| Histone proteins packages newly synthesized DNA into higher organizations. During DNA synthesis in metazoans, there is a high demand for biosynthesis of histone proteins. Strict regulation of histone protein expression in the S phase is achieved by unique structure of histone mRNA. Unlike other eukaryotic mRNAs, histone mRNAs end with a stem-loop, rather than a polyA tail. The stem-loop binds the stem loop binding protein (SLBP), a novel RNA binding protein that is conserved among metazoans. Xenopus laevis has two forms of SLBP. xSLBP1 is the homolog of hSLBP and N-terminus of the both proteins are shown to stimulate translation of histone mRNA. xSLBP2, the oocyte specific form, maintains high similarity to hSLBP only in the RBD. xSLBP2 does not stimulate histone mRNA translation, but instead represses histone mRNAs during early oogenesis. xSLBP2 is degraded during oocyte maturation, and then xSLBP1 binds histone mRNAs coinciding with the timing of translational activation. I have characterized a motif in the N-terminus of xSLBP1 and hSLBP which is responsible for translational activity. The motif, DWX(3-4)VEE, is highly conserved among vertebrates. I demonstrated that this motif interacts with a novel factor, named as SLBP-Interacting Protein 1 (SLIP1). Interestingly, SLIP1 stimulates translation of histone mRNA to higher levels in the presence of SLBP. SLBP mutants that can not stimulate histone mRNA translation are also incapable of interacting with SLBP.; Previously, it had been reported that eukaryotic polyadenylated mRNAs were translationally stimulated by circularization via protein-protein interactions. I showed that SLIP1 interacts with translation initiation factors IF4GI and II. Possibly, histone mRNA forms a closed loop similar to polyadenylated mRNAs during translation. Together with Jeremy Kupsco and Bob Duronio, I genetically investigated the role of SLBP and SLIP1 in Drosophila melanogaster . We identified a region in the N-terminus of dSLBP which has an essential post-processing function in Drosophila during early development. We demonstrated that dSLIP1 interacts with dSLBP. We characterized the SLIP1 interaction motif in the N-terminus of SLBP, (KFX(2-3)VEKE), which is similar to DWX(3-4)VEE motif in vertebrates. Both dSLBP and SLIP1 null mutant flies are embryonically lethal, probably due to a defect in translation of histone mRNAs. We conclude that the SLIP1-SLBP interaction is crucial for viability of Drosophila during early development. |
