Xenopus oocytes store large quantities of translationally dormant mRNA in the cytoplasm as storage messenger ribonucleoprotein particles (mRNPs). The Y-box proteins, mRNP3 and FRGY2/mRNP4, are major RNA binding components of maternal storage mRNPs in oocytes. In this study, we show that the FRGY2 proteins form complexes with mRNA, which leads to mRNA stabilization and translational repression. Visualization of the FRGY2-mRNA complexes by electron microscopy reveals that FRGY2 packages mRNA into a compact RNP. Our results are consistent with a model that the Y-box proteins function in packaging of mRNAs to store them stably for a long time in the oocyte cytoplasm. (C) 2003 Elsevier Science (USA). All rights reserved.