S1-1 nuclear domains: characterization and dynamics as a function of transcriptional activity.
BACKGROUND
The RNA-binding protein S1-1, also called RBM10 (RNA-binding motif 10), is a paralogue of putative tumour suppressor RBM5 and has been correlated with cancer proliferation and apoptosis. In the present study, we have investigated the cell biology of S1-1.


RESULTS
In the extranucleolar nucleoplasm, S1-1 occurred in hundreds of punctate and irregular domains. Some 10-40 of these domains were larger than 0.5 mum and prominent for S1-1 immunostaining. These domains (S1-1 nuclear bodies) were commonly present in tissue cells and in cultured cells. When cellular transcription was globally reduced by heat shock, serum starvation, culture at high cell densities or inhibition with RNA polymerase II inhibitors, small S1-1 domains (S1-1 granules), with weak immunostaining signals, reduced in number, whereas S1-1 nuclear bodies became prominent and increased in size. These altered S1-1 domains were returned to initial states when the cells were placed under normal conditions. Similar to paraspeckles, S1-1 nuclear bodies occurred closely adjacent to nuclear speckles or IGCs (interchromatin granule clusters), as determined by immunoelectron microscopy. However, the S1-1 nuclear bodies did not correspond to paraspeckles or IGAZs (interchromatin-granule-associated zones), but coincided with TIDRs (transcription-inactivation-dependent RNA domains), which we had characterized previously at the RNA level. The enlarged S1-1 nuclear bodies/TIDRs accumulated the S1-1 protein and microinjected primary and spliced mRNAs, presumably for later elevation of gene expression. In addition, electron microscopy revealed that S1-1 was also present on perichromatin fibrils, suggesting the structure of S1-1 granules seen at higher resolution.


CONCLUSIONS
S1-1 constitutes hundreds of nuclear domains, which dynamically change their structures in a reversible manner. Upon globally reducing RNA polymerase II transcription, S1-1 nuclear bodies enlarge and decrease in number. They are novel domains different from paraspeckles or IGAZs, despite their similar occurrence adjacent to nuclear speckles. We discuss S1-1 granules in terms of their association with gene expression. In addition, this is the first report of a TIDR-localized protein.
Inoue
Akira
A
Molecular Mechanisms of Biological Regulation, Osaka City University Graduate School of Medicine, 1-4-3 Asahi-machi, Abeno-ku, Osaka 545-8585, Japan. ainoue@med.osaka-cu.ac.jp
Tsugawa
Katsuji
K
Tokunaga
Kazuaki
K
Takahashi
Kenichi P
KP
Uni
Shigehiko
S
Kimura
Masatsugu
M
Nishio
Koji
K
Yamamoto
Naoki
N
Honda
Ken-ichi
K
Watanabe
Takanori
T
Yamane
Hideo
H
Tani
Tokio
T
D000818:Animals; D002460:Cell Line; D002467:Cell Nucleus / Q000737:chemistry / Q000378:metabolism*; D012321:DNA-Directed RNA Polymerases / Q000037:antagonists & inhibitors / Q000378:metabolism; D003609:Dactinomycin / Q000494:pharmacology; D006801:Humans; D051379:Mice; D019384:Nucleic Acid Synthesis Inhibitors / Q000494:pharmacology; D017434:Protein Structure, Tertiary; D016601:RNA-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D014158:Transcription, Genetic / Q000187:drug effects
D019384:Nucleic Acid Synthesis Inhibitors; C512260:RBM10 protein, human; C531317:RBM10 protein, mouse; D016601:RNA-Binding Proteins; D003609:Dactinomycin; D012321:DNA-Directed RNA Polymerases