Nuclear phosphoinositides and phase separation: Important players in nuclear compartmentalization.

Nuclear phosphoinositides are recognized as regulators of many nuclear processes including chromatin remodeling, splicing, transcription, DNA repair and epigenetics. These processes are spatially organized in different nuclear compartments. Phase separation is involved in the formation of various nuclear compartments and molecular condensates separated from surrounding environment. The surface of such structures spatiotemporally coordinates formation of protein complexes. PI(4,5)P2 (PIP2) integration into phase-separated structures might provide an additional step in their spatial diversification by attracting certain proteins with affinity to PIP2. Our laboratory has recently identified novel membrane-free PIP2-containing structures, so called Nuclear Lipid Islets (NLIs). We provide an evidence that these structures are evolutionary conserved in different organisms. We hypothesize that NLIs serve as a scaffolding platform which facilitates the formation of transcription factories, thus participating in the formation of nuclear architecture competent for transcription. In this review we speculate on a possible role of NLIs in the integration of various processes linked to RNAPII transcription, chromatin remodeling, actin-myosin interaction, alternative splicing and lamin structures.