Transcription-dependent nucleocytoplasmic distribution of hnRNP A1 proteinin early mouse embryos

A unique feature of certain members of the heterogeneous nuclear ribonucleo protein (hnRNP) family of proteins is that they shuttle continuously betwee n nucleus and cytoplasm and their accumulation in the nucleus is transcript ion-dependent. An extensively characterised protein of this group is hnRNP A1. To date, most studies addressing the transcription-dependent transport of hnRNP A1 have been performed on cultured cell lines treated with transcr iption inhibitors. Here we have analysed the nucleocytoplasmic distribution of hnRNP A1 in early mouse embryos, where the haploid pronuclei remain tra nscriptionally inactive for a period of several hours. Consistent with its small molecular size (36 kDa), the hnRNP A1 protein diffuses passively thro ugh the nuclear pores and equilibrates between the nucleus and the cytoplas m of transcriptionally inactive embryos. In contrast, following transcripti onal activation the A1 protein becomes accumulated in the nucleus, This acc umulation of the A1 protein in the nucleus is blocked by the lectin wheat g erm agglutinin (WGA), which binds to nuclear pore proteins and prevents tra nslocation of receptor-cargo complexes through the pores. This indicates th at a carrier-mediated transport pathway is required for the concentration o f A1 in transcriptionally active nuclei. To further analyse how transcripti on is coupled to nucleocytoplasmic transport, we transplanted transcription ally inactive pronuclei into the cytoplasm of transcriptionally active embr yos. The results show that the presence of newly synthesised RNAs in the cy toplasm is not sufficient to induce the accumulation of hnRNP A1 in the nuc leus. Rather, the appearance of nascent transcripts in the nucleus appears to be the crucial event. Since hnRNP A1 is a shuttling protein, an increase in its steady state nuclear concentration could be the result of either fa ster nuclear import or slower export to the cytoplasm, We propose that bind ing of A1 to nascent transcripts retards its export to the cytoplasm and th erefore contributes to its concentration in the nucleus.