Unique structure and stability of HmuY, a novel heme-binding protein of Porphyromonas gingivalis
Infection, survival, and proliferation of pathogenic bacteria in humans depend on their capacity to impair host responses and acquire nutrients in a hostile environment. Among such nutrients is heme, a co-factor for oxygen storage, electron transport, photosynthesis, and redox biochemistry, which is indispensable for life. Porphyromonas gingivalis is the major human bacterial pathogen responsible for severe periodontitis. It recruits heme through HmuY, which sequesters heme from host carriers and delivers it to its cognate outer-membrane transporter, the TonB-dependent receptor HmuR. Here we report that heme binding does not significantly affect the secondary structure of HmuY. The crystal structure of hemebound HmuY reveals a new all-b fold mimicking a right hand. The thumb and fingers pinch heme iron through two apical histidine residues, giving rise to highly symmetric octahedral iron co-ordination. The tetrameric quaternary arrangement of the protein found in the crystal structure is consistent with experiments in solution. It shows that thumbs and fingertips, and, by extension, the bound heme groups, are shielded from competing heme-binding proteins from the host. This may also facilitate heme transport to HmuR for internalization. HmuY, both in its apo- and in its heme-bound forms, is resistant to proteolytic digestion by trypsin and the major secreted proteases of P. gingivalis, gingipains K and R. It is also stable against thermal and chemical denaturation. In conclusion, these studies reveal novel molecular properties of HmuY that are consistent with its role as a putative virulence factor during bacterial infection. © 2009 Wójtowicz et al. ; This study was supported by grants from Spanish ministries (BIO2006-02668, BFU2006-09593, BIO2009-XXXXX, PSE-010000-2007-1, and CONSOLIDER-INGENIO 2010 Project "La Factoría de Cristalización" (CSD2006-00015)) and by grant 2005SGR00280 from the Generalitat de Catalunya. Additional funding was obtained from grants N401 029 32/0742 and 1642/B/P01/2008/35 from the Department of Scientific Research, Polish Ministry of Science and Higher Education; from US National Institutes of Health grant DE 09761; and from the European Union through EU FP6 Strep Project LSHG-2006-018830 "CAMP" and EU FP7 Collaborative Project 223101 "AntiPathoGN". Further funding was provided by ESRF (http://www.esrf.eu/) for data collection ; Peer Reviewed
