Histone hairpin binding protein, an RNA binding protein, essential for development
Histones are proteins found in the nuclei of eukaryotic cells where they are complexed to DNA in chromatin. Rephcation-dependent histones are expressed only during S-phase. Regulation of expression of replication-dependent histone genes requires a highly conserved hairpin RNA element in the 3' untranslated region of histone mRNAs. Replication-dependent histone mRNAs are not polyadenylated; their 3' end is formed by an endonucleolytic cleavage event, 3' of a hairpin element, which is recognised by the Hairpin Binding Protein, HBP (also known as Stem-Loop Binding Protein, SLBP). This protein-RNA interaction is important for the endonucleolytic cleavage that generates the mature mRNA 3' end. The 3' hairpin, and presumably HBP, are also required for nucleocytoplasmic transport, translation and stability of histone mRNAs. It is therefore important to understand this interaction. The hairpin is highly conserved and I have demonstrated that residues in the hairpin loop are important for binding the HBP. This complimented structural studies that showed that the same residues are involved in stacking interactions in the RNA loop. In cell culture, expression of replication-dependent histone genes is S phase specific as is the expresion of HBP. Here I demonstrated that in Caenorhabditis elegans the HBP promoter is active in dividing cells during embryonic and postembryonic development. Depletion of HBP by RNAi leads to an embryonic lethal phenotype associated with defects in chromosome condensation. Postembryonic depletion of HBP results in defects in cell fate during late larval development, specifically in vulval development. A similar phenotype was obtained when histone H3 and H2A were depleted by RNAi suggesting that the phenotype of the hbp (RNAi) worms was due to a lack of histone proteins. I have confirmed this by showing that histone proteins are indeed reduced in hbp (RNAi) worms. I have also shown that depletion of HBP leads to a change in expression of a number of other proteins and specifically an up-regulation of a histone H3 like protein with an apparent molecular mass of 34 kDa. I have evidence that suggests that this protein is the centromer specific protein, CENP-A. As this protein was up-regulated when RNAi was used to deplete histones proteins, this suggests that there could be a compensatory mechanism that helps the animal to deal with the shortage of histone proteins.