The major core protein of messenger ribonucleoprotein particles (p50) promotes initiation of protein biosynthesis in vitro.

The major core protein of cytoplasmic messenger ribonucleoprotein particles (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 regulator of both transcription and translation. Here we show that binding or removal of p50 from rabbit reticulocyte lysate by monospecific antibodies to p50 strongly inhibits translation of endogenous and exogenous globin mRNAs as well as prokaryotic beta-galactosidase mRNA in a rabbit reticulocyte cell-free system. Thus, depending on the conditions, p50 not only may act as a translational repressor, but may also be required for protein synthesis. Translation inhibition with anti-p50 antibodies is not a result of mRNA degradation or its functional inactivation. The inhibition does not change the ribosome transit time, and therefore, it does not affect elongation/termination of polypeptide chains. The inhibition with anti-p50 antibodies is followed by a decay of polysomes and accumulation of the 48 S preinitiation complex. These results suggest that p50 participates in initiation of protein biosynthesis. Although uninvolved in the formation of the 48 S preinitiation complex, p50 is necessary either for attachment of the 60 S ribosomal subunit or for previous 5'-untranslated region scanning by the 43 S preinitiation complex.

[The major cytoplasmic mRNP protein, p50, is required for efficient mRNA translation in vitro]. / Mazhornyi belok tsitoplazmaticheskikh mRNP, p50, neobkhodim dlia éffektivnoi transliatsii mRNK in vitro.

The major cytoplasmic mRNP protein of somatic cells, p50, is the member of the Y-box-binding transcription factor family and can control gene expression at two levels including mRNA transcription and translation. It has been demonstrated that p50 is responsible for the inactive state of mRNA within free mRNPs. In this work, we show that the Y-box-containing DNA (Y-DNA) predominantly binds to p50 in rabbit reticulocyte lysates and causes translation inhibition of the endogenous and exogenous globin mRNA and prokaryotic beta-galactosidase mRNA. Preincubation of Y-DNA with purified p50 prevents the inhibition. Inhibition of protein biosynthesis by the Y-DNA is not due to the degradation or functional inactivation of mRNA. The inhibition is associated with the decay of polyribosomes and dissociation of a newly synthesized protein from the ribosomes. The data indicate that Y-DNA inhibits protein biosynthesis predominantly at the initiation stage and that p50 is an essential component of the translation initiation apparatus.