THE MAJOR CORE PROTEIN OF MESSENGER-RIBONUCLEOPROTEIN PARTICLES (P50)PROMOTES INITIATION OF PROTEIN-BIOSYNTHESIS IN-VITRO

The major core protein of cytoplasmic messenger ribonucleoprotein part icles (p50) has been shown previously to inhibit protein synthesis in vitro and in vivo, Furthermore, p50 is highly homologous to the Y-box- binding transcription factor family of proteins, binds DNA containing the Y-box motif, and thus may have a dual function in cells as a regul ator of both transcription and translation, Here we show that binding or removal of p50 from rabbit reticulocyte lysate by monospecific anti bodies to p50 strongly inhibits translation of endogenous and exogenou s globin mRNAs as well as prokaryotic beta-galactosidase mRNA in a rab bit reticulocyte cell-free system, Thus, depending on the conditions, p50 not only may act as a translational repressor, but may also be req uired for protein synthesis, Translation inhibition with anti-p50 anti bodies is not a result of mRNA degradation or its functional inactivat ion, The inhibition does not change the ribosome transit time, and the refore, it does not affect elongation/termination of polypeptide chain s. The inhibition with anti-p50 antibodies is followed by a decay of p olysomes and accumulation of the 48 S preinitiation complex, These res ults suggest that p50 participates in initiation of protein biosynthes is, Although uninvolved in the formation of the 48 S preinitiation com plex, p50 is necessary either for attachment of the 60 S ribosomal sub unit or for previous 5'-untranslated region scanning by the 43 S prein itiation complex.