Fig 1. Staining of nucleoli rim by NPM1 (green) with tubules (red) in A-431 cells.

Fig 2. Nucleoli anatomy showing the four main components of nucleolar organisation.

It's time for another Image of the week! This week's image was brought to us by citizen scientists in Project Discovery, and specifically by Yadaryon who pointed out several nice examples of proteins localized to the rim of the nucleoli.

The nucleolus is an organelle within the nucleus of your cells. There may be one or more nucleoli in each nucleus, but they usually occur in small numbers. The primary job of the nucleoli is to assemble ribosomes which in turn make proteins within your cells.

In Project Discovery, citizen scientists are refining our annotations for proteins within the nucleoli by labeling the rim and the fibrillar center in addition to more classically evenly labeled nucleoli.

The protein stained in Fig 1. is an image of NPM1 or Nucleophosmin (nucleolar phosphoprotein B23, numatrin). This staining is in A-431 cells, human epidermoid carcinoma, however this phenotype is visible for all cell types for the antibody CAB012983.

Unlike some other organelles such as the mitochondria, endoplasmic reticulum or nucleus, the nucleoli do not have a membrane surrounding them.

Despite not being membrane bound, nucleoli still exhibit a remarkable amount of organization. There are 4 main parts to a nucleolus (Németh and Längst 2011). These components are very roughly diagramed in Fig 2 and include the fibrillar center, dense fibrillar component, and granular component and the perinucleolar chromatin from center out.

Proteins localized to the nucleolar rim are likely associated with either the granular component or perinucleolar heterochromatin.

This timely post follows a recent review from Feb 2016 by Dr. Burke of the role of NPM1 as biomarker of cancer, specifically, leukemia and is used to determine patient prognosis.

Functionally, recent studies have shown that NPM1 is involved in both ribosome biogenesis and chromatin remodeling (Mikael S. Lindström 2011). As mentioned above, it is likely that nucleoli rim proteins are associated with the heterochromatin surrounding the nucleoli and NPM1 has been found in association with heterochromatin protein 1 via co-IP and impact nucloelar morphology based on siRNA experiments (Olausson et al. 2014).

Thanks to the citizen scientists and Yadaryon for helping us further characterize proteins with this subcellular location.

We would like to again thank Yadaryon and all the citizen scientists participating in Project Discovery for their contribution to science!