Phosphorylation at serine 482 affects stability of NF90 and its functional role in mitosis.

Objectives: NF90 is a multifunctional double-strand RNA binding protein with documented roles in transcription, mRNA stability, translation, RNA processing and transport, and mitosis. It is a phosphoprotein that interacts with, and is a substrate for, several protein kinases. The study described here was initiated to gain better understanding of specific NF90 phosphorylation sites and their relationship to mechanisms by which NF90 performs its various functions.

Materials And Methods: Phosphoproteomic studies have identified NF90 serine 482 (S482) as a major phosphorylation site in vivo. Site-specific mutations were introduced at this site and the mutated proteins were expressed in MCF7 cells by transfection. Western blotting was used to examine NF90 expression, stability, and responsiveness to protein kinase activators and inhibitors. Flow cytometry was used to examine effects of NF90 mutation on cell cycle progression.

Results: Non-phosphorylatable mutant S482A was unstable compared to phosphomimetic S482E mutant. NF90-S482A expression was greatly enhanced by inhibiting proteasomal degradation or by activating PKC. Identical treatments had little effect on NF90-S482E. In contrast to WT NF90 or NF90-S482E, cells stably expressing NF90-S482A accumulated in M phase when treated with TPA.

Conclusions: Phosphorylation at S482 is important for NF90 stability and in regulating its functional role during mitosis. Based on the sequence surrounding S482, mitotic kinase PLK1 is a strong candidate for the enzyme that phosphorylates NF90 at this site.