OVEREXPRESSION IN COS CELLS OF P50, THE MAJOR CORE PROTEIN ASSOCIATEDWITH MESSENGER-RNA, RESULTS IN TRANSLATION INHIBITION

p50, the major core protein of messenger ribonucleoprotein particles ( mRNPs) in the cytoplasm of somatic mammalian cells, has been character ized previously as a member of the Y-box binding transcription factor family of proteins (YB-protein) by both high structural homology and a bility 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 m RNA from translation, Western blot analysis of the distribution of p50 in subcellular fractions of COS-1 cells shows that p50 is a cytoplasm ic protein quantitatively associated with mRNA, both in polyribosomes and in free mRNPs, The level of p50 in COS-1 cells determined by Weste rn immunoblotting is 0.10% of total protein, which is nearly equimolar to that of ribosomes and is similar to 5-10-fold higher than the mRNA level, Transient transfection of COS-1 cells with a p50-expressing ve ctor 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 protei n level is responsible for the inhibitory effect in these cells.