Functional specificity of the IP6 kinase isoforms

The inositol polyphosphate family of molecules has a prominent place in the field of intracellular signalling, and inositol pyrophosphates are the most recent addition to this large family. First discovered in 1993, they have since been identified in all eukaryotic organisms studied. In addition to their unique ‘high energy’ pyrophosphate bond, their concentration in the cell is tightly regulated with an extremely rapid turnover. This, together with the function of other inositol polyphosphates, makes it likely that they have an important role in intracellular signalling through some basic cellular processes. In mammals there are three isoforms of the IP_6 kinase enzyme responsible for the synthesis of 5PP-IP_5 (IP_7), the most well-characterised inositol pyrophosphate. In order to study the functional specificity of the IP_6 kinase isoforms, three independent yeast-two hybrid screens were performed using a less well-conserved region of the enzymes as bait in order to identify specific interactors. I have identified a hydrogen peroxide-stimulated interaction between IP_6 kinase 2 and Inorganic Pyrophosphatase 2. This protein is mitochondrial localized and IP_6 kinase 2 was shown to translocate to mitochondria after hydrogen peroxide treatment. I also present the functional characterization of the interaction between IP_6 kinase 1 and a newly identified jumonji domain-containing histone lysine demethylase (JMJD2C) in mammalian cells. This protein is involved in transcriptional regulation through the epigenetic regulation of chromatin states. I present data demonstrating a role for IP_6 kinase 1 in regulating histone post-translational modifications. This effect is dependent on the catalytic activity of IP_6 kinase 1 suggesting a role for IP_7 signalling in transcriptional regulation. I have also demonstrated that the demethylase binds specifically to monophosphorylated phosphoinositides via its dual C–terminal PHD domains