Nuclear speckles: New insights into structure and function.

Nuclear speckles are prominent biomolecular condensates enriched in RNA-processing factors. Although historically viewed as reservoirs for spliceosomal components, recent work has demonstrated that nuclear speckles actively promote splicing efficiency, especially for genes containing short guanine-cytosine (GC)-rich introns. Nuclear speckles also participate in RNA 3' end formation and mRNA export. Advances in imaging and biophysical studies further reveal that speckles are hierarchically organized heterogeneous condensates formed through multivalent protein-RNA interactions and regulated by phosphorylation-dependent signaling. These discoveries redefine nuclear speckles as central regulators of gene expression.