Characterisation of the localisation of histone hairpin binding protein and its role in translation of histone mRNA
Histones are proteins required for packaging of DNA in the nucleus. Metazoan replication-dependent histone genes are expressed only during S-phase, intronless, and their mRNAs do not end in a poly(A) tail but rather a highly conserved hairpin structure. Hairpin binding protein (HBP or Stem-Loop Binding Protein) specifically binds to the hairpin structure of histone pre-mRNA. This interaction is important during the endonucleolytic cleavage reaction to generate the mature histone mRNA. I have performed preliminary work to identify nucleotides in the hairpin structure that are involved in the interaction with HBP. HBP expression occurs concurrently with expression of histone genes during S-phase. A hairpin RNA binding activity, presumed to be HBP has been previously demonstrated in both nuclear and cytoplasmic extracts. I have used a specific antibody to show that hHBP has a histone RNA-independent localisation in both the nucleus and cytoplasm only during S-phase. Also, I have examined the localisation of an exclusively nuclear alternative splice variant of HBP which arises from skipping of exon 3 of mouse and human HBP mRNA. Efficient translation of histone mRNA is dependent on HBP. I show that a minor fraction of cytoplasmic hHBP associates with ribosomes in an RNA-dependent manner and show that HBP stimulates translation. I further participated in mapping of the HBP region required for the translation stimulation by a motif encoded by xSLBP1 exon 3 found in xSLBP1-Nt and characterised interactions between HBP and translation initiation factors, eIF3h and PAIP1, mediated by hHBP RBD and Ct, respectively.