A TESTIS CYTOPLASMIC RNA-BINDING PROTEIN THAT HAS THE PROPERTIES OF ATRANSLATIONAL REPRESSOR

Translation of the mouse protamine 1 (Prm-1) mRNA is repressed for sev eral days during male germ cell differentiation. With the hope of clon ing genes that regulate the translational repression of Prm-1, we scre ened male germ cell cDNA expression libraries with the 3' untranslated region of the Prm-1 RNA. From this screen we obtained two independent clones that encode Prbp, a Prm-1 RNA-binding protein. Prbp contains t wo copies of a double-stranded-RNA-binding domain. In vitro, the prote in binds to a portion of the Prm-1 3' untranslated region previously s hown to be sufficient for translational repression in transgenic mice, as well as to poly(I). poly(C). Prbp protein is present in multiple f orms in cytoplasmic extracts prepared from wild-type mouse testes and is absent from testes of germ cell-deficient mouse mutants, suggesting that rbp is restricted to the germ cells of the testis. Immunocytoche mical localization confirmed that Prbp is present in the cytoplasmic c ompartment of late-stage meiotic cells and haploid round spermatids. R ecombinant Prbp protein inhibits the translation of multiple mRNAs in a wheat germ lysate, suggesting that Prbp acts to repress translation in round spermatids. While this protein lacks complete specificity for Prm-1-containing RNAs in vitro, the properties of Prbp are consistent with it acting as a general repressor of translation.