Fibrillarin

The name “fibrillarin” is used to refer to several proteins in different organisms , including archaea, although their fibrillarin is very different from eucaryotic fibrillin . The literature contains many synonyms of the name "fibrillarin", including 34 kDa scleroderma nucleolar antigen weighing 34 kDa ( eng. 34 kDa nucleolar scleroderma antigen ), Dmel_CG9888, CG9888, Dmel \ CG9888ri, GCR-6, GCR, syrcoderma, cyCR, cymel, Cr9888ri, GCR-6, GCR, syrmoderma antigen; , FIB, FBL, Fbl, FIB1, FLRN, RNU3IP1, fibrillar rRNA 2'-O-methyltransferase ( eng. RRNA 2'-O-methyltransferase fibrillarin ), NOP1, nop1, fibM, afib. The name NOP1 is often used to designate yeast fibrillarin, and aFIBs for archaeal fibrillarin ^[2] .

For the first time fibrillarin was described in the nucleolus of the slystvik in 1977. Fibrillarin is found not only in eukaryotes, but also in archaea, and, since archaea do not have a nucleolus, it is assumed that the functions of fibrillarin of archaea are somewhat different from the functions of fibrillarin eukaryotes. The yeast cells Saccharomyces cerevisiae contained therein fibrillarin NOP1 possible without compromising these cells replace fibrillarin plant Arabidopsis thaliana , frog Xenopus laevis or human, but not fibrillarin ciliate Tetrahymena thermophila (perhaps due to this different structure N-terminal domain of the latter). The given data testify in favor of high conservatism of the structure and functions of fibrillarin. However, the GAR domain, enriched with arginine and glycine , has a low conservatism of amino acid composition, and when NOP1 is replaced by human fibrillarin, X. laevis or A. thaliana , the parameters of the nucleolus and the growth of yeast cells change, therefore some functions of fibrillarin in different organisms, apparently somewhat different ^[2] .

The conservatism of the amino acid sequences of fibrillarin is different in different groups of organisms. The fibrillarins of plants (63% similarity between the most distant members of the group) and vertebrates (61%) demonstrate the greatest conservatism, while the fibrillins of invertebrates , fungi and archaea are the least conservative (33, 27 and 20%, respectively) ^[2] .

On the basis of phylogenetic data, several families of fibrillars can be distinguished, but so far biochemical and genetic data are insufficient for such a separation ^[2] .

In humans, the FBL gene encoding fibrillarin is located on chromosome 19 at locus 19q13.1 and contains 9 exons ^[1] . The gene is located on the negative chain and has a length of 11962 base pairs ^[3] .

Fibrillarin belongs to the family of SAM-methyltransferases containing Rossman styling . Depending on the organism, the molecular weight of fibrillarin can range from 34 to 38 kDa . In humans, fibrillarin consists of 321 amino acid residues and has a mass of 33,784 Da ^[3] . Fibrillarin can be divided into an N-terminal domain and a domain possessing methyltransferase activity. In Arabidopsis thaliana, the N-terminal domain is divided into two regions: a domain enriched with glycine and arginine (GAR) and consisting of 77 amino acid residues, and a spacer region, including 61 amino acid residues. The GAR domain is involved in the interaction with various cellular and viral proteins, and also contains a signal that holds the protein in the nucleolus together with the RNA-binding motif . GAR is not a necessary domain for the localization of fibrillarin in Cajal bodies , but in humans and Arabidopsis thaliana it is absolutely necessary for the localization of protein in the nucleus. The GAR domain can be methylated on several arginine residues. Fibrillarin can be methylated on arginine residues by the protein, and methylated residues make up 45% of all arginine fibrillarin residues. Methylation can promote the specific binding of fibrillarin to certain proteins, for example, . In humans, the GAR domain and the spacer region interact with the SF2A-p32 protein, and the methyltransferase domain interacts with ^[2] .

Methyltransferase domain is divided into two sections: R-, or the central section, which includes 87 amino acid residues, and the 95-amino acid residue length, enriched in α-helices . Binding of fibrillarin with RNA requires several sites. Firstly, this is the GCVYAVCF motif located inside the R-region, secondly, the sequence in the R-region between amino acid residues 138 and 179 and, third, the sequence between amino acid residues 225 and 281, localized in the α-helix-rich region . The last two RNA binding sites interact with different RNAs independently of each other and deletion of one of them does not adversely affect the ability of fibrillarin to bind RNA, however, when these two sites work together, a synergistic effect is manifested. In the C-terminal region of the protein there is a conservative structure consisting of 7 α-helices and 7 β-sheets . In addition, in the C-terminal region is the binding site of S-adenosylmethionine, surrounded by three conservative amino acid residues. The same region interacts with the protein Nop56 ^[2] .

Surprisingly, the structure of fibrillarin is highly conserved in a number of organisms from archaea to humans. However, archaea fibrillarin lacks the GAR domain, and therefore shorter eukaryotic fibrillin ^[2] .

As noted above, of post-translational modifications, fibrillarin can undergo methylation on arginine residues. In addition, it can be phosphorylated on a Ser124 serine residue ^[4] .

Like many other nuclear proteins, fibrillarin is a very dynamic protein, probably due to the fact that the process of formation of ribosomal particles, in which it participates, requires high mobility. Fibrillarin can quickly move between the nucleoli and the nucleoplasm , and also appear in the bodies of Cajal. Perhaps fibrillarin actively moves around the nucleus in search of a substrate for methylation ^[2] .

In the interphase of the cell cycle, fibrillarin is localized in the dense fibrillar component of the nucleolus, and on going from G1 to the G2 phase, its concentration can double. At the beginning of the prophase, the nucleoli begin to be disassembled, and fibrillarin together with other elements of rRNA processing (pre-rRNA, nucleolin , small nucleolar RNA and ) moves to the periphery of the chromosomes, where it forms part of the perichromosomal membrane or perichromosomal compartment. In addition, fibrillarin was noted in the cytoplasm of cells undergoing mitosis ; therefore, it is possible that during mitosis, the rRNA processing complexes are disassembled and destroyed in the cytoplasm. In telophase, fibrillarin is already assembled into rRNA processing complexes in pre-nucleolar bodies (PNB). After that, PNBs are associated with nucleolar organizers (NORs), and fibrillarin is one of the first factors in rRNA processing, which are transferred from PNBs to NORs. It has been suggested that kinases and phosphatases that regulate the transition from mitosis to interphase can also regulate the transfer of fibrillarin to NORs before the start of transcription of rRNA genes ( pDNA ) ^[2] .

Fibrillarin is directly involved in the post-transcriptional processing of rRNA - in particular, pre-rRNA cutting, rRNA methylation and ribosome assembly. Methylation of rRNA occurs in more than 100 sites with small deviations depending on the organism. In archaea, the methylating complex consists of the proteins aFIB, Nop5 and L7Ae. In eukaryotes, the and proteins replace Nop5, and the 15.5K protein replaces L7Ae. The N-terminal domain of Nop5 interacts with the aFIB, and the C-terminal domain of Nop5 binds to L7Ae, and the complex becomes active upon binding to the guide RNA. In eukaryotes, small nucleolar RNA act as guide RNAs. Small nucleolar RNAs containing the C / D box are involved in rRNA methylation. The C-box, which is the RUGAUGA (R- purine ) sequence, is near the 5'-end of the RNA, and the D-box (CUGA) is near the 3'-end. The small nucleolar RNA guide contains a sequence of 10-21 nucleotides complementary to the target RNA, and methylation occurs 5 nucleotides above the D-box. The following small RNAs interact with fibrillarin: U3, U8, U13, U14, U60, x, y, snR3, snR4, snR8, snR9, snR10, snR11, snR30, snR189 and snR190. Of all the numerous methylations occurring in the processing of rRNA, apparently, not one is responsible for any specialized function. The complex of eukaryotic fibrillarin with Nop56, Nop58, 15.5K and small nucleolar RNA has a mass of about 400 kDa. Fibrillarin can be a part of it in the form of a , which changes its conformation in order to methylate different regions of rRNA. Nop56 / 58 plays an important role in locating the catalytic subunit on the target RNA, while interacting with fibrillarin. Small nucleolar RNAs act as guides and define the immediate editing site ^[2] .

It was shown that fibrillarin can participate in the methylation of histone H2A by the glutamine residue Gln105 in yeast and Gln104 in humans. This modification occurs only in the nucleolus, where there is a high concentration of fibrillarin. This is the first epigenetic modification of histones, characteristic only of the nucleolus. This modification prevents the binding of the complex of chromatin remodeling and is characteristic of the 35S rDNA locus, and therefore has an epigenetic effect on the promoters of ^[2] .

The role of fibrillarin-mediated methylation has been investigated on multicellular organisms (in mice ) using the knockdown of the corresponding gene. Wild-type fibrillarin was replaced by a form devoid of methyltransferase and N-terminal domains, and thus consisting of only the GAR domain. The embryos homozygous for this form did not develop due to numerous apoptosis , but the heterozygotes did not have any abnormalities. In addition, it was shown that fibrillarin is necessary for the normal development of zebrafish , and in plants a reduction in fibrillarin expression using RNA interference led to the development of a dwarf phenotype ^[2] .

Cell Protein Interactions

In addition to the aforementioned proteins Nop56, Nop58 and 15.5K, which together with fibrillarin are involved in the processing of rRNA, fibrillin interacts with many other cellular proteins. Thus, proteins p32 and Nop52 interact with fibrillarin not simultaneously, but, presumably, in the same sites. p32 is involved in the regulation of splicing , and Nop52 is involved in the late stages of formation of 60S ribosomal subunits. Since both proteins cannot interact with fibrillarin at the same time, it is likely that the interaction of each of them with fibrillarin is necessary at a certain stage of ribosome biogenesis . Fibrillarin can interact with DDX5 ^[5] . In Cajal bodies, fibrillarin can interact with the motor neuron survival protein ( English Survival of Motor Neuron protein (SMN) ). In plants, fibrillarin 2 is part of a transcriptional complex mediated by RNA polymerase II , and can interact with the major transcription factors of RNA polymerase II, such as ^[2] .

Virus Protein Interactions

Some viruses in the life cycle of which there is a nuclear phase ( , virus , HIV and others) interact with proteins localized in the cajal body and nucleoli during the implementation of their replication and transcription. These proteins include fibrillarin, which moves between the nucleoli and Cajal bodies. For example, fibrillarin interacts with the ORF3 protein encoded by a virus from the group of umbraviruses that infect plants. Fibrillarin directly interacts with its arginine-enriched domain with the lysine- enriched domain ORF3, due to which the viral protein can move between the nucleolus and Cajal body. In addition, during viral infection, fibrillarin is transferred to the cytoplasm, where it participates in the formation of viral particles ^[2] .

Some animal viruses also interact with fibrillarin. For example, the multifunctional NS1 protein inhibits the processing of the mRNA of the host cell and prevents the development of an antiviral response. NS1 binds to fibrillarin and nucleolin through its C-terminal nuclear / nucleolar localization signal . It is assumed that the HIV Tat protein interacts with fibrillarin in complex with the small nucleolar U3 RNA and thereby disrupts the process of pre-rRNA ripening in the nucleoli. This, in turn, disrupts the vital activity of the affected cells and can lead to apoptosis. In other viruses, for example, in a mumps- causing , nucleocapsid proteins interact with fibrillarin during the life cycle ^[2] . It appears that fibrillin plays a crucial role in the life cycle of the , a highly pathogenic virus carried by bats . In cells lacking fibrillarin, significant disturbances in the synthesis of RNA and proteins of this virus were observed ^[6] .

Like other nuclear proteins, fibrillarin is involved in the regulation of cell division and cell growth, therefore, violations of its expression often lead to the development of cancer. Fibrillarin is an oncogene . Overexpression of fibrillarin is observed in the case of prostate in mice in humans, which can develop into prostate cancer . In the case of human adenocarcinoma, the amount of fibrillarin is related to the amount of Myc protein, a well-studied oncogene that can interact with fibrillarin. Excessive expression of fibrillarin was detected twice in leukemia and lymphoma cells of various types. It has been shown that p53 decreases the expression of fibrillarin, interacting with the intron 1 sequence in the fibrillarin gene. In breast cancer cells, a decrease in the p53 content leads to an increased expression level of fibrillarin, and this, in turn, increases the amount of incorrectly methylated rRNA. This disrupts the work of the ribosome and increases the expression of oncogenes through the IRES ^[2] .

Fibrillarin autoantibodies are found in approximately 8% of patients suffering from systemic scleroderma ^{[1] [7] [8]} .

• Proteins of the Nucleolus / O'Day, Danton H, Catalano, Andrew. - Netherlands: Springer, 2013. - ISBN 978-94-007-5818-6 . - DOI : 10.1007 / 978-94-007-5818-6 .

• The Nucleolus / Mark OJ Olson. - New York: Springer, 2011. - ISBN 978-1-4614-0514-6 . - DOI : 10.1007 / 978-1-4614-0514-6 .