Phosphorylated Nucleolin Interacts with Translationally Controlled Tumor Protein during Mitosis and with Oct4 during Interphase in ES Cells

Background: Reprogramming of somatic cells for derivation of either embryonic stem (ES) cells, by somatic cell nuclear transfer (SCNT), or ES-like cells, by induced pluripotent stem (iPS) cell procedure, provides potential routes toward non-immunogenic cell replacement therapies. Nucleolar proteins serve as markers for activation of embryonic genes, whose expression is crucial for successful reprogramming. Although Nucleolin (Ncl) is one of the most abundant nucleolar proteins, its interaction partners in ES cells have remained unidentified. Methodology: Here we explored novel Ncl-interacting proteins using in situ proximity ligation assay (PLA), colocalization and immunoprecipitation (IP) in ES cells. Principal Findings: We found that phosphorylated Ncl (Ncl-P) interacted with translationally controlled tumor protein (Tpt1) in murine ES cells. The Ncl-P/Tpt1 complex peaked during mitosis and was reduced upon retinoic acid induced differentiation, signifying a role in cell proliferation. In addition, we showed that Ncl-P interacted with the transcription factor Oct4 during interphase in human as well as murine ES cells, indicating of a role in transcription. The Ncl-P/Oct4 complex peaked during early stages of spontaneous human ES cell differentiation and may thus be involved in the initial differentiation event(s) of mammalian development. Conclusions: Here we described two novel protein-protein interactions in ES cells, which give us further insight into the complex network of interacting proteins in pluripotent cells.