Major constituent of messenger ribonucleoprotein particles (mRNPs). Involved in the regulation of the stability and/or translation of germ cell mRNAs. Binds to Y-box consensus promoter element. Binds to full-length mRNA with high affinity in a sequence-independent manner. Binds to short RNA sequences containing the consensus site 5'-UCCAUCA-3' with low affinity and limited sequence specificity. Its binding with maternal mRNAs is necessary for its cytoplasmic retention. May mark specific mRNAs (those transcribed from Y-box promoters) in the nucleus for cytoplasmic storage, thereby linking transcription and mRNA storage/translational delay. Found in a mRNP complex with PABPC1 and YBX3. Expressed in meiotic and postmeiotic male germ cells and oocytes; poorly expressed in two cell stage embryos (at protein level). Not detected in preimplantation embryos. Phosphorylated during oocyte maturation and dephosphorylated following egg activation. Phosphorylated in vitro by a kinase activity associated with testicular mRNPs. Dephosphorylation leads to a decrease in its affinity to bind RNA in vitro. Knockout mice for this gene exhibited reduced level of mRNA and protein synthesis in fully grown oocytes and decrease fertility. In PubMed:10772793 isoform 2 not detected in oocytes.