Newly assembled snRNPs associate with coiled bodies before speckles, suggesting a nuclear snRNP maturation pathway.
BACKGROUND
Small nuclear ribonucleoproteins (snRNPs), which are essential components of the mRNA splicing machinery, comprise small nuclear RNAs, each complexed with a set of proteins. An early event in the maturation of snRNPs is the binding of the core proteins - the Sm proteins - to snRNAs in the cytoplasm followed by nuclear import. Immunolabelling with antibodies against Sm proteins shows that splicing snRNPs have a complex steady-state localisation within the nucleus, the result of the association of snRNPs with several distinct subnuclear structures. These include speckles, coiled bodies and nucleoli, in addition to a diffuse nucleoplasmic compartment. The reasons for snRNP accumulation in these different structures are unclear.


RESULTS
When mammalian cells were microinjected with plasmids encoding the Sm proteins B, D1 and E, each tagged with either the green fluorescent protein (GFP) or yellow-shifted GFP (YFP), a pulse of expression of the tagged proteins was observed. In each case, the newly synthesised GFP/YFP-labelled snRNPs accumulated first in coiled bodies and nucleoli, and later in nuclear speckles. Mature snRNPs localised immediately to speckles upon entering the nucleus after cell division.


CONCLUSIONS
The complex nuclear localisation of splicing snRNPs results, at least in part, from a specific pathway for newly assembled snRNPs. The data demonstrate that the distribution of snRNPs between coiled bodies and speckles is directed and not random.
Sleeman
J E
JE
Department of Biochemistry University of Dundee Wellcome Trust Building, Dow Street, Dundee, DD1 5EH, UK.
Lamond
A I
AI
D001324:Autoantigens / Q000378:metabolism; D001426:Bacterial Proteins / Q000378:metabolism; D002466:Cell Nucleolus / Q000502:physiology; D002467:Cell Nucleus / Q000378:metabolism*; D020541:Coiled Bodies / Q000378:metabolism*; D049452:Green Fluorescent Proteins; D006367:HeLa Cells; D006801:Humans; D007700:Kinetics; D008164:Luminescent Proteins / Q000378:metabolism; D018613:Microscopy, Confocal; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D017411:Ribonucleoproteins, Small Nuclear / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins; D013997:Time Factors; D014407:Tumor Cells, Cultured; D055517:snRNP Core Proteins
D001324:Autoantigens; D001426:Bacterial Proteins; D008164:Luminescent Proteins; D011993:Recombinant Fusion Proteins; D017411:Ribonucleoproteins, Small Nuclear; C112007:SME1 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D055517:snRNP Core Proteins; C054562:yellow fluorescent protein, Bacteria; D049452:Green Fluorescent Proteins