Protein synthesis termination

[keikoblue2]

Are there two methods of termination depending upon the type of protein?

TBR talks about a stop codon on the mRNA that will be recognized by a protein release factor that will bind to it, releasing the popypeptide chain. Are these for proteins that float around in the cytoplasm?

Then for proteins that are destined to be secreted to other parts, the mRNA is translated until a signal recognition peptide (SRP) binds to the SRP receptor? And then the SRP drags the peptide back to the ER, peptide crosses the ER membrane into the lumen where the SRP is cleaved off and the peptide folds inside the lumen?

Do all proteins have the SRP receptor?

I think my understanding is completely off... Thanks in advance for your answers!

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[jetsfan1234]

ALL mRNAs have a stop codon. Like you said, this is what allows translation to stop, and the mRNA is released from the ribosome.

The signal peptide is something different -- it is a special sequence on the amino end of some proteins (those that will be transmembrane or secreted). Translation of ALL proteins begins in the cytoplasm. If the amino end of the growing peptide has a signal peptide sequence, it binds to the SRP (which is floating around the cytoplasm). The SRP takes the growing peptide+ribosome+mRNA complex to the ER membrane, where translation continues. Once the ribosome reaches a stop codon, translation stops. At this point, you have a long protein with one or more transmembrane sequences in the ER membrane. The protein can be cleaved to become a free protein (will be secreted), or it can stay in the membrane (will become a transmembrane protein).

 

[jmbusclas]

ALL mRNAs have a stop codon. Like you said, this is what allows translation to stop, and the mRNA is released from the ribosome.

The signal peptide is something different -- it is a special sequence on the amino end of some proteins (those that will be transmembrane or secreted). Translation of ALL proteins begins in the cytoplasm. If the amino end of the growing peptide has a signal peptide sequence, it binds to the SRP (which is floating around the cytoplasm). The SRP takes the growing peptide+ribosome+mRNA complex to the ER membrane, where translation continues. Once the ribosome reaches a stop codon, translation stops. At this point, you have a long protein with one or more transmembrane sequences in the ER membrane. The protein can be cleaved to become a free protein (will be secreted), or it can stay in the membrane (will become a transmembrane protein).

Deleted. Misread your post.

 

[keikoblue2]

ALL mRNAs have a stop codon. Like you said, this is what allows translation to stop, and the mRNA is released from the ribosome.

The signal peptide is something different -- it is a special sequence on the amino end of some proteins (those that will be transmembrane or secreted). Translation of ALL proteins begins in the cytoplasm. If the amino end of the growing peptide has a signal peptide sequence, it binds to the SRP (which is floating around the cytoplasm). The SRP takes the growing peptide+ribosome+mRNA complex to the ER membrane, where translation continues. Once the ribosome reaches a stop codon, translation stops. At this point, you have a long protein with one or more transmembrane sequences in the ER membrane. The protein can be cleaved to become a free protein (will be secreted), or it can stay in the membrane (will become a transmembrane protein).

THANK YOU. My gosh, my Bio foundation is crappy... so all mRNAs have a stop codon but only some (targeted for transmembrane/secretetion) will have the signal peptide, gotcha!