosol, mitochondria etc. as opposed to the RER --> golgi pathway. The fact that it is a hormone confirms that it is in fact going to default go through the RER pathway since it is released from the cell and thus would go through the SRP/translocation to RER. However, in cell biology when they use the term "free ribosome" they generally are referring to the non-RER protein synthesis pathway.

Thanks man for clearing this up. I only took general bio. I get confused about the names sometimes. I mistook microsome for the enzyme cleaving the protein.

 

[2Pints]

Thanks man for clearing this up. I only took general bio. I get confused about the names sometimes. I mistook microsome for the enzyme cleaving the protein.

"ase" is used for enzymes like ligase, tautomerase, etc. "some" means body like in ribosome, lysosome.