Identification of an evolutionary conserved SURF-6 domain in a family of nucleolar proteins extending from human to yeast

Human nucleolar protein SURF6/RRP14 participates in early steps of pre-rRNA processing

The biogenesis of ribosomes requires tightly controlled transcription and processing of pre-rRNA which comprises ribosomal RNAs forming the core of large and small ribosomal subunits. Early steps of the pre-rRNA processing and assembly of the ribosomal subunits require a large set of proteins that perform folding and nucleolytic cleavage of pre-rRNAs in the nucleoli. Structure and functions of proteins involved in the pre-rRNA processing have been extensively studied in the budding yeast S. cerevisiae. Functional characterization of their human homologues is complicated by the complexity of mammalian ribosomes and increased number of protein factors involved in the ribosomal biogenesis. Homologues of human nucleolar protein SURF6 from yeast and mouse, Rrp14 and Surf6, respectively, had been shown to be involved in the early steps of pre-rRNA processing. Rrp14 works as RNA chaperone in complex with proteins Ssf1 and Rrp15. Human SURF6 knockdown and overexpression were used to clarify a role of SURF6 in the early steps of pre-rRNA processing in human cell lines HeLa and HTC116. By analyzing the abundance of the rRNA precursors in cells with decreased level or overexpression of SURF6, we demonstrated that human SURF6 is involved in the maturation of rRNAs from both small and large ribosomal subunits. Changes in the SURF6 level caused by knockdown or overexpression of the protein do not result in the death of HeLa cells in contrast to murine embryonic fibroblasts, but significantly alter the distribution of cells among the phases of the cell cycle. SURF6 knockdown in both p53 sufficient and p53 deficient HCT116 human cancer cells results in elongation of G0/G1 and shortening of G2/M phase. This surprising result suggests p53 independence of SURF6 effects on the cell cycle and possible multiple functions of SURF6. Our data point to the shift from pathway 1 to pathway 2 of the rRNA biogenesis caused by the SURF6 knockdown and its likely association with p53 pathway.

Rrp14 controls rRNA transcription via facilitating the translocation of Pol5 into the nucleolus

Rrp14 is a conserved protein that plays an important role in rRNA processing and ribosomal biogenesis. In Schizosaccharomyces pombe, the rrp14 gene is split into SPAC8C9.10 c (rrp14) and SPBC947.07 (rrp1402). Although the SPAC8C9.10 c gene is not essential for S. pombe survival, deletion of the gene causes the yeast cells to grow sick and to exhibit decreased rRNA transcription. We identified a novel Pol5 protein that physically interacts with the Rrp14 protein. Taking advantage of the Pil1 co-tethering assay, we found that Rrp14 facilitates the nucleolus translocation of Pol5, and the 7-RINAWN-12 motif of the Rrp14 protein is responsible for the interaction between Pol5 and Rrp14. Since deletion of the 7-RINAWN-12 motif affects rRNA transcription, we thus propose that Rrp14 affects rRNA transcription by facilitating the nucleolus translocation of Pol5.

A Higher Level of Expression of the Nucleolar Protein SURF6 in Human Normal Activated Lymphocytes and in Lymphocytes of Patients with Lymphoproliferative Disorders

Proliferation of mammalian cells is often accompanied by an increase in the content of the nucleolar proteins, which allows researchers to consider such proteins as potential activation markers. To test this assumption experimentally, we examined the expression pattern of the nucleolar rRNA processing factor SURF6 in normal (resting) peripheral blood lymphocytes, lymphocytes activated for proliferation in vitro, and in blood samples from patients with lymphoproliferative diseases. Using two methods (immunofluorescence and immunoblotting), we for the first time showed that the SURF6 protein is not detected in normal lymphocytes but can easily be visualized in lymphocytes after PHA activation and in lymphocytes of lymphocytic leukemia patients. The level of SURF6 expression in patients correlated with the aggressiveness of the disease development determined by the content of Ki-67-positive lymphocytes. These results allow the SURF6 nucleolar protein to be considered as a putative activation marker of lymphocytes in human blood disorders.

Involvement of the specific nucleolar protein SURF6 in regulation of proliferation and ribosome biogenesis in mouse NIH/3T3 fibroblasts

The nucleolar proteins which link cell proliferation to ribosome biogenesis are regarded to be potentially oncogenic. Here, in order to examine the involvement of an evolutionary conserved nucleolar protein SURF6/Rrp14 in proliferation and ribosome biogenesis in mammalian cells, we established stably transfected mouse NIH/3T3 fibroblasts capable of conditional overexpression of the protein. Cell proliferation was monitored in real-time, and various cell cycle parameters were quantified based on flow cytometry, Br-dU-labeling and conventional microscopy data. We show that overexpression of SURF6 accelerates cell proliferation and promotes transition through all cell cycle phases. The most prominent SURF6 pro-proliferative effects include a significant reduction of the population doubling time, from 19.8 ± 0.7 to 16.2 ± 0.5 hours (t-test, p < 0.001), and of the length of cell division cycle, from 17.6 ± 0.6 to 14.0 ± 0.4 hours (t-test, p < 0.001). The later was due to the shortening of all cell cycle phases but the length of G1 period was reduced most, from 5.7 ± 0.4 to 3.8 ± 0.3 hours, or by ∼30%, (t-test, p < 0.05). By Northern blots and qRT-PCR, we further showed that the acceleration of cell proliferation was concomitant with an accumulation of rRNA species along both ribosomal subunit maturation pathways. It is evident, therefore, that like the yeast homologue Rrp14, mammalian SURF6 is involved in various steps of rRNA processing during ribosome biogenesis. We concluded that SURF6 is a novel positive regulator of proliferation and G1/S transition in mammals, implicating that SURF6 is a potential oncogenic protein, which can be further studied as a putative target in anti-cancer therapy.

Development of novel mouse hybridomas producing monoclonal antibodies specific to human and mouse nucleolar protein SURF-6

SURF-6 is an evolutionarily conserved nucleolar protein that is important for cell viability; however, its function in mammals still remains uncertain. The aim of this study is to generate monoclonal antibodies to human SURF-6 protein suitable for fundamental and biomedical research. The full-size human SURF-6 was expressed as a recombinant GST-fusion protein and used as an antigen to generate monoclonal antibodies, S79 and S148, specific for SURF-6. The monoclonal antibody produced by hybridoma clone S79 specifically recognizes endogenous SURF-6 by Western and immunofluorescence analyses in various cultured human cells, and by immunohistochemistry in paraffin-embedded sections of human breast cancer samples. Moreover, S79 immunoprecipitates protein complexes containing SURF-6 from HeLa cells extracts. The antibody S79 recognizes SURF-6 only in human cells; however, the antibody produced by hybridoma clone S148 can detect SURF-6 of human and mouse origin. Monoclonal antibodies to the nucleolar protein SURF-6 described in this work can be a useful tool for studies of ribosome biogenesis in normal and cancer cells.

[Overexpression of the nucleolar protein SURF-6 in mouse fibroblasts NIH/3T3 leads to stabilisation of intragenic transcribed spacers of the pre-rRNA]

SURF-6 is an evolutionary conserved nucleolar protein that is required for maintenance of cell viability, but its functional significance in mammals still remains illusive. In the present work we examined effects of SURF-6 overexpression in mouse NIH/3T3 fibroblasts transfected with two plasmids. The plasmid pUHrT62-1 encodes a tetracycline-dependant trans-activator, the protein rtTA, the plasmid pBI-SURF6--the genes of EGFP (enhanced green fluorescent protein) and of mouse SURF-6 which expression was controlled by the rtTA-responsive bi-directorial promoter. Western blot analysis showed that the SURF-6 level was severely augmented in cells transfected with pUHrT62-1 and pBI-SURF6 and incubated with the inducer--doxycycline opposed to the transfected but not-induced cells. The increase of SURF-6 was observed in 24 and 48 h after adding the inducer doxycycline. Dot-hybridization of isolated RNA with biotinilated oligonucleotide probes to various regions of mouse primarily pre-rRNA transcripts showed that overexpression of SURF-6 enhanced levels of the second intragenic transcribed spacer ITS2 in about seven folds and of the 5' external transcribed spacer 5'ETS in two folds. Amounts of fragments corresponding to 18S, 5.8S and 28S rRNA remained almost unchanged. These observations for the first time demonstrated that mammalian SURF-6 helps to stabilize or prevents premature cleavage of the pre-rRNA intragenic transcribed spacers, particularly of ITS2, similar to its homologue in S. cerevisiae the protein Rrp14. Today metazoan proteins that play a similar role in ribosome biogenesis, are not described.

Early expression of nucleolar SURF-6 protein in mouse spleen lymphocytes activated for proliferation in vitro

Using specific antibodies we studied the content of nucleolar SURF-6 protein, which participates in rRNA processing, in mouser spleen lymphocytes activated for proliferation with concanavalin A and compared it with the content of nucleolar nucleophosmin/B23 protein and DNA replication factor PCNA, well-known markers of proliferating cells. Using immunocytochemistry and immunoblotting methods we demonstrate that the concentration of all these proteins increases simultaneously with increasing the proportion of proliferating cells. Unlike nucleophosmin/B23, SURF-6 protein was not revealed in quiescent lymphocyte nucleoli, while the increase of its level in activated lymphocytes preceded elevation of PCNA level. These observations suggest that nucleolar protein SURF-6 can act as a marker of early T lymphocyte activation for proliferation and that it could participate in cell cycle regulation in mammals.

Yeast Rrp14p is a nucleolar protein involved in both ribosome biogenesis and cell polarity

We previously cloned RRP14/YKL082c, whose product exhibits two-hybrid interaction with Ebp2p, a regulatory factor of assembly of 60S ribosomal subunits. Depletion of Rrp14p results in shortage of 60S ribosomal subunits and retardation of processing from 27S pre-rRNA to 25S rRNA. Furthermore, 35S pre-rRNA synthesis appears to decline in Rrp14p-depleted cells. Rrp14p interacts with regulatory factors of 60S subunit assembly and also with Utp11p and Faf1p, which are regulatory factors required for assembly of 40S ribosomal subunits. We propose that Rrp14p is involved in ribosome synthesis from the beginning of 35S pre-rRNA synthesis to assembly of the 60S ribosomal subunit. Disruption of RRP14 causes an extremely slow growth rate of the cell, a severe defect in ribosome synthesis, and a depolarized localization of cortical actin patches throughout the cell cycle. These results suggest that Rrp14p has dual functions in ribosome synthesis and polarized cell growth.

Yeast Rrp14p is required for ribosomal subunit synthesis and for correct positioning of the mitotic spindle during mitosis

Here we report that Rrp14p/Ykl082p is associated with pre-60S particles and to a lesser extent with earlier 90S pre-ribosomes. Depletion of Rrp14p inhibited pre-rRNA synthesis on both the 40S and 60S synthesis pathways. Synthesis of the 20S precursor to the 18S rRNA was largely blocked, as was maturation of the 27SB pre-rRNA to the 5.8S and 25S rRNAs. Unexpectedly, Rrp14p-depleted cells also showed apparently specific cell-cycle defects. Following release from synchronization in S phase, Rrp14p-depleted cells uniformly arrested in metaphase with short mitotic spindles that were frequently incorrectly aligned with the site of bud formation. In the absence of Bub2p, which is required for the spindle orientation checkpoint, this metaphase arrest was not seen in Rrp14p-depleted cells, which then arrested with multiple buds, several SPBs and binucleate mother cells. These data suggest that Rrp14p may play some role in cell polarity and/or spindle positioning, in addition to its function in ribosome synthesis.

The nucleolar protein SURF-6 is essential for viability in mouse NIH/3T3 cells

SURF-6 is a bona fide nucleolar protein comprising an evolutionary conserved family that extends from human to yeast. The expression of the mammalian SURF-6 has been recently found to be regulated during the cell cycle. In order to determine the importance of SURF-6 in mammalian cells, we applied the Tet-On system to regulate conditionally, in response to tetracycline, the expression of an antisense RNA (asRNA) that targets Surf-6 mRNA in mouse NIH/3T3 cells. Induced Surf-6 asRNA caused an effective depletion of SURF-6 protein resulted in cell death and in an apparent arrest in the G1 phase of the cell cycle. These results provide for the first time evidence that expression of SURF-6 is essential for mammalian cell viability, and suggest that SURF-6 might participate in the progression of cell cycle.

Implication of nucleolar protein SURF6 in ribosome biogenesis and preimplantation mouse development

The step-wise assembly of a functional nucleolus, which occurs over the first few cell cycles during preimplantation development, is poorly understood. In this study, we examined the function of the evolutionary conserved nucleolar protein SURF6 in preimplantation mouse embryo development. Immunocytochemical analyses revealed that the localization of SURF6 was similar but not identical to those of fibrillarin and B23/nucleophosmin 1, which are involved in rRNA processing and ribosome biogenesis in mammalian somatic cells. Surf6 mRNA, which is expressed in oocytes and maternally inherited in the zygote, reached a peak level of expression during the 8-cell stage of embryo development, at which time rDNA is highly transcribed. Knock-down of Surf6 mRNA by RNAi led to a decrease in both the mRNA and protein levels, and resulted in developmental arrest at the 8-cell/morula stage, as well as a decrease in the level of 18S rRNA. These results suggest that Surf6 is essential for mouse preimplantation development, presumably by regulating ribosome biogenesis.

[Properties and functions of a new nucleolar protein, Surf-6, in 3T3 mouse cells]

The localization of the specific protein Surf-6 from nucleoli of eukaryotic cells in mitosis and its sensitivity to the treatment of cells with RNase A and DNase I in situ were studied. It was shown that, in interphase nucleoli of 3T3 mouse cells, Surf-6 is probably associated with RNA and practically is not associated with DNA. In mitosis, Surf-6 appears in forming nucleoli after the known RNA-binding proteins fibrillarin and B23/nucleofozmin, which are involved in the early and late stages of the assembly of ribosomal particles, respectively. These observations and the regularities of migration of early and late proteins of ribosome assembly to nucleoli in the telophase of mitosis led us to the presumption that Surf-6 is involved in the terminal stages of the assembly of ribosomal particles in murine cells. An immunoblot analysis of the Surf-6 content in synchronized 3T3 cells showed for the first time that Surf-6 is present at all stages of the cell cycle but its content markedly decreases when cells enter the G0 period. Conversely, the activation of cells for proliferation is accompanied by an increase in the Surf-6 content. These observations allow one to regard Surf-6 as a marker of the cell proliferative state and suggest its implication in the regulation of the cell cycle. The English version of the paper: Russian Journal of Bioorganic Chemistry, 2005, vol. 31, no. 6; see also http://www.maik.ru.