RNA. 2013 Dec;19(12):1669-83. doi: 10.1261/rna.038356.113. Epub 2013 Oct 15.
Visualization of the joining of ribosomal subunits reveals the presence of 80S ribosomes in the nucleus.
Al-Jubran K, Wen J, Abdullahi A, Roy Chaudhury S, Li M, Ramanathan P, Matina A, De S, Piechocki K, Rugjee KN, Brogna S.
Abstract
In eukaryotes the 40S and 60S ribosomal subunits are assembled in the nucleolus, but there appear to be mechanisms preventing mRNA binding, 80S formation, and initiation of translation in the nucleus. To visualize association between ribosomal subunits, we tagged pairs of Drosophila ribosomal proteins (RPs) located in different subunits with mutually complementing halves of fluorescent proteins. Pairs of tagged RPs expected to interact, or be adjacent in the 80S structure, showed strong fluorescence, while pairs that were not in close proximity did not. Moreover, the complementation signal is found in ribosomal fractions and it was enhanced by translation elongation inhibitors and reduced by initiation inhibitors. Our technique achieved 80S visualization both in cultured cells and in fly tissues in vivo. Notably, while the main 80S signal was in the cytoplasm, clear signals were also seen in the nucleolus and at other nuclear sites. Furthermore, we detected rapid puromycin incorporation in the nucleolus and at transcription sites, providing an independent indication of functional 80S in the nucleolus and 80S association with nascent transcripts.
In short, accumulating evidence says translation occurs in the nucleus.
This has been an old chestnut since early 1950s when protein synthesis was discovered. Over the last 60 years or so of this controversy, the major scepticism against evidence for nuclear translation comes from the possibility of ribosomes leaking into nucleus from cytoplasm while processing samples. This scepticism is not unfounded because most translation occurs on the rough endoplasmic reticulum which is proximal to the nuclear membrane to the extent of being continuous with it - so any permeabilization of cells while processing samples for the experiments, may contaminate the nucleus with cytoplasmic ribosomes.
In spite of stringently controlled experiments (http://www.sciencemag.org/content/293/5532/1139.long) to categorically address this chestnut, there is yet to be consensus on occurrence of translation in nucleus. One would expect that incorporation of amino acids, sensitive to translation inhibitors, in nuclei of intact cells, to take care of the existing scepticism against nuclear translation. We have recently done the needful in: (http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0099346) and followed it up with a mini-review: (https://www.landesbioscience.com/journals/nucleus/article/29577/). Pulses as short as 5 seconds assure us that we observe nascent peptides at their sites of synthesis, and the prominent nuclear signal using 3 independent methods convince us that this is no background.
The serendipitous finding of rapid turnover of most peptides synthesized in cells suggests us to be the reason why the short pulses could illuminate this new world of peptides, which was in 'dark' till now!