Perinucleolar compartment
The perinucleolar compartment is a subnuclear body characterized by its location at the periphery of the nucleolus. The PNC participates in the patterned compartmentalization inside the nucleus to organize the specialized functions. It is almost exclusively found in oncogenic cells and enriched with RNA binding proteins as well as RNA polymerase III transcripts.
History
The perinucleolar compartment was first discovered on the periphery of the nucleus in 1992 by Andrea Getti et al. while studying the hnRNPI/PTB protein. Getti found that in addition to the nucleoplasm, the hnRNPI was staining a "discrete unidentified structure" always opposite of the nucleoli. In 1995, A. Gregory Matera et al. first gave the structure its name "perinucleolar compartment" after finding several RNA polymerase III transcripts as well as hnRNPI at the nucleolar rim. Sui Huang et al. has extensively researched the perinucleolar compartment and in 1997 were the first to study the PNC in a large number of human cancer cells.Structure
The PNC is a dynamic and irregular structure composed of multiple dense strands found primarily in transformed cancer cells. Electron microscopy on HeLa cells confirmed that the thick strands are 0.25 - 4 μm in length and 80 - 180 nm in diameter. These strands form a meshwork directly in contact with the nucleolus, and in some instances extend into the nucleolus.RNA
The perinucleolar compartment relies on RNA binding proteins and RNA polymerase III transcripts to stabilize its structure. Therefore, the continuous production of these transcripts is pivotal. During permeabilization of cells, RNase, but not DNase, destroys the PNC establishing the importance of RNA to the structure. Upon inhibition of RNA pol III transcripts, mature RNA pol III is not altered. Therefore, the PNC only relies on newly transcribed RNA pol III transcripts.DNA
The perinucleolar compartment also relies on DNA integrity as well as a DNA locus for stability. DNA studies with DNA-intercalators, DNA-binding molecules, and DNA-damagers established that the PNC dissociates with certain DNA damage. Additionally, DNA damage inhibitors do not prevent the disassembly of PNC, but they do prevent the reformation of the PNC proving the importance of DNA integrity.The reliance on the DNA structure as well as the fact that daughter cells are exact replicates of the mother cells suggests the PNC is associated with a DNA locus, although, the exact locus is undetermined. In the S-phase of the cell cycle, the PNC nucleates at a DNA locus and replicates undisturbed with DNA. This shows a direct correlation between PNC and DNA replication cycles, and further distinguishes the reliance of the perinucleolar compartment on DNA.