The degradation of the stem-loop binding protein at the late 2-cell stage of mouse embryogenesis

The efficient processing of replication-dependent histone mRNA requires the Stem-Loop Binding Protein (SLBP). SLBP is also involved in regulating histone mRNA half-life, their nucleocytoplasmic transport, and their translation. Unlike somatic cells, where SLBP protein accumulates only in S-phase, SLBP protein is present throughout the first two embryonic cell cycles in mice. We report here that in late 2-cell mouse embryos there is a substantial, proteasome-dependent decrease in SLBP throughout the cell. Based on chromosome morphology, the degradation of SLBP protein in late 2-cell embryos is most likely a late G2-phase event. The degradation of SLBP protein is not simply a zygotic clock event, but requires development to the late 2-cell stage. Furthermore, SLBP protein degradation in 2-cell mouse embryos requires cyclin-dependent kinase (Cdk) activity, DNA replication, and zygotic genome activation. A model for SLBP protein degradation is proposed based on observations made in both early mouse embryos and somatic cells.