A TRANSLATION REGULATORY PARTICLE CONTAINING THE XENOPUS OOCYTE Y-BOXPROTEIN MRNP3+4

In oocytes, nontranslated maternal mRNAs are packaged by protein into messenger ribonucleoprotein particles (mRNPs) that are masked from tra nslation by protein-RNA interactions. Proteins associated with such ma sked states of mRNAs are particularly abundant in amphibian oocytes, O ne of these mRNP proteins from Xenopus oocytes, mRNP3+4 (also called F RG Y2a/b or p54/p56), binds to diverse mRNAs independent of their sequ ence and is the germ line member of the evolutionarily conserved Y box protein multigene family. Xenopus oocytes contain soluble pools of mR NP3+4 6 S oligomers, probably dimers, and larger similar to 15 S parti cles containing mRNP3+4 and additional proteins. Here we report the pu rification of this larger form as an similar to 320-kDa particle that contains mRNP3+4 and nine additional polypeptides, including mRNA-bind ing polypeptides of 34 and 36 kDa and a doublet of 110/105 kDa that pr oved to be nucleolin, The particle has a protein kinase activity that phosphorylates its own mRNP3+4, nucleolin, and a 31-kDa polypeptide co mponent and exhibits translational inhibition in both the wheat germ e xtract and rabbit reticulocyte lysate systems. The presence of mRNP3+4 and nucleolin in this large translation regulatory particle suggests that it participates in an early step of mRNP assembly and masking.