Fibrillarin is a highly conserved nucleolar methyltransferase responsible for ribosomal RNA methylation across evolution from Archaea to humans. It has been reported that fibrillarin is involved in the methylation of histone H2A in nucleoli and other processes, including viral progression, cellular stress, nuclear shape, and cell cycle progression. We show that fibrillarin has an additional activity as a ribonuclease. The activity is affected by phosphoinositides and phosphatidic acid and insensitive to ribonuclease inhibitors. Furthermore, the presence of phosphatidic acid releases the fibrillarin-U3 snoRNA complex. We show that the ribonuclease activity localizes to the GAR (glycine/arginine-rich) domain conserved in a small group of RNA interacting proteins. The introduction of the GAR domain occurred in evolution in the transition from archaea to eukaryotic cells. The interaction of this domain with phospholipids may allow a phase separation of this protein in nucleoli.

• Klíčová slova
• fibrillarin, nucleolus, phosphoinositides, rRNA, ribonucleolar particle, viral progression,
• MeSH
• chromozomální proteiny, nehistonové chemie   genetika   metabolismus   MeSH
• fosfolipidy metabolismus   MeSH
• HeLa buňky MeSH
• lidé MeSH
• malá jadérková RNA metabolismus   MeSH
• mutace genetika   MeSH
• proteinové domény MeSH
• rekombinantní proteiny metabolismus   MeSH
• ribonukleasy chemie   genetika   metabolismus   MeSH
• ribonukleoproteiny metabolismus   MeSH
• vztahy mezi strukturou a aktivitou MeSH
• Check Tag
• lidé MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Názvy látek
• chromozomální proteiny, nehistonové MeSH
• fibrillarin MeSH Prohlížeč
• fosfolipidy MeSH
• malá jadérková RNA MeSH
• rekombinantní proteiny MeSH
• ribonukleasy MeSH
• ribonukleoproteiny MeSH
• RNA, U3 small nucleolar MeSH Prohlížeč

Fibrillarin is one of the most important nucleolar proteins that have been shown as essential for life. Fibrillarin localizes primarily at the periphery between fibrillar center and dense fibrillar component as well as in Cajal bodies. In most plants there are at least two different genes for fibrillarin. In Arabidopsis thaliana both genes show high level of expression in transcriptionally active cells. Here, we focus on two important differences between A. thaliana fibrillarins. First and most relevant is the enzymatic activity by AtFib2. The AtFib2 shows a novel ribonuclease activity that is not seen with AtFib1. Second is a difference in the ability to interact with phosphoinositides and phosphatidic acid between both proteins. We also show that the novel ribonuclease activity as well as the phospholipid binding region of fibrillarin is confine to the GAR domain. The ribonuclease activity of fibrillarin reveals in this study represents a new role for this protein in rRNA processing.

• Klíčová slova
• fibrillarin, glycine-arginine rich domain, nucleoli, phosphatidic acid, phosphoinositides, ribonuclease,
• Publikační typ
• časopisecké články MeSH

Fibrillarin is a well conserved methyltransferase involved in several if not all of the more than 100 methylations sites in rRNA which are essential for proper ribosome function. It is mainly localized in the nucleoli and Cajal bodies inside the cell nucleus where it exerts most of its functions. In plants, fibrillarin binds directly the guide RNA together with Nop56, Nop58, and 15.5ka proteins to form a snoRNP complex that selects the sites to be methylated in pre-processing of ribosomal RNA. Recently, the yeast counterpart NOP1 was found to methylate histone H2A in the nucleolar regions. Here we show that plant fibrillarin can also methylate histone H2A. In Brassica floral meristem cells the methylated histone H2A is mainly localized in the nucleolus but unlike yeast or human cells it also localize in the periphery of the nucleus. In specialized transport cells the pattern is altered and it exhibits a more diffuse staining in the nucleus for methylated histone H2A as well as for fibrillarin. Here we also show that plant fibrillarin is capable of interacting with H2A and carry out its methylation in the rDNA promoter.

• Klíčová slova
• Brassica, RNA polymerase I, histones, methylation, phosphoinositide,
• Publikační typ
• časopisecké články MeSH