The Sm core domain mediates targeting of U1 snRNP to subnuclear compartments involved in transcription and splicing

In the mammalian cell nucleus pre-mRNA splicing factors such as U snRNPs are concentrated in distinct subnuclear compartments named perichromatin fibrils (PFs), interchromatin granules (IGs), interchromatin granule-associated zones (IG-associated zones), and coiled bodies (CBs). The structural requirement for the localization of U snRNPs to these domains was investigated by microinjection of digoxygenin-labeled in vitro-reconstituted U1 snRNPs and mutants thereof and subsequent analysis by immunoelectron microscopy. Wild-type U1 snRNP was targeted, after injection into the cytoplasm, to the nucleus and localized in PFs, IGs, IG-associated zones, and CBs. Thus, microinjected U1 snRNP particles exhibited a subnuclear localization similar to that previously observed for endogenous U1 snRNPs. Specific U snRNP proteins were shown not to be essential for subnuclear targeting since U1 snRNP mutants that did not bind to 70K, A, or C peptides were distributed in the cell nucleus in a pattern indistinguishable from that of wild-type U1 snRNP. Moreover, the Sm core domain, common to all spliceosomal U snRNPs, was shown to be sufficient for appropriate subnuclear distribution. Thus, these observations indicate that the Sm core domain, previously shown to be essential for nuclear import of spliceosomal U1 snRNPs, is also important for mediating the targeting to distinct nuclear subcompartments.