Nuclear phosphatidylinositol 4,5-bisphosphate islets contribute to efficient RNA polymerase II-dependent transcription

Abstract

This paper describes a novel type of nuclear structures – nuclear lipid islets (NLIs). They are of 40–100 nm with a lipidic interior, and PtdIns(4,5)P2 molecules comprise a significant part of their surface. Most of NLIs have RNA at the periphery. Consistently with that, RNA is required for their integrity. NLI periphery is associated with Pol II transcription machinery, including the Pol II largest subunit, transcription factors, and NM1. The PtdIns(4,5)P2–NM1 interaction is significant for Pol II transcription, since NM1 knock-down reduces the Pol II transcription level, and the overexpression of wild-type NM1 (but not NM1 mutated in the PtdIns(4,5)P2-binding site) rescues the transcription. Importantly, Pol II transcription is dependent on NLI integrity, because an enzymatic reduction of the PtdIns(4,5)P2 level results in a decrease of the Pol II transcription level. Furthermore, about a half of nascent transcripts localise to NLIs, and transcriptionally active transgene loci preferentially colocalise with NLIs. We hypothesize that NLIs serve as a structural platform, which facilitates the formation of Pol II transcription factories, thus participating in the formation of nuclear architecture competent for transcription.