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IUCrJ. 2019 Feb 28;6(Pt 2):317-330. doi: 10.1107/S2052252519001568. eCollection 2019 Mar 1.

Structure of mammalian plasma fetuin-B and its mechanism of selective metallopeptidase inhibition.

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Proteolysis Laboratory, Department of Structural Biology, Molecular Biology Institute of Barcelona, CSIC, Barcelona Science Park, Helix Building, c/o Baldiri Reixac 15-21, E-08028 Barcelona, Catalonia, Spain.
Institute of Molecular Physiology, Cell and Matrix Biology, Johannes Gutenberg-University Mainz, Johann-Joachim-Becher-Weg 7, D-55128 Mainz, Germany.
Department of Biosciences and Nutrition and Center for Innovative Medicine, Karolinska Institutet, Blickagången 16, SE-141 83 Huddinge, Sweden.
Biointerface Laboratory, Helmholtz Institute for Biomedical Engineering, RWTH Aachen University Medical Faculty, Pauwelsstrasse 30, D-52074 Aachen, Germany.
ESRF - The European Synchrotron, 71 Rue Jules Horowitz, F-38000 Grenoble, France.
Institute of Molecular Pathobiochemistry, Experimental Gene Therapy and Clinical Chemistry, RWTH Aachen University Hospital, Pauwelsstrasse 30, D-52074 Aachen, Germany.


Mammalian fetuin-A and fetuin-B are abundant serum proteins with pleiotropic functions. Fetuin-B is a highly selective and potent inhibitor of metallo-peptidases (MPs) of the astacin family, which includes ovastacin in mammals. By inhibiting ovastacin, fetuin-B is essential for female fertility. The crystal structure of fetuin-B was determined unbound and in complex with archetypal astacin, and it was found that the inhibitor has tandem cystatin-type modules (CY1 and CY2). They are connected by an exposed linker with a rigid, disulfide-linked 'CPDCP-trunk', and are followed by a C-terminal region (CTR) with little regular secondary structure. The CPDCP-trunk and a hairpin of CY2 form a bipartite wedge, which slots into the active-site cleft of the MP. These elements occupy the nonprimed and primed sides of the cleft, respectively, but spare the specificity pocket so that the inhibitor is not cleaved. The aspartate in the trunk blocks the catalytic zinc of astacin, while the CY2 hairpin binds through a QWVXGP motif. The CY1 module assists in structural integrity and the CTR is not involved in inhibition, as verified by in vitro studies using a cohort of mutants and variants. Overall, the inhibition conforms to a novel 'raised-elephant-trunk' mechanism for MPs, which is reminiscent of single-domain cystatins that target cysteine peptidases. Over 200 sequences from vertebrates have been annotated as fetuin-B, underpinning its ubiquity and physiological relevance; accordingly, sequences with conserved CPDCP- and QWVXGP-derived motifs have been found from mammals to cartilaginous fishes. Thus, the raised-elephant-trunk mechanism is likely to be generally valid for the inhibition of astacins by orthologs of fetuin-B.


X-ray crystallography; enzyme mechanisms; mammalian fertilization; metallopeptidase; multi-protein complexes; polyspermy; protein inhibitor; protein structure; sperm–egg fusion; structure determination

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