Do 40S and 60S come together sooner than we thought?

The ribosomal subunits 40S and 60S are produced and assembled whilst they are still within the nucleus. However, there seem to be several different levels of control which prevent their formation into a fully functioning 80S ribosome, which is only relieved once they are exported into the cytoplasm. This has often led researchers to believe that 80S ribosomes can be found only within the cytoplasm. However, recent research has brought this theory into question. 

The synthesis of proteins is required for the viability of all living cells, with the code for these proteins contained within chromosomes as a sequence of bases on DNA. For this sequence to be developed into functioning proteins, a number of complex processes must first take place, chief amongst which is the transcription of DNA into messenger RNA (mRNA). This mRNA can then be used to assemble simple amino acids into complex proteinacious structures. However, the process of transcription is not possible without the large molecular machine called the ribosome, which acts as the primary site of protein synthesis.

During the production of a ribosome, the subunits 40S and 60S must join together to form a fully mature and transcribing ribosome (80S), the formation of which acts as a main indicator that transcription is occurring within a cell.  Much previous research has suggested that the 60S and 40S subunits are synthesised in the nucleus through various complex mechanisms, with the 40S and 60S subunits being incapable of associating with mRNA, preventing their proper functioning until they are exported into the cytoplasm. Once in the cytoplasm they are able to form the fully functioning 80S ribosome, which is then capable of transcribing DNA and producing functionally mature mRNA .

There are several levels of control which are capable of controlling each of these components. Studies in saccharomyces cerevisiae indicate that this level of repression can be controlled by nonribosomal assembly factors (AFs) which bind to pre-40S and pre-60S subunits preventing their activation and assembly into the 80S ribosome until they are exported into the cytoplasm. In addition to this, it is also believed that other proteins are crucial for the translocation of these ribosomal subunits through the nuclear pore, the control of which may also be involved in preventing the assembly of 80S before they are required. There are also several lines of evidence which suggest that the 40S ribosome is not processed properly whilst it still resides within the nucleus, further preventing it from forming a mature ribosome, unless exported into the cytoplasm.

However, with a recent study indicating that immature 40S subunits can actually initiate translation whilst still residing in the nucleus, as well as numerous other studies confirming this fact, it is now clear that 40S is capable of interacting with 60S and forming an 80S-like structure, which is able to produce a low level of inefficient translation.

With this information in mind it was the goal of a recent paper by Al-Jubran et al (2013) to fully determine the cellular location of the functional 80S ribosome within Drosophila. To do this a number of ribosomal proteins (RPs) were identified, with their molecular position determined both when in their 40 and 60S subunits, as well as when contributing to the mature 80S structure. If it was found that these RPs are situated closely to each other in the 80S ribosome, then they were tagged in such a way as to make them fluoresce when they are within close proximity to each other. This would give a clear indication of exactly where 80S ribosomes were situated within a cell.

The results from this visualisation indicated that the majority of ribosomes were in fact located primarily in the cytoplasm of the cell. However, there were also a number of signals which suggest that a properly functioning ribosome can be found within the nucleus, with higher levels of intensity found at the nuclear periphery and the nucleolus.

However, these results had to be further confirmed to ensure their authenticity. One such confirmation was gained through treatment with puromycin. Usually, when a cell is treated with puromycin the 80S ribosomes become inactive and non-translating. When these non-translating ribosomes were tagged in the same way, no signal was produced. This indicates that the 80S ribosomes that were visualised without the puromycin treatment must have been actively transcribing, giving further weight to the idea that transcribing ribosomes can be found within the nucleus.

Therefore, this research confirms findings by previous research which indicates the presence of 80S ribosomes within the nucleus, as well as cementing a strong technique that is capable of visualising these ribosomes, in a simple and clear cut way. This research could be further developed through quantifying the level of transcription that is being produced by these nuclear 80S ribosomes. This would make it much clearer whether the transcription of mRNA before cytoplasmic export is essential for the proper functioning of the cell, or whether this process simply occurs at this stage to kick start transcription of all genes.