The characterization of the nuclear dynamics of syntenin-2, a PIP2 binding PDZ protein

Cellular signaling is largely dependent on the presence, that is, assembly/disassembly, of supramolecular complexes. Postsynaptic density protein, Discs-large, Zona occludens (PDZ) domains play important roles in the assembly of various signaling complexes. Syntenin-2 (S2) is a PDZ protein that interacts with nuclear phosphatidylinositol 4,5-bisphosphate (PIP2 ). Although nuclear lipids emerge as key players in nuclear processes, the global significance of nuclear phosphoinositide-protein interactions is still poorly understood. Those phosphoinositide-protein interactions that have been studied in detail appear to have profound physiological effects. To our knowledge none of these were investigated by dynamic studies such as Fluorescence Correlation Spectroscopy (FCS), Fluorescence Cross-Correlation Spectroscopy (FCCS), or Fluorescence Recovery After Photobleaching (FRAP). Although the exact function of S2 is unknown, siRNA experiments suggest that this PDZ protein plays a role in the organization of nuclear PIP2 , cell division, and cell survival. As a consequence of its PIP2 interaction, its reported self-association in a yeast two-hybrid study and its speculated interaction with many, yet unidentified, proteins one can hypothesize that S2 plays an important role in cell signaling. Therefore, we studied the dynamics of S2 using FCS, FCCS, and FRAP, utilizing an active truncated form deleted for the first 94 amino acids (S2-ΔN). We showed that S2-ΔN self-associates and is distributed in three groups. One immobile group, one slow diffusing group, which interacts with the nuclear environment and one fast diffusing group, which is not incorporated in high molecular weight complexes. In addition, our FCS and FRAP measurements on S2-ΔN mutants affected in their PIP2 binding showed that PIP2 plays an important role in the distribution of S2-ΔN among these groups, and favors the enrichment of S2-ΔN in the slow diffusing and immobile group. This work indicates that S2 relies on nuclear PIP2 to interact with practically immobile structures, possibly chromatin.