Inositol phosphates (InsPs) are signaling molecules with multiple roles in cells. In particular Insð1,2,3,4,5,6ÞP 6 (InsP 6 ) is involved in mRNA export and editing or chromatin remodeling among other events. InsP 6 accumulates as mixed salts (phytate) in storage tissues of plants and plays a key role in their physiology. Human diets that are exclusively grain-based provide an excess of InsP 6 that, through chelation of metal ions, may have a detrimental effect on human health. Insð1,3,4,5,6ÞP 5 2-kinase (InsP 5 2-kinase or Ipk1) catalyses the synthesis of InsP 6 from InsP 5 and ATP, and is the only enzyme that transfers a phosphate group to the axial 2-OH of the myo-inositide. We present the first structure for an InsP 5 2-kinase in complex with both substrates and products. This enzyme presents a singular structural region for inositide binding that encompasses almost half of the protein. The key residues in substrate binding are identified, with Asp368 being responsible for recognition of the axial 2-OH. This study sheds light on the unique molecular mechanism for the synthesis of the precursor of inositol pyrophosphates.InsP5 2-kinase | IP5 | IP6 | ipk1 | crystal structure

Uracil-DNA glycosylase (UDG) is a key repair enzyme responsible for removing uracil residues from DNA. Interestingly, UDG is the only enzyme known to be inhibited by two different DNA mimic proteins: p56 encoded by the Bacillus subtilis phage ϕ29 and the well-characterized protein Ugi encoded by the B. subtilis phage PBS1/PBS2. Atomic-resolution crystal structures of the B. subtilis UDG both free and in complex with p56, combined with site-directed mutagenesis analysis, allowed us to identify the key amino acid residues required for enzyme activity, DNA binding and complex formation. An important requirement for complex formation is the recognition carried out by p56 of the protruding Phe191 residue from B. subtilis UDG, whose side-chain is inserted into the DNA minor groove to replace the flipped-out uracil. A comparative analysis of both p56 and Ugi inhibitors enabled us to identify their common and distinctive features. Thereby, our results provide an insight into how two DNA mimic proteins with different structural and biochemical properties are able to specifically block the DNA-binding domain of the same enzyme.