Mycobacterial and Human Nitrobindins: Structure and Function

Abstract

<b><i>Aims:</i></b> Nitrobindins (Nbs) are evolutionary conserved all-β-barrel heme-proteins displaying a highly solvent-exposed heme-Fe(III) atom. The physiological role(s) of Nbs is almost unknown. Here, the structural and functional properties of ferric <i>Mycobacterium tuberculosis</i> Nb (<i>Mt</i>-Nb(III)) and ferric <i>Homo sapiens</i> Nb (<i>Hs</i>-Nb(III)) have been investigated and compared with those of ferric <i>Arabidopsis thaliana</i> Nb (<i>At</i>-Nb(III), <i>Rhodnius prolixus</i> nitrophorins (<i>Rp</i>-NP(III)s), and mammalian myoglobins. <b><i>Results:</i></b> Data here reported demonstrate that <i>Mt</i>-Nb(III), <i>At</i>-Nb(III), and <i>Hs</i>-Nb(III) share with <i>Rp</i>-NP(III)s the capability to bind selectively nitric oxide, but display a very low reactivity, if any, toward histamine. Data obtained overexpressing <i>Hs</i>-Nb in human embryonic kidney 293 cells indicate that <i>Hs</i>-Nb localizes mainly in the cytoplasm and partially in the nucleus, thanks to a nuclear localization sequence encompassing residues Glu124-Leu154. Human <i>Hs</i>-Nb corresponds to the <i>C</i>-terminal domain of the human nuclear protein THAP4 suggesting that Nb may act as a sensor possibly modulating the THAP4 transcriptional activity residing in the <i>N</i>-terminal region. Finally, we provide strong evidence that both <i>Mt</i>-Nb(III) and <i>Hs</i>-Nb(III) are able to scavenge peroxynitrite and to protect free l-tyrosine against peroxynitrite-mediated nitration. <b><i>Innovation:</i></b> Data here reported suggest an evolutionarily conserved function of Nbs related to their role as nitric oxide sensors and components of antioxidant systems. <b><i>Conclusion:</i></b> Human THAP4 may act as a sensing protein that couples the heme-based Nb(III) reactivity with gene transcription. <i>Mt</i>-Nb(III) seems to be part of the pool of proteins required to scavenge reactive nitrogen and oxygen species produced by the host during the immunity response.