SKAR is a specific target of S6 kinase 1 in cell growth control.
BACKGROUND
The mammalian target of rapamycin (mTOR) and phosphatidylinositol 3-kinase (PI3K) signaling pathways promote cell growth and cell cycle progression in response to nutritional, energy, and mitogenic cues. In mammalian cells, the ribosomal protein S6 kinases, S6K1 and S6K2, lie downstream of mTOR and PI3K, suggesting that translational control through the phosphorylation of S6 regulates cell growth. Interestingly, genetic experiments predict that a substrate that is specific to S6K1 but not S6K2 regulates cell growth.


RESULTS
Here we identify SKAR as a novel and specific binding partner and substrate of S6K1 but not S6K2. We find that serines 383 and 385 of human SKAR are insulin-stimulated and rapamycin-sensitive S6K1 phosphorylation sites. Quantitative mass spectrometry reveals that serine 383/385 phosphorylation is sensitive to RNA interference (RNAi)-mediated S6K1 reduction, but not S6K2 reduction. Furthermore, RNAi-mediated reduction of SKAR decreases cell size. SKAR is nuclear protein with homology to the Aly/REF family of RNA binding proteins, which has been proposed to couple transcription with pre-mRNA splicing and mRNA export.


CONCLUSIONS
We have identified a novel and specific target of S6K1, SKAR, which regulates cell growth. The homology of SKAR to the Aly/REF family links S6K1 with mRNA biogenesis in the control of cell growth.
Richardson
Celeste J
CJ
Department of Cell Biology, Harvard Medical School, Boston, MA 02115, USA.
Bröenstrup
Mark
M
Fingar
Diane C
DC
Jülich
Kristina
K
Ballif
Bryan A
BA
Gygi
Steven
S
Blenis
John
J
D000595:Amino Acid Sequence; D015152:Blotting, Northern; D015153:Blotting, Western; D002455:Cell Division / Q000502:physiology; D048429:Cell Size; D019084:Fluorescent Antibody Technique, Indirect; D005786:Gene Expression Regulation; D005982:Glutathione Transferase; D006801:Humans; D047468:Immunoprecipitation; D013058:Mass Spectrometry; D008969:Molecular Sequence Data; D009687:Nuclear Proteins / Q000378:metabolism; D010449:Peptide Mapping; D010766:Phosphorylation; D011485:Protein Binding; D034622:RNA Interference; D012333:RNA, Messenger / Q000378:metabolism*; D016601:RNA-Binding Proteins / Q000235:genetics* / Q000378:metabolism*; D019893:Ribosomal Protein S6 Kinases / Q000378:metabolism*; D017422:Sequence Analysis, DNA; D015398:Signal Transduction; D020798:Two-Hybrid System Techniques
D009687:Nuclear Proteins; C495327:POLDIP3 protein, human; D012333:RNA, Messenger; D016601:RNA-Binding Proteins; D005982:Glutathione Transferase; D019893:Ribosomal Protein S6 Kinases