Cytosolic- vs ER-translated proteins

[Dagl1]

I have recently started looking back at the function of the Golgi complex and am now aware not all proteins pass through the Golgi Complex (cytoplasmic translation). However I was always under the impression that most post-translational modifications (PTM) happen in the ER (co-translational) or Golgi complex.

 

Additionally, some types of PTMs happen in the mitochondria or at other organelles (transmembrane receptors come to mind).

 

My questions are fourfold:

1. Are there specific PTM's exclusive to the Golgi complex.

2. By what mechanism can proteins translated by cytosolic (floating) ribosomes be folded and modified (in the ER the concentration/location of chaperone proteins and PTM-proteins will be much higher, but I can't imagine how that works for cytosolic ribosomes).

3. Are (some) proteins translated by cytosolic ribsomes still transported to the Golgi?

4. In case cytosolic ribosomes and ER ribosomes produce different (types) of proteins, how do they know which mRNA's to translate, or how does the mRNA reach the right ribosomes (escape the ER in case of cytosolic ribosomes)?

Especially the last question is one that I find very curious (in case there are proteins that go to either the ER or Golgi, but not to both).
I suppose mRNA export pathways together with specific RNA binding molecules could lead to no translation in the ER, but I haven't found clear data on this yet.

Thanks in advance
-Dagl

[CharonY]

These questions are not easy to answer in a short post- one could hold a full lecture on it in depth. Based on the questions I think you have still a "static" view of the cell, which is typically taught in highschool and often in the first semesters of uni. But be prepared to modify it quite a bit, in order to better understand what is going on. Obviously only short pointers can be given here and only a deeper reading will provide you with the proper context. 

 

1) depends on you define specific. Certain glycosylations of membrane proteins are generally only added in the Golgi systems before directed to the cell membrane. However, other forms of glycosylation are also added elsewhere.

2) obviously you have the sponteneous folding in an aqueous environment, but there are also plenty of chaperones in the cytosol- they are highly abundant proteins.

3) technically all eukaryotic ribosomes are cytosolic- they are either free or associated with a membrane. The pathways proteins can take are complex but they can e.g. be directed via endosomal pathways to the Golgi. However, proteins synthesized in the cytosol can also be directed to the ER and from there to the Golgi. There are also a number of non-canonical vesicular pathways, but let's just say that understanding intracellular protein trafficking requires serious reading.

4) There are again multiple models for how ER-localized translation works. In the classical, mRNA-ribosomal complexes are directed to the ER after initiation utilizing so called signal recognition particles. In other words, initiation is assumed to be a fully cytosolic action, the translocation to a membrane occurs afterward. However, there is recent evidence that ER-bound ribosomes also produce non-membrane targeted (i.e. cytosolic) proteins, which challenges some prior assumptions. As a whole, there is more recognition of ribosomal dynamics, which respond to a vast number of cellular cues (such as stress).

 

 

[Dagl1]

Thank you, I never realised all ribosomes are cytosolic and then can transfer to the ER, thanks, I'll go read some stuff about it.

51 minutes ago, CharonY said:

 As a whole, there is more recognition of ribosomal dynamics, which respond to a vast number of cellular cues (such as stress).

 

 

Than recognition of? Do you mean, there is more recognition of ER-ribosomal dynamics, which respond...

Thanks again

[CharonY]

Perhaps replace "more" with "better". I was alluding to the increasing information we have of molecular and organelle dynamics, organization and trafficking.

[Dagl1]

 Ahh I see, my bad.