Abstract
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.
Highlights
The nucleolus is the largest structure inside the cell nucleus
The comparison between human, yeast, A. thaliana and B. oleracea reveal that the arginine glycine rich domain (GAR) domain contains the lowest degree of conservation with 31.82% of similarity between Arabidopsis thaliana fibrillarin 1 (AtFib1) and nucleolar protein 1 (Nop1) as the lowest and with 49.32% of similarity between Arabidopsis thaliana fibrillarin 2 (AtFib2) and Brassica oleracea fibrillarin (BoFib) as the highest
The alpha helix rich domain differs by 63.44% of similarity between Homo sapiens fibrillarin (HsFib) and Nop1 as the lowest and with 90.72% of similarity between AtFib1 and AtFib2 as the highest, and is known to interact with other proteins in mammalian cells like survival of motor neuron (SMN) (Pellizzoni et al, 2001)
Summary
The main function of this structure is ribosome biogenesis This process involves transcription of rDNA, processing of rRNA and assembly of ribosomal proteins (Kressler et al, 1999). Overproduction of fibrillarin in mammalian cells can lead to alteration on ribosomal methylation and as a result there is an alteration in the process of translation. In RNA pol I transcription FACT interacts preferentially with the methylated H2A to reorganized nucleosomes in the active promoters for rRNA (Tessarz et al, 2014). Our results showed that in vivo methylated histone H2A in B. oleracea can be found at other locations besides the nucleolar regions, this modification in plants may have additional epigenetic roles than what is found in animal cells
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