The mouse Y-box protein, MSY2, is associated with a kinase on non-polysomal mouse testicular mRNAs

In male germ cells many mRNAs are sequestered by proteins into translationally silent messenger ribonucleoprotein (mRNP) particles. These masked paternal mRNAs are stored and translated at specific times of germ cell development. Little is known about the mammalian testicular mRNA masking proteins bound to non-polysomal mRNAs. In this report, the major proteins binding to non-polysomal testicular mRNAs were isolated and analyzed. The two predominant proteins identified were: a Y-box protein (MSY2), the mammalian homolog to the Xenopus oocyte masking protein FRGY2/mRNP(3+4), and a poly(A) binding protein. A kinase activity was also found associated with these non-polysomal RNAs. The kinase co-immunoprecipitates with MSY2 and phosphorylates MSY2 in vitro. The MSY2 associated kinase is not casein kinase 2, the kinase believed to phosphorylate mRNP(3+4) in oocytes, but a yet unidentified kinase. MSY2 was found to be phosphorylated in vivo and MSY2 dephosphorylation led to a decrease in its affinity to bind RNA as judged by northwestern blotting. Therefore, testicular masked mRNAs may be regulated by the phosphorylation state of MSY2. Reconstitution experiments in which nonpolysomal mRNA-binding proteins are dissociated from their RNAs and allowed to bind to exogenous mRNAs suggest that MSY2 binds RNA in a sequence-independent fashion. Furthermore, association of the non-polysomal derived proteins to exogenous nonspecific mRNAs led to their translational repression in vitro.