Interchromatin Granule Clusters of Mouse Preovulatory Oocytes are Enriched with Some Components of mRNA Export Machinery

At the diplotene stage of meiotic prophase, the nucleus of mouse preovulatory oocytes contains multiple and prominent interchromatin granule clusters (IGCs). IGCs, also known as nuclear speckles, splicing factor compartments, or SC35-domains, are one of the most universal nuclear organelles. We have used mouse oocytes as an experimental model to study nuclear distribution of some factors involved in mRNA export: the core protein of the exon-exon junction complex (EJC) Y14, EJC shell protein Aly, and the essential mRNA export factor NXF1/TAP. RNA export proteins were detected by confocal laser microscopy and immunogold-labeling electron microscopy. We found that Y14, Aly and NXF1/TAP are located in the same IGCs together with actin and SC35 used as an IGC marker. The relationships of these proteins with IGCs occur in an RNA-dependent manner, since RNase treatment results in disappearance of the identified antigens from IGCs. After the suppression of transcription by DRB, intranuclear localization of the studied proteins varied considerably: NXF1/TAP, Y14 and Aly were detected only in the nucleoplasm, but not in the IGCs. The concept of the formation and retention of mRNA export complex and nuclear actin in the IGCs of mouse oocyte is discussed.