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J. Biol. Chem., Vol. 281, Issue 49, 37803-37812, December 8, 2006
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1
From the
Department of Biomolecular Sciences and Biotechnology, and CNR-INFM, University of Milano, I-20131 Milano, Italy, the Department of Physics, CNR-INFM and Center for Excellence in Biomedical Research, University of Genova, I-16146 Genova, Italy, the ¶Departement de Biochimie et de Microbiologie, Pavillon Marchand, Université Laval, Faculté des Sciences et de Génie, Quebec G1K 7P4, Canada, the ||Department of Biology and Interdepartmental Laboratory for Electron Microscopy, University "Roma Tre," Roma I-00146, and the **National Institute for Infectious Diseases I.R.C.C.S."Lazzaro Spallanzani," Roma I-00149, Italy
Truncated hemoglobins (trHbs) constitute a distinct lineage in the globin superfamily, distantly related in size and fold to myoglobin and monomeric hemoglobins. Their phylogenetic analyses revealed that three groups (I, II, and III) compose the trHb family. Group I and II trHbs adopt a simplified globin fold, essentially composed of a 2-on-2 
-helical sandwich, wrapped around the heme group. So far no structural data have been reported for group III trHbs. Here we report the three-dimensional structure of the group III trHbP from the eubacterium Campylobacter jejuni. The 2.15-Å resolution crystal structure of C. jejuni trHbP (cyano-met form) shows that the 2-on-2 trHb fold is substantially conserved in the trHb group III, despite the absence of the Gly-based sequence motifs that were considered necessary for the attainment of the trHb specific fold. The heme crevice presents important structural modifications in the C-E region and in the FG helical hinge, with novel surface clefts at the proximal heme site. Contrary to what has been observed for group I and II trHbs, no protein matrix tunnel/cavity system is evident in C. jejuni trHbP. A gating movement of His(E7) side chain (found in two alternate conformations in the crystal structure) may be instrumental for ligand entry to the heme distal site. Sequence conservation allows extrapolating part of the structural results here reported to the whole trHb group III.
Received for publication, July 31, 2006 , and in revised form, September 26, 2006.
The atomic coordinates and structure factors (code 2IG3) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).
* This work was supported in part by Natural Sciences and Engineering Research Council of Canada Grant 46306-01 (2005-2010) and National Institutes of Health Grant 1-R01-AI052258 (2004-2007) (to M. G.) and the Italian Ministry for University and Scientific Research FIRB Project "Biologia Strutturale" Contract RBLA03B3KC (to M. B.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
The on-line version of this article (available at http://www.jbc.org) contains supplemental data, Figs. S1-S2, and Table S1.
1 To whom correspondence should be addressed: Via Celoria 26, I-20131 Milano, Italy. Tel.: 39-02-50314893; Fax: 39-02-50314895; E-mail: martino.bolognesi{at}unimi.it.
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