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As far as I am aware, all eukaryotic membranes consist of a lipid bilayer. Smooth endoplasmic reticulum (ER) and rough ER are distinguished by the presence of ribosome on the rough ER surface, but are apparently otherwise similar. So what causes ribosomes to associate with the membrane of rough ER but not that of smooth ER. Or do ribosomes convert smooth ER to rough ER by binding to their membranes?

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  • $\begingroup$ probably because there aren't ribosome translocation complexes on other membranes $\endgroup$ – KingBoomie Dec 11 '16 at 18:33
  • $\begingroup$ there is a specific binding protein that attaches the ribosome to the rough ER. There are other such binding proteins for say the cell membrane. There are free floating ribosomes as well. But the RER is a specialized structure that produces the proteins in an controlled environment separate from the rest of the cell, reducing the potential damage by improper folding of the proteins and allowing for easier modification and transport of those proteins. the binding proteins also create an additional layer of control/regulation on protein synthesis. $\endgroup$ – John Dec 11 '16 at 19:45
  • $\begingroup$ @John — Please read the message that comes up when you click in the comment box and do what it says: "Use comments to ask for more information or suggest improvements. Avoid answering questions in commnets." This site has a particular modus operandum underpinned by a clear rationale that is explained when you join. $\endgroup$ – David Dec 11 '16 at 22:11
  • $\begingroup$ I think your question is good, but I made it more precise and compact because English is obviously not your mother tongue. If I have altered the meaning, do say so. $\endgroup$ – David Dec 11 '16 at 22:17
  • $\begingroup$ Thank you for refining my question. I have problem expressing ideas even in my mother tongue haha $\endgroup$ – Tho H. Ho Dec 12 '16 at 0:41
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Peptides that are destined to be either secreted or included in the cell membrane have a signal sequence that binds a protein called Signal Recognition Particle (SRP). The SRP will in turn bind to translocons--basically peptide tunnels in the RER membrane. You need to have this interaction between the translocon and the SRP in order to have stable ribosome attachment. That's why you won't have ribosomes attaching to other membranes. After attachment the ribosome will continue translating mRNA through the translocon into the RER lumen.

Now here's where things get a bit dicey. If the resulting structure in the RER lumen is largely hydrophilic then it will stay in the RER lumen where it will later be targeted to wherever it needs to go based on other signal sequences.

However, if there are large stretches of hydrophobic residues, then those don't like to be in the aqueous environment of the ER lumen. Ergo they will incorporate into the RER membrane. Thus they become transmembrane proteins. A good rule of thumb is that if you have 10 hydrophobic residues in a row, then there's a good chance that is going to get incorporated into a membrane.

Once protein transcription is complete the ribosome will no longer be associated with the RER and will re-enter the pool of free ribsomes (ready to pick up a new mRNA).

EDIT: In response to question in comments about why translocons are found in RER and not in other membranes

You can certainly ask the question, but you're getting close to falling into that beautiful genesis of basic science research: something I call the 'why-hole'. I searched around, and I don't think we have a full understanding of membrane organization throughout the cell, though I would defer to any cell biologists who have a better understanding. We do have some idea of the mechanisms. What follows is a stitching together of hypothesis and some facts I was able to find.

It stands to reason that since the translocon is a complex of transmembrane proteins, its parts were at some point synthesized by a ribosome through a previous translocon. That would get it into the RER membrane. Now in order for a piece of membrane (or anything within the ER lumen, for that matter) to get to its final destination it has to have another signal sequence. This signal sequence will be recognized by unique signal recognition protein. This will localize the protein to a part of the RER that has several proteins that will form a vesicle (one of which being clathrin) and a protein attached to it called a v(esicle)-SNARE. Once the vesicle buds off it will go through the cell until its v-SNARE runs into a t(arget)-SNARE. This interaction will ultimately lead to fusion of the vesicle into the target membrane (e.g. lysosome, plasma membrane, etc). I double checked all of my facts here, where you can read more about it.

All of this is to say that if a transmembrane protein doesn't have a signal sequence it won't get targeted for transport through this system and should stay in the endoplasmic reticulum.

At this point it would be good to remind everyone that the ER--both rough and smooth--is a continuous structure with the RER located closer to the nucleus and the SER closer to the cell membrane. So it would be reasonable to ask why translocons stay in the RER and doesn't diffuse into the SER. That's where we get into more emerging science. From what I gather there are many proteins that are involved in the strutrue and organization of the ER. You can look into "the ability of NDPK-B to form microdomains at the membrane level" or reticulon2, but I think there's still a fair amount of work to do in this field. I encourage you to look into these resources, and if this is something that you're interested in consider doing some research. There is some evidence that membrane dysfunction is associated with neuronal disease, so any advances would be well received.

As always, keep asking questions!

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  • $\begingroup$ This seems quite a good answer, but it needs a supporting reference to the translocons, so people can check it independently. If you do that and it seeems correct, I'll upvote it. The other problem with your answer is that it goes off topic. The lipid raft thing is surely not relevant and therefore detracts from your answer. You would have done better to integrate your PS into the answer. Also, How about a spelling check? $\endgroup$ – David Dec 11 '16 at 22:21
  • $\begingroup$ Thanks for your feedback! I've adjusted the answer accordingly. $\endgroup$ – kingfishersfire Dec 12 '16 at 13:07
  • $\begingroup$ May I ask why the translocons are on the RER but not other membrane? $\endgroup$ – Tho H. Ho Dec 13 '16 at 10:18
  • $\begingroup$ Of course, always! I've expanded my answer in response to your query. It seems that you're starting to get close to the limits of my (perhaps even science's) knowledge. But that just means that the area is ripe for further research in my opinion. $\endgroup$ – kingfishersfire Dec 13 '16 at 16:38
  • $\begingroup$ Ok, as I am aware that we're on the boundary of knowledge, your expanded answer has fulfilled my curiosity. Thank you very much for your effort. Accepted and upvoted. $\endgroup$ – Tho H. Ho Dec 13 '16 at 17:21

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