EK Bio Lecture question 34 (ch2)

[fizzgig]

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translation in eukaryotic cell is associated with all the following organelles or locations except:

mitochondrial matrix
cytosol
nucleus
rough ER

answer says 'nucleus' because translation does not occur in the nucleus.

the chapter text after talking about signal peptides that can drag a ribosome to the rough ER: 'signal peptides may also be attached to polypeptides to target them to mitochondria, nucleus, or other organelles'

ok, so can a signal peptide actually
option1) cause ribosome movement to and translation at mitochondria and nucleus, or does that only happen for the rough ER, and
option2)there are other signal peptides that can direct completed proteins to other organelles once they're released into the cytosol?

and whichever is the answer, it seems like it would be true for nucleus AND mitomatrix, so i don't get how the distinction between the two comes up in this question...

either translation 'occurs at' both via signal peptides or it occurs at neither it would seem...

 

[StIGMA]

The answer to the question is nucleus. Translation happens within mitochondria (on different ribosomes), in the cytosol, and at the rough ER.

Mitochondria make their own ~70S ribosomes (with components from nuclear DNA), which make specific mitochondrial proteins within the mitochondrial matrix.

Mitochondrial proteins that are made in the cytoplasm (the majority of mitochondrial proteins) are able to get into the mitochondria because of a signal peptide, but this signal peptide is different than the signal peptide used to make proteins co-translationally at the rough ER.

The only place in the cell where a true signal peptide / SRP is used is at the rough ER; the mitochondria and nucleus do not use co-translational transport like the rough ER system uses.

Signal peptides are not needed for nuclear transport of proteins because the nuclear pores are large enough to transmit macromolecules - only a nuclear localization/export sequence is needed to bind a transport protein if the protein is large enough to need a co-transporter. Another note, that peroxisomes/some other cellular organelles (chloroplasts in plants) have similar transport mechanisms to mitochondria, where a protein is made in the cytoplasm and then trafficked by a signal sequence post-translationally.

Proteins that are trafficked to organelles that are not part of the ER/golgi system (ER golgi system = proteins that are endocytosed or made at rough ER) will have signal sequences in their 3D structure indicating where and how they will be trafficked. So proteins moving to the nucleus or mitochondria are made in the cytosol, and proteins may also be made within the mitochondrial matrix from ribosomes in the mitochondria (the mitochondrial DNA codes for the rRNA of its ribosome).

 

[fizzgig]

Very thorough, thank you so much!! I definitely needed to understand that there were 2 separate 'signal' mechanisms at work, and I didn't know the details about the mitochondria.

Either I misread some EK text or it completely skipped telling me that mitochondria made ribosomes and could do translation itself. Maybe it thought that was obvious or something but I'm bio deficient.

 

[MCAT4life]

The answer to the question is nucleus. Translation happens within mitochondria (on different ribosomes), in the cytosol, and at the rough ER.

Mitochondria make their own ~70S ribosomes (with components from nuclear DNA), which make specific mitochondrial proteins within the mitochondrial matrix.

Mitochondrial proteins that are made in the cytoplasm (the majority of mitochondrial proteins) are able to get into the mitochondria because of a signal peptide, but this signal peptide is different than the signal peptide used to make proteins co-translationally at the rough ER.

The only place in the cell where a true signal peptide / SRP is used is at the rough ER; the mitochondria and nucleus do not use co-translational transport like the rough ER system uses.

Signal peptides are not needed for nuclear transport of proteins because the nuclear pores are large enough to transmit macromolecules - only a nuclear localization/export sequence is needed to bind a transport protein if the protein is large enough to need a co-transporter. Another note, that peroxisomes/some other cellular organelles (chloroplasts in plants) have similar transport mechanisms to mitochondria, where a protein is made in the cytoplasm and then trafficked by a signal sequence post-translationally.

Proteins that are trafficked to organelles that are not part of the ER/golgi system (ER golgi system = proteins that are endocytosed or made at rough ER) will have signal sequences in their 3D structure indicating where and how they will be trafficked. So proteins moving to the nucleus or mitochondria are made in the cytosol, and proteins may also be made within the mitochondrial matrix from ribosomes in the mitochondria (the mitochondrial DNA codes for the rRNA of its ribosome).

Thats an MD/PHD answer right there lol... seriously if you wrote all that from memory thats pretty impressive.

 

[murfettie]

I think the book mentions somewhere that mitochondrial has some ribosome and makes some of their own stuff.

 

[fizzgig]

it may well be. i apparently missed it. twice. did i mention i love microbio and genetics stuff :/

 

[PurefighaUCD]

I had the same question! thanks that makes perfect sense

 

[medemic]

TPRH bio book does a good job of explaining this if you can get your hands on. Proteins destined for mitochondria, nucleus, and peroxisome have what they refer to as "localization" signals, different from the signal sequences located on proteins destined for ER, Golgi, lysosome, secretory pathway. Secretory pathway is the default pathway for those that have signal sequences. So your next question should be well how does an enzyme with a signal sequence divert from the default pathway? These are called targeting signals. So you got signal sequences, targeting signals, and localization signals. An example of a targeting signal would be the mannose-6-phosphate tag on proteins destined for the lysosome. This gets added on during the Golgi processing. Hope that didn't confuse more

Medemic.