A Y-box protein/RNA helicase complex links mRNP assembly on the gene to mRNA translation

Abstract

Immediately upon transcription, messenger RNA molecules become associated with proteins to form ribonucleoprotein (RNP) complexes, usually called mRNP particles. The mRNPs are the actual targets for regulation and dictate the fate of an mRNA in the gene expression pathway. To understand the molecular basis of mRNP formation on the gene, export and translation the behaviour of the mRNP proteins have to be investigated. In the salivary glands of the dipteran Chironomus tentans, it is possible to visualize the co-transcriptional assembly of a specific giant mRNP (Balbiani ring mRNP, BR mRNP) on the gene and follow it during the nucleocytoplasmic transport and the formation of polysomes in the cytoplasm. We have now used this system to characterize two evolutionarily conserved RNA-binding proteins: a Y-box protein and a DEAD box RNA helicase.

In vertebrates, cytoplasmic mRNPs contain an abundant Y-box protein called YB1, which regulates translation, presumably by affecting the packaging of the mRNA. We have identified the C.tentans YB-1 (ctYB-1) homologue and revealed that ctYB-1 is added along the nascent transcripts and is present in the released BR RNP particles in the nucleoplasm as well as in the polysomes. Thus, ctYB-1 accompanies mRNA from the site of transcription in the nucleus to the place for translation in the cytoplasm.

We have shown that the ctYB-1 appears in two splice isoforms, p40 and p50, which differ only in the region close to the C-terminus. Except for tissue culture cells, both variants are present in all tested tissues but in different proportions. Protein composition analysis of isolated BR RNPs revealed that both isoforms are associated with nuclear and cytoplasmic BR mRNA and could be located on the same mRNA transcript.

Ded1 is a DEAD box RNA helicase essential for the initiation of mRNA translation in yeast. We have identified the C.tentans Ded1 homologue, hrp84, and shown that it binds co-transcriptionally to the nascent pre-mRNA transcript and remains associated with mRNA from the gene to the polysome. Injection of an hrp84 antibody into the nucleus did not affect mRNP assembly or transport, while injection into the cytoplasm blocked mRNA translation. Thus, hrp84 accompanies mRNA from the nucleus to the cytoplasm, where it exerts its function during translation.

In studies of C.tentans cultured cells, we have revealed by immunoprecipitation that hpr84 is associated with ctYB-1 both in the nucleus and cytoplasm, and the two proteins also appear together in polysomes. The interaction is likely to be direct as shown by in vitro binding of purified components. We conclude that the mRNA-bound hrp84/ctYB-1 complex is formed in the nucleus and is translocated with mRNA into the cytoplasm and further into polysomes.

In general, we conclude that two evolutionarily conserved translational regulators become bound to mRNA already during transcription, which indicates that translational competence of the mRNA in cytoplasm can be decided already cotranscriptionally during mRNP assembly.