Nuclear compartmentalization is crucial for spatiotemporal regulation of key processes such as transcription and splicing. Recently, we have identified an F-actin regulatory protein (MPRIP) in nucleus by using microscopy and biochemistry methods. Having two PH domains, MPRIP has an affinity to Phosphoinositols and as a key regulator of protein phosphatase 1, it is a potential mediator for dephosphorylation of Pol II and splicing machinery (Mulder et al., 2003). Methods: We have first investigated the localization of the endogenous MPRIP by super-resolution microscopy (STED) and visualized that it forms nanoscale foci in the nucleoplasm, which partly coalesce with PIP2 containing structures in nucleus, particularly with nuclear speckles and sub-population of Nuclear Lipid Islets (NLIs) (Sobol et al., 2018). To study the dynamics of endogenous MPRIP nanoscale foci is extremely challenging, therefore we employ GFP-MPRIP overexpression, in order to address question about the inner dynamics and mobility of MPRIP induced condensates. Results: The transient transfections experiments showed that highly expressing protein undergoes phase separation by accumulating in nucleus where it forms condensates that are up to 5 µm in diameter. We presume that this formation is mediated by the C Terminal- Intrinsically Disordered Region (IDR). The Fluorescence recovery after photobleaching (FRAP) and live-cell imaging experiments uncovered their rapid motion, of which we have quantified the diffusion coefficients as a way to comprehend how the protein might act at endogenous levels. Conclusion: Here, we showed that MPRIP, an actin regulatory protein, is present in mammalian nucleus and partly localize to proximity of PIP2 rich structures. We hypothesize that MPRIP might regulate transcription by recruiting its known interactor, Protein phosphatase 1, to the vicinity of NLIs. Altogether, these data indicates that MPRIP might be an important regulator of Pol II transcription, while the exact pathway remains to be enlightened. References: Mulder, J., Poland, M., Gebbink, M.F.B.G., Calafat, J., Moolenaar, W.H., Kranenburg, O., 2003. p116Rip Is A Novel Filamentous Actin-binding Protein. J. Biol. Chem. 278, 27216–27223. Sobol, M., Hozak, P., 2018. Nuclear phosphatidylinositol 4,5-bisphosphate islets contribute to efficient RNA polymerase II-dependent transcription. Journal of Cell Science 131, jcs211094. https://doi.org/10.1242/jcs.211094 Funding: This study was supported by the Czech Academy of Sciences (JSPS-18-18); Grant Agency of the Czech Republic (16-03346S, 17-09103S, 15-08738S).