Expression, phosphorylation, and mRNA-binding of heterogeneous nuclear ribonucleoprotein K in Xenopus oocytes, eggs, and early embryos

Here we show that heterogeneous nuclear ribonucleoprotein K (hnRNP K), a member of the K homology domain-containing proteins, is expressed in Xenopus immature oocytes, unfertilized eggs, and early embryos. Fertilization or egg activation treatment involving upregulation of the egg tyrosine kinase Src promotes a rapid and transient tyrosine phosphorylation of hnRNP K. HnRNP K is also phosphorylated on serine/threonine residues in unfertilized eggs, dephosphorylated after fertilization, and re-phosphorylated during the premitotic phase of early embryogenesis. In vitro, Src and mitogen-activated protein kinase (MAPK) were capable of phosphorylating hnRNP K on tyrosine and serine/threonine residues, respectively. In support of this, pretreatment of oocytes, eggs, or embryos with inhibitors for Src (PP2) and MAPK (U0126) blocked effectively the phosphorylation of hnRNP K. We also identify some maternal mRNAs that coimmuno-precipitate with hnRNP K in unfertilized eggs. Specific binding of these mRNAs to hnRNP K was verified by reverse transcriptase-polymerase chain reaction (RT-PCR). In addition, real-time PCR analyses revealed a subset of the mRNAs whose binding to hnRNP K might be up or downregulated in activated eggs. In vitro binding assay with the use of poly U monopolymeric RNA-coupled beads demonstrated that the RNA-binding property of hnRNP K is negatively regulated by tyrosine phosphorylation and positively or neutrally regulated by serine/threonine phosphorylation. Taken together, it is attractive to suggest that hnRNP K is in association with certain pools of maternal mRNAs whose translational activation are modulated by the Src/MAPK phosphorylation of hnRNP K during oocyte-egg-embryo transition.