Mammalian nuclear speckles exhibit stable association with chromatin: a biochemical study.

Nuclear Speckles (NS) are phase-separated condensates of protein and RNA whose components dynamically coordinate RNA transcription, splicing, transport and DNA repair. NS, probed largely by imaging studies, remained historically well known as Interchromatin Granule Clusters, and biochemical properties, especially their association with Chromatin have been largely unexplored. In this study, we tested whether NS exhibit any stable association with chromatin and show that limited DNAse-1 nicking of chromatin leads to the collapse of NS into isotropic distribution or aggregates of constituent proteins without affecting other nuclear structures. Further biochemical probing revealed that NS proteins were tightly associated with chromatin, extractable only by high-salt treatment just like histone proteins. NS were also co-released with solubilised mono-dinucleosomal chromatin fraction following the MNase digestion of chromatin. We propose a model that NS-chromatin constitutes a "putative stable association" whose coupling might be subject to the combined regulation from both chromatin and NS changes. NS: Nuclear speckles; DSB: double strand breaks; PTM: posttranslational modifications; DDR: DNA damage repair; RBP-RNA binding proteins; TAD: topologically associated domains; LCR: low complexity regions; IDR: intrinsically disordered regions.