Protein synthesis site

[Melomare17]

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It was my understanding that protein synthesis in eukaryotic cells occurred at the rough ER, including its initiation? In the TPR book (passage 30 #1) it says that it is initiated in the cytoplasm--which is it??

 

[MedPR]

It was my understanding that protein synthesis in eukaryotic cells occurred at the rough ER, including its initiation? In the TPR book (passage 30 #1) it says that it is initiated in the cytoplasm--which is it??

Depends what kind of protein. Some are synthesized on cytoplasmic ribosomes, others on rough ER.

 

[Melomare17]

Depends what kind of protein. Some are synthesized on cytoplasmic ribosomes, others on rough ER.

do u know what kind is synthesized where? in the question it doesnt specify just says proteins

 

[MedPR]

do u know what kind is synthesized where? in the question it doesnt specify just says proteins

I believe all we need to know for the MCAT is that proteins destined to leave the cell are synthesized on the rough ER so that they can be passed to the golgi to be secreted. What's the whole question? Perhaps there's a hint in there.

 

[Melomare17]

I believe all we need to know for the MCAT is that proteins destined to leave the cell are synthesized on the rough ER so that they can be passed to the golgi to be secreted. What's the whole question? Perhaps there's a hint in there.

hm ok

Which of the following is a site of initiation of protein synthesis in eukaryotic cells?
I. nucleus
II. cytoplasm
III. rough endoplasmic reticulum

a. I only
b. II only
c. III only
d. II and III only

 

[mcloaf]

Initiation is always in the cytoplasm. This is where free floating ribosomes assemble and begin to work. If the protein is bound for the nucleus or secretion then the protein begins with a short sequence called a signal peptide. This sequence is recognized by signal recognition particles (SRPs) and an SRP binds, stopping protein synthesis. The rough ER has SRP receptors, which allow the SRP to bring the mRNA/ribosome complex to the rough ER where the SRP can leave and the protein is synthesized through the pore into the rough ER lumen, where the signal peptide is then cleaved off and recycled.

 

[Melomare17]

oh ok that makes sense now--thanks!

 

[MedPR]

Initiation is always in the cytoplasm. This is where free floating ribosomes assemble and begin to work. If the protein is bound for the nucleus or secretion then the protein begins with a short sequence called a signal peptide. This sequence is recognized by signal recognition particles (SRPs) and an SRP binds, stopping protein synthesis. The rough ER has SRP receptors, which allow the SRP to bring the mRNA/ribosome complex to the rough ER where the SRP can leave and the protein is synthesized through the pore into the rough ER lumen, where the signal peptide is then cleaved off and recycled.

Ah that's right. Thank you.

Sorry for the confusion OP!

 

[hellocubed]

Which of the following is a site of initiation of protein synthesis in eukaryotic cells?
I. nucleus
II. cytoplasm
III. rough endoplasmic reticulum

a. I only
b. II only
c. III only
d. II and III only

OP, was the answer to this question B?

 

[hellocubed]

Initiation is always in the cytoplasm. This is where free floating ribosomes assemble and begin to work. If the protein is bound for the nucleus or secretion then the protein begins with a short sequence called a signal peptide. This sequence is recognized by signal recognition particles (SRPs) and an SRP binds, stopping protein synthesis. The rough ER has SRP receptors, which allow the SRP to bring the mRNA/ribosome complex to the rough ER where the SRP can leave and the protein is synthesized through the pore into the rough ER lumen, where the signal peptide is then cleaved off and recycled.

Oh wow.

so
1.) mRNA is released from the Nuclear pores into the cytoplasm
2.) rRNA assemble around the mRNA in the cytoplasm
3.) tRNA begin to build until it hits an SRP
4.) Protein synthesis stops as the incomplete peptide is taken to the rough ER
5.) rough ER snips SRP (which leaves)
6.) Synthesis now continues in the rough ER until completion


is this correct?

 

[chiddler]

How is the SRP sequence snipped? Anybody remember the mechanism for that?

 

[MedPR]

How is the SRP sequence snipped? Anybody remember the mechanism for that?

Something to do with snrnps and spliceosomes iirc.

 

[mcloaf]

Something to do with snrnps and spliceosomes iirc.

Nah, those guys are for cutting up mRNAs (introns/exons, alternative splice sites, etc.). I don't know the specific enzyme off the top of my head, but you won't need to, it's just some nameless enzyme that chops of signal peptides in the ER lumen--certainly won't need a mechanism either. 😉

@hellocubed, yeah, pretty much. I'm not positive if the SRP dissociates during or after the completion of translation, but I believe once it's brought the mRNA/ribosome complex to the ER it's allowed to peace out.

 

[rjosh33]

Nah, those guys are for cutting up mRNAs (introns/exons, alternative splice sites, etc.). I don't know the specific enzyme off the top of my head, but you won't need to, it's just some nameless enzyme that chops of signal peptides in the ER lumen--certainly won't need a mechanism either. 😉

Signal peptidase. 🙂

@hellocubed, yeah, pretty much. I'm not positive if the SRP dissociates during or after the completion of translation, but I believe once it's brought the mRNA/ribosome complex to the ER it's allowed to peace out.

Yeah, the SRP is released from its receptor and complex pretty much as soon as the nascent peptide enters the ER

 

[mcloaf]

Signal peptidase. 🙂

This is why I loved biochem. When it doubt just add -ase. 😎

 

[pm1]

so protein synthesis is always initiated in the cytoplasm oNLY? answer B?

 

[Melomare17]

so protein synthesis is always initiated in the cytoplasm oNLY? answer B?

yup answer is B