I understand that rRNA is made in the nucleus, but the ribosomal proteins are made in the cytosol following export of the mRNA from the nucleus. Since each nuclear transport step requires energy, it seems energetically unfavourable to import the ribosomal proteins, assemble the subunits (40S and 60S), and then export the massive complexes. Why does the cell not simply export rRNA and have the assembly take place in the cytosol? There must be an advantage to doing it the way it is done but I can't seem to figure it out.
This is an excellent question and we can only speculate, at this point. Here are some points that we may consider.
1) Protein translation occurs in the cytoplasm . . . so it seems that ribosomal proteins must be transported into the nucleus to participate in ribosome maturation.
2) Compartmentalization of ribosomal proteins, snoRNAs, and rRNA may be essential for the proper maturation of ribosomes. It is a hierarchical process and stoichiometry of such constituents during certain steps of maturation may be controlled via their subcellular localization. Further, compartmentalization is also important for dictating whether or not mature ribosomes can participate in translation. eiF6 is an anti-association factor that prevents free 60S subunits from binding 48S initiation complexes. This factor, once released from 60S in the cytoplasm, is recycled back to the nucleolus where it contributes to 60S subunit maturation and is suggested to play a role in 60S subunit nuclear export. It seems to me that this may be the most efficient way to modulate both subunit maturation and function.
My two cents . . . and again . . . complete speculation . . . not an ipse dixit fallacy.
The best,
Joe
28S, 18S and 5.8S are processed from one common precursor, and the processing of rRNA is happening in the nucleus. The rRNA precursor needs to be modified by snoRNA before its processing. Ribosomal proteins are required during the processing, and some ribosomal proteins bind to intermediate precursors to facilitate their cleavage and proper folding.
Excellent question, again. I'm here to learn. I guess the splicing of mRNA has it branded for export, which is not a property of those ribosome related non-coding RNAs, meaning the ribosome subunit maturation needs to happen in nucleus. Maybe the genes of prokaryotes are intronless and prokaryotes have no nuclear envelop is not just coincident. The processing of rRNA doesn't only require ribosomal proteins - a lot RNA binding proteins go back to nucleus even nucleolus to help. I'm working on one of them.
This question is such a mind-activaing one. Exceptional biochemical reasoning has given rise to many hypotheses which in turn has led to profound breakthrough in Biochemistry and Molecular biology.
To the question, one must consider the Bioenergetics involved in the whole process of transportation of molecules membrane and those involved in protein synthesis. Is it thermodynamically favourable for the cell to export rRNA in their numbers into the cytosol? what of other components (transcription factors, translation factors) involved in the synthesis of a protein? I think if we take a critical look at these processes, we will come to a better answer to the inquisition
I always wonder how ribosomal protein subunits to be translated again require ribosomes? any suggestions..is it the chciken and egg situation
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