Localization of poly(A)(+) RNA and mRNA export factors in interchromatin granule clusters of two-cell mouse embryos

Interchromatin granule clusters (IGCs), also known as nuclear speckles, splicing factor compartments, or SC35-domains, are one of the most universal nuclear organelles of the cell. We have used two-cell mouse embryos as an experimental system to study the possible association of poly(A)(+) RNA and factors involved in RNA export (Tip-associated protein [TAP] and heterogenous nuclear ribonucleoprotein A/B [hnRNP A/B]) with IGCs. Poly(A)(+) RNA was localized by microinjections of 2'-O-Me(U)(22) probes conjugated with tetramethylrhodamine. RNA export proteins were detected by immunofluorescence confocal laser microscopy and immunogold-labeling electron microscopy. We found that poly(A)(+) RNA was located in IGCs regardless of the transcriptional state of the nuclei. hnRNP A/B was also found to characterize IGCs of the embryo nuclei with various levels of transcription activity. In transcriptionally active embryo nuclei, TAP was detected in the vicinity of IGCs rather than inside them; however, when transcription was inhibited by drugs, TAP was localized to IGCs. Our data support the idea that IGCs not only serve as splicing factor reservoirs, but also take part in mRNA retention and export.