The SOUL family of heme-binding proteins: Structure and function 15 years later


The SOUL, or heme-binding protein HBP/SOUL, family represents a group of evolutionary conserved putative heme-binding proteins that contains a number of members in animal, plant and bacterial species. The structures of the murine form of HEBP1, or p22HBP, and the human form of HEBP2, or SOUL, have been determined in 2006 and 2011 respectively. In this work we discuss the structures of HEBP1 and HEBP2 in light of new X-ray data for heme bound murine HEBP1. The interaction between tetrapyrroles and HEBP1, initially proven to be hydrophobic in nature, was thought to also involve electrostatic interactions between heme propionate groups and positively charged amino acid side chains. However, the new X-ray structure, and results from murine HEBP1 variants and human HEBP1, confirm the hydrophobic nature of the heme-HEBP1 interaction, resulting in K-d values in the low nanomolar range, and rules out any electrostatic stabilization. Results from NMR relaxation time measurements for human HEBP1 describe a rigid globular protein with no change in motional regime upon heme binding. X-ray structures deposited in the PDB for human HEBP2 are very similar to each other and to the new heme-bound murine HEBP1 X-ray structure (backbone rmsd ca. 1 A). Results from a HSQC spectrum centred on the histidine side chain N delta-proton region for HEBP2 confirm that HEBP2 does not bind heme via H42 as no chemical shift differences were observed upon heme addition for backbone NH and Nd protons. A survey of the functions attributed to HEBP1 and HEBP2 over the last 20 years span a wide range of cellular pathways. Interestingly, many of them are specific to higher eukaryotes, particularly mammals and a potential link between heme release under oxidative stress and human HEBP1 is also examined using recent data. However, at the present moment, trying to relate function to the involvement of heme or tetrapyrrole binding, specifically, makes little sense with our current biological knowledge and can only be applied to HEBP1, as HEBP2 does not interact with heme. We suggest that it may not be justified to call this very small family of proteins, heme-binding proteins. The family may be more correctly called the SOUL family of proteins related to cellular fate as, even though only HEBP1 binds heme tightly, both proteins may be involved in cell survival and/or proliferation. (C) 2021 Elsevier B.V. All rights reserved.



subject category

Chemistry, Inorganic & Nuclear


Goodfellow, BJ; Freire, F; Carvalho, AL; Aveiro, SS; Charbonnier, P; Moulis, JM; Delgado, L; Ferreira, GC; Rodrigues, JE; Poussin-Courmontagne, P; Birck, C; McEwen, A; Macedo, AL

our authors


Financial support was received from the Fundacao para a Ciencia e a Tecnologia, FEDER, COMPETE, and QREN through grants SFRH/BD/30239/2006 (FF) and SFRH/BD/64519/2009 (SSA) and projects: PTDC/QUI/64203/2006 (BJG), UIDP/04378/2020 and UIDB/04378/2020 of the Research Unit on Applied Molecular Biosciences -UCIBIO, LA/P/0140/2020 of the Associate Laboratory Institute for Health and Bioeconomy -i4HB and UIDB/50011/2020 of the CICECO-Aveiro Institute of Materials.; JMM thanks the French Plan Cancer (n~ 18CB010-00) for partial support and JMM and BJG also thank the Luso-French Pessoa Program for financial support.; The authors acknowledge the ESRF, Grenoble (beamline ID23EH1) for access and technical support during data collection (HEBP2). Special recognition is given to Dr Jose Trincao and Catarina Coelho for X-ray data collection. Access to the high Throughput Crystallization facility of the Partnership for Structural Biology in Grenoble (PSB) ( was supported by the European Community-Research Infrastructure Action PCUBE under the FP7 ``Capacitiesspecific program. Financial support was also provided by Instruct-ERIC (PID 1046, 1797 and 3808) as a funding source (BJG) and by the grant ANR-10-LABX-0030-INRT, a French State fund managed by the Agence Nationale de la Recherche under the frame program Investissements d'Avenir ANR-10IDEX-0002-02, and by the French Infrastructure for Integrated Structural Biology (FRISBI) ANR-10-INSB-05-01 and Instruct-ERIC. We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities, and we would like to thank Christoph Mueller-Dieckmann for assistance in using beamline ID30B.; The NMR spectrometers used in this work are part of the Portuguese NMR Network (PTNMR) and are partially supported by Infrastructure Project No 022161 (co-financed by FEDER through COMPETE 2020, POCI and PORL and FCT through PIDDAC).

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