Nuclear speckles can buffer the availability of splicing factors in the nucleoplasm to regulate rates of gene expression

Nuclear speckles are membrane-less nuclear bodies that harbor splicing factor (SFs), but their function is unclear. It is suggested that SFs can shuttle between nuclear speckles and transcribing genes, to participate in intron removal from pre-mRNAs. After splicing, SFs return to the nuclear speckles and are re-phosphorylated for the next cycle. To test the influence of the nuclear speckle structure on transcription and splicing kinetics, we used a detectable, transcriptionally active gene in living cells. We previously found that an mRNA undergoing many splicing events was retained at this gene until the completion of mRNA processing. To determine whether the reason for this retention was SF availability, we disassembled nuclear speckles by Clk1 kinase overexpression, and measured SF dynamics in the nucleus. Disassembly of nuclear speckles increased the diffusing fraction of SFs in the nucleoplasm, and reduced their residence times on the active gene. Importantly, the mRNA that was previously retained on the gene until completion of processing was now rapidly released. In contrast, a mutant inactive form of Clk1 and other treatments tested, did not affect the dynamics of mRNA release from the gene. Rather, faster release of the mRNA from the gene mediated by increased availability of splicing factors, was dependent on the RS domain of the splicing factors and its phosphorylation state. We suggest that nuclear speckles can buffer the availability of splicing factors in the nucleoplasm, and as membrane-less structures can rapidly respond and regulate the kinetics of mRNA release from the gene after processing.