Figure 1. Staining of NOLC1 (green) with DNA (blue) and microtubules (red) in U-2 OS cells.

Figure 2. Enlarged staining of NOLC1 (green) with DNA (blue) and microtubules (red) in U-2 OS cells showing localization to the fibrillar center of the nucleoli.

This week, image of the week highlights another organelle, the nucleoli fibrillar center!! This week's contribution is brought to us by Lovisa Åkesson, who works on the Subcellular protein atlas, and specializes in understanding nuclear function.

A few months ago, an excellent example of a protein located in the nucleoli rim was shown in another blog post. Today, we are diving deeper into the function of the nucleoli and specifically the fibrillar center, another nucleolar compartment.

As mentioned before, the nucleolus is a non-membrane bound structure located within the nucleus and there is usually more than one in each nucleus. Its main function is to synthesize and assemble ribosomes, which are the protein creation factories of the cell. Interestingly, nucleoli increase in size when the cell grows and a lot of new proteins need to be synthesized and decrease in size when the cells are resting!

The process for making proteins in the cell is complex, but the main idea is that the DNA is "transcribed" into messenger RNA (mRNA) that acts as a template for how the protein should look like. The translation of mRNA into proteins is facilitated by ribosomes and after the ribosome has been assembled in the nucleoli, the ribosomes and the mRNA are transported to the cytoplasm where proteins are synthesized. This process is referred to as the central dogma of biology. Each ribosome consists of two units of ribosomal RNA (rRNA), one small and one large, that are joined together. The mRNA is run in between these subunits and read sequentially when the protein is synthesized.

The nucleolus itself consists of four regions that all have different functions. These are the fibrillar center, dense fibrillar component, the granular component and the perinucleolar region. The border between the fibrillar center and the dense fibrillar component contains proteins that convert DNA into so-called pre-rRNA (Scheer & Hock 1999). This pre-rRNA contains everything needed to be a functional ribosome unit but they also contain some unnecessary parts, so-called introns. These introns are removed from the pre-rRNA in the dense fibrillar component of the nucleolus and are after that fully functional rRNAs. When this is done, the large and the small ribosomal subunits are joined together in the granular component and are then ready to be transported into the cytoplasm for protein production. (http://bscb.org/learning-resources/softcell-e-learning/ribosome/).

Many proteins are localized to the whole nucleolus and might therefore have a general role in the ribosome synthesis, while some proteins localize to just one of the nucleolar compartments. Figure 1 shows an example of the protein called nucleolar and coiled-body phosphoprotein 1 (NOLC1), which is a protein thought to maintain the structure of the fibrillar center and dense fibrillar components but has also been found to interact with RNA polymerase I, a protein complex that transcribes the DNA into pre-rRNA (Chen et al. 1999). Using the antibodies HPA050388 and HPA037366 NOLC1 has been shown to localize to the fibrillar center of the nucleolus as seen in the enlarged cells in Figure 2. This localization makes sense considering the protein's previously described function!

This example highlights two things; the importance of having proteins that regulate the structure of the nucleolus, since it has no membrane that maintains its shape and integrity but also that the more refined subcellular location, the better clue of what function the protein might have in the cell!

We would like to thank all the members of the Subcellular Human Protein Atlas who generate these images and especially to Lovisa Åkesson (aka HPA_Darkfield) for contributing this article about the nucleolar fibrillar center.