Nek9, a Novel FACT-associated Protein, Modulates Interphase Progression

The heterodimeric Spt16-Pob3/DUF/FACT complex is a class of chromatin structure modulators with important roles in replication and transcription. Although regarded as a transcription elongator for chromatin template, little is known about the mode of action and involvement in other molecular processes of the mammalian FACT. Here we report the identification of a novel interacting and functional partner of FACT, Nek9. Nek9 forms a stable, ∼600-kDa complex with FACT in the interphase nuclei. Its active form is characterized by phosphorylation-dependent electrophoretic mobility shift and phosphorylation at a conserved residue within the activation loop (Thr²¹⁰). When complexed with FACT, Nek9 exhibits markedly elevated phosphorylation on Thr ²¹⁰. Cell cycle analysis on the Nek9^dsRNAi cells directly implicated Nek9 in maintaining proper G₁ and S progression, a role temporally correlated to the formation of a phospho-Nek9-FACT complex. Collectively, these observations provide evidence that Nek9, potentially as an active enzymatic partner of FACT, mediates certain FACT-associated cellular processes, which are ultimately essential for interphase progression.