A mouse Y box protein, MSY1, is associated with paternal mRNA in spermatocytes

We have isolated a mouse cDNA clone, which encodes the protein MSY1 (mouse Y box protein 1), a new member of the Y box family of proteins. Northern analysis indicates that MSY1 mRNA accumulates over 100-fold more in testis than in other tissues. Moreover, MSY1 mRNA is developmentally regulated, initially appearing at the pachytene stage of spermatogenesis. This is the stage of maximal transcription and translation in the spermatocyte. In Xenopus laevis, homologous Y box proteins, FRGY1 and FRGY2, positively regulate transcription from promoters containing a Y box (CTGATTGGCCAA). In addition, the germ cell-specific Y box protein FRGY2 binds maternal mRNA within 60-80 S mRNP storage particles and in doing so regulates translation in the developing oocyte and embryo (Smith, L. D., Richter, J. D., and Taylor, M. A. (1984) in Molecular Biology of Development (Davidson, E. R., and Firtel, R. A., eds) pp. 129-141, Alan R. Liss, New York). The MSY1 protein can be isolated from a 60-80 S mRNP fraction of testis which like the frog oocyte contains stored, untranslated mRNAs. Furthermore, cross-linking experiments demonstrate that MSY1 is bound to mRNAs of this fraction. Finally, mobility shift analysis performed using the isolated protein indicates that MSY1 has nucleic acid binding properties similar to those of the FRGY proteins. These data suggest that the mouse Y box protein, MSY1, functions similarly to the FRGY2 protein in regulating the storage and translation of germ cell RNAs.