Overexpression in COS cells of p50, the major core protein associated with mRNA, results in translation inhibition.

p50, the major core protein of messenger ribonucleoprotein particles (mRNPs) in the cytoplasm of somatic mammalian cells, has been characterized previously as a member of the Y-box binding transcription factor family of proteins (YB-protein) by both high structural homology and ability to bind specifically the Y-box sequence in double-stranded DNA. YB proteins are present in a whole range of cell types and some have been identified as germ-specific cytoplasmic proteins masking stored mRNA from translation. Western blot analysis of the distribution of p50 in subcellular fractions of COS-1 cells shows that p50 is a cytoplasmic protein quantitatively associated with mRNA, both in polyribosomes and in free mRNPs. The level of p50 in COS-1 cells determined by Western immunoblotting is 0.10% of total protein, which is nearly equimolar to that of ribosomes and is approximately 5-10-fold higher than the mRNA level. Transient transfection of COS-1 cells with a p50-expressing vector results in a dramatic inhibition of protein synthesis. A control transfection with a vector expressing a frameshift mutant of p50 does not cause translation inhibition. Therefore the increase in p50 protein level is responsible for the inhibitory effect in these cells.