THE MAJOR PROTEIN OF MESSENGER-RIBONUCLEOPROTEIN PARTICLES IN SOMATIC-CELLS IS A MEMBER OF THE Y-BOX BINDING TRANSCRIPTION FACTOR FAMILY

A cDNA encoding the major core protein, p50, of cytoplasmic messenger ribonucleoprotein particles (mRNPs) of somatic cells was cloned from a rabbit reticulocyte cDNA library. From the derived 324-amino acid seq uence, p50 is identified as a member of the Y-box binding transcriptio n factor family. The protein was earlier described as a repressor of g lobin mRNA translation. These findings suggest that p50 may affect pro tein biosynthesis at two levels: mRNA transcription in the nucleus and mRNA translation in the cytoplasm. Together with recently published r esults showing that masked mRNA in germ cells also is associated with proteins of the Y-box binding protein family, the present finding indi cates that these proteins are universal core proteins responsible for the formation of cytoplasmic mRNPs in eukaryotes. Highly purified p50 forms large 18 S homomultimeric complexes with a molecular mass of abo ut 800 kilodaltons and melts RNA secondary structure. This suggests th at p50 may affect translation by changing the overall structure of the mRNA.