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Picornavirus genome replication takes place in specialized intracellular membrane compartments that concentrate viral RNA and proteins as well as a number of host factors that also participate in the process. The core enzyme in the replication machinery is the viral RNA-dependent RNA polymerase (RdRP) 3Dpol. Replication requires the primer protein 3B (or VPg) attached to two uridine molecules. 3B uridylylation is also catalysed by 3Dpol. Another critical interaction in picornavirus replication is that between 3Dpol and the precursor 3AB, a membrane-binding protein responsible for the localization of 3Dpol to the membranous compartments at which replication occurs. Unlike other picornaviruses, the animal pathogen foot-and-mouth disease virus (FMDV), encodes three non-identical copies of the 3B (3B1, 3B2, and 3B3) that could be specialized in different functions within the replication complex. Here, we have used a combination of biophysics, molecular and structural biology approaches to characterize the functional binding of FMDV 3B1 to the base of the palm of 3Dpol. The 1.7 Å resolution crystal structure of the FMDV 3Dpol -3B1 complex shows that 3B1 simultaneously links two 3Dpol molecules by binding at the bottom of their palm subdomains in an almost symmetric way. The two 3B1 contact surfaces involve a combination of hydrophobic and basic residues at the N- (G5-P6, R9; Region I) and C-terminus (R16, L19-P20; Region II) of this small protein. Enzyme-Linked Immunosorbent Assays (ELISA) show that the two 3B1 binding sites play a role in 3Dpol binding, with region II presenting the highest affinity. ELISA assays show that 3Dpol has higher binding affinity for 3B1 than for 3B2 or 3B3. Membrane-based pull-down assays show that 3B1 region II, and to a lesser extent also region I play essential roles in mediating the interaction of 3AB with the polymerase and its recruitment to intracellular membranes.

Populations of RNA viruses are composed of complex and dynamic mixtures of variant genomes that are termed mutant spectra or mutant clouds. This applies also to SARS-CoV-2, and mutations that are detected at low frequency in an infected individual can be dominant (represented in the consensus sequence) in subsequent variants of interest or variants of concern. Here we briefly review the main conclusions of our work on mutant spectrum characterization of hepatitis C virus (HCV) and SARS-CoV-2 at the nucleotide and amino acid levels and address the following two new questions derived from previous results: (i) how is the SARS-CoV-2 mutant and deletion spectrum composition in diagnostic samples, when examined at progressively lower cut-off mutant frequency values in ultra-deep sequencing; (ii) how the frequency distribution of minority amino acid substitutions in SARS-CoV-2 compares with that of HCV sampled also from infected patients. The main conclusions are the following: (i) the number of different mutations found at low frequency in SARS-CoV-2 mutant spectra increases dramatically (50- to 100-fold) as the cut-off frequency for mutation detection is lowered from 0.5% to 0.1%, and (ii) that, contrary to HCV, SARS-CoV-2 mutant spectra exhibit a deficit of intermediate frequency amino acid substitutions. The possible origin and implications of mutant spectrum differences among RNA viruses are discussed., This work was supported by Instituto de Salud Carlos III, Spanish Ministry of Science and Innovation (COVID-19 Research Call COV20/00181), and co-financed by European Development Regional Fund ‘A way to achieve Europe’. The work was also supported by grants CSIC-COV19-014 from Consejo Superior de Investigaciones Científicas (CSIC), project 525/C/2021 from Fundació La Marató de TV3, PID2020-113888RB-I00 from Ministerio de Ciencia e Innovación, BFU2017-91384-EXP from Ministerio de Ciencia, Innovación y Universidades (MCIU), PI18/00210 and PI21/00139 from Instituto de Salud Carlos III, and S2018/BAA-4370 (PLATESA2 from Comunidad de Madrid/FEDER). C.P., M.C., and P.M. are supported by the Miguel Servet programme of the Instituto de Salud Carlos III (CPII19/00001, CPII17/00006, and CP16/00116, respectively) cofinanced by the European Regional Development Fund (ERDF). CIBERehd (Centro de Investigación en Red de Enfermedades Hepáticas y Digestivas) is funded by Instituto de Salud Carlos III. Institutional grants from the Fundación Ramón Areces and Banco Santander to the CBMSO are also acknowledged. The team at CBMSO belongs to the Global Virus Network (GVN). B.M.-G. is supported by predoctoral contract PFIS FI19/00119 from Instituto de Salud Carlos III (Ministerio de Sanidad y Consumo) cofinanced by Fondo Social Europeo (FSE). R.L.-V. is supported by predoctoral contract PEJD-2019-PRE/BMD-16414 from Comunidad de Madrid. C.G.-C. is supported by predoctoral contract PRE2018-083422 from MCIU. P.S. is supported by postdoctoral contract “Margarita Salas” CA1/RSUE/2021 from MCIU. B.S. was supported by a predoctoral research fellowship (Doctorados Industriales, DI-17-09134) from Spanish MINECO.

Viral RNA-dependent RNA polymerases (RdRps) are major determinants of high mutation rates and generation of mutant spectra that mediate RNA virus adaptability. The RdRp of the picornavirus foot-and-mouth disease virus (FMDV), termed 3D, is a multifunctional protein that includes a nuclear localization signal (NLS) in its N-terminal region. Previous studies documented that some amino acid substitutions within the NLS altered nucleotide recognition and enhanced the incorporation of the mutagenic purine analogue ribavirin in viral RNA, but the mutants tested were not viable and their response to lethal mutagenesis could not be studied. Here we demonstrate that NLS amino acid substitution M16A of FMDV serotype C does not affect infectious virus production but accelerates ribavirin-mediated virus extinction. The mutant 3D displays polymerase activity, RNA binding, and copying processivity that are similar to those of the wild-type enzyme but shows increased ribavirin-triphosphate incorporation. Crystal structures of the mutant 3D in the apo and RNA-bound forms reveal an expansion of the template entry channel due to the replacement of the bulky Met by Ala. This is a major difference with other 3D mutants with altered nucleotide analogue recognition. Remarkably, two distinct loop β9-α11 conformations distinguish 3Ds that exhibit higher or lower ribavirin incorporation than the wild-type enzyme. This difference identifies a specific molecular determinant of ribavirin sensitivity of FMDV. Comparison of several polymerase mutants indicates that different domains of the molecule can modify nucleotide recognition and response to lethal mutagenesis. The connection of this observation with current views on quasispecies adaptability is discussed., Work in Madrid was supported by grants BFU2011-23604, SAF2014-52400-R, and AGL2014-52395-C2-1-R from the Spanish MINECO and S2013/ABI-2906 (PLATESA from Comunidad de Madrid/FEDER). CIBERehd (Centro de Investigación en Red de Enfermedades Hepáticas y Digestivas) is funded by Instituto de Salud Carlos III. Work in Barcelona was supported by grants BIO2017-83906-P and Maria de Maeztu Unit of Excellence MDM-2014-0435, from the Spanish MINECO. X-ray data were collected at ALBA Synchrotron (Cerdanyola de Valles, Spain) XALOC beamline with the collaboration of ALBA staff and at ESRF beamline ID29 (Grenoble, France). Financial support was provided by ALBA and ESRF. The kinetic studies at the University of Missouri were partially supported by grant AI076119 (National Institutes of Health, USA) to S.G.S. C.P. is supported by the Miguel Servet program of the Instituto de Salud Carlos III (CP14/ 00121) cofinanced by the European Regional Development Fund (ERDF). Institutional grants from the Fundación Ramón Areces and Banco Santander to the CBMSO are also acknowledged.

The selective pressures acting on viruses that replicate under enhanced mutation rates are largely unknown. Here, we describe resistance of foot-and-mouth disease virus to the mutagen 5-fluorouracil (FU) through a single polymerase substitution that prevents an excess of A to G and U to C transitions evoked by FU on the wild-type foot-and-mouth disease virus, while maintaining the same level of mutant spectrum complexity. The polymerase substitution inflicts upon the virus a fitness loss during replication in absence of FU but confers a fitness gain in presence of FU. The compensation of mutational bias was documented by in vitro nucleotide incorporation assays, and it was associated with structural modifications at the N-terminal region and motif B of the viral polymerase. Predictions of the effect of mutations that increase the frequency of G and C in the viral genome and encoded polymerase suggest multiple points in the virus life cycle where the mutational bias in favor of G and C may be detrimental. Application of predictive algorithms suggests adverse effects of the FU-directed mutational bias on protein stability. The results reinforce modulation of nucleotide incorporation as a lethal mutagenesis-escape mechanism (that permits eluding virus extinction despite replication in the presence of a mutagenic agent) and suggest that mutational bias can be a target of selection during virus replication, Work in Madrid was supported by grants [BFU2011-23604, SAF2014-52400-R, S2013/ABI2906 (PLATESA from Comunidad de Madrid/FEDER)] and Fundacion R. Areces and grant BFU2012-40020 to U.B. CIBERehd is funded by Instituto de Salud Carlos III. Work in Barcelona was supported by grants [BIO2011-24333, BIO2014-54588-P and MDM-2014-0435 from the Spanish MINECO]. X-ray data were collected at ALBA Synchrotron (Cerdanyola de Valles, Spain) XALOC beamline with the collaboration of ALBA staff. Financial support was provided by ALBA. C.P. is supported by the Miguel Servet program of the Instituto de Salud Carlos III (CP14/00121) cofinanced by the European Regional Development Fund (ERDF). The kinetic studies at the University of Missouri were partially supported by grant [AI076119] (National Institute of Health, USA) to S.G.S.

Increasing amounts of data show that conformational dynamics are essential for protein function. Unveiling the mechanisms by which this flexibility affects the activity of a given enzyme and how it is controlled by other effectors opens the door to the design of a new generation of highly specific drugs. Viral RNA-dependent RNA polymerases (RdRPs) are not an exception. These enzymes, essential for the multiplication of all RNA viruses, catalyze the formation of phosphodiester bonds between ribonucleotides in an RNA-template-dependent fashion. Inhibition of RdRP activity will prevent genome replication and virus multiplication. Thus, RdRPs, like the reverse transcriptase of retroviruses, are validated targets for the development of antiviral therapeutics. X-ray crystallography of RdRPs trapped in multiple steps throughout the catalytic process, together with NMR data and molecular dynamics simulations, have shown that all polymerase regions contributing to conserved motifs required for substrate binding, catalysis and product release are highly flexible and some of them are predicted to display correlated motions. All these dynamic elements can be modulated by external effectors, which appear as useful tools for the development of effective allosteric inhibitors that block or disturb the flexibility of these enzymes, ultimately impeding their function. Among all movements observed, motif B, and the B-loop at its N-terminus in particular, appears as a new potential druggable site. © 2013 Elsevier Ltd. All rights reserved., Primary research described in this review was supported by grants BIO2008-02556 and BIO2011-24333, from the Spanish Ministerio de Ciencia e Innovacion and Ministerio de Economía y Competitividad, respectively, and by the SILVER Cooperation project GA No. 260644 of the European Union, Seventh Framework Program.

Picornavirus RNA replication is initiated by the covalent attachment of a UMP molecule to the hydroxyl group of a tyrosine in the terminal protein VPg. This reaction is carried out by the viral RNA-dependent RNA polymerase (3D). Here, we report the X-ray structure of two complexes between foot-and-mouth disease virus 3D, VPg1, the substrate UTP and divalent cations, in the absence and in the presence of an oligoadenylate of 10 residues. In both complexes, VPg fits the RNA binding cleft of the polymerase and projects the key residue Tyr3 into the active site of 3D. This is achieved by multiple interactions with residues of motif F and helix α8 of the fingers domain and helix α13 of the thumb domain of the polymerase. The complex obtained in the presence of the oligoadenylate showed the product of the VPg uridylylation (VPg-UMP). Two metal ions and the catalytic aspartic acids of the polymerase active site, together with the basic residues of motif F, have been identified as participating in the priming reaction. © 2006 European Molecular Biology Organization | All Rights Reserved., Work in Barcelona was supported by Grants BIO2002-00517 and BFU2005-02376/BMC. Work in Madrid by Grants BMC 2001.1823.C02-01, BFU2005-00863/BMC and Fundación R Areces. CF and AA were supported by I3P fellowships from Ministerio de Educación y Ciencia. RA was supported by an FPI fellowship from Comunidad de Madrid. The financial support was provided by the ESRF

RNA-dependent RNA polymerases (RdRPs) are the central players in both transcription and viral genome replication. Using NMR spectroscopy, Yang and colleagues (in this issue of Structure) show that the conformational changes in the structural motif D of poliovirus RdRP correlate with the nature of the bound nucleotide (correct versus incorrect), with a conserved lysine within this motif playing a key role. © 2012 Elsevier Ltd.

Passage of poliovirus (PV) or foot-and-mouth disease virus (FMDV) in the presence of ribavirin (R) selected for viruses with decreased sensitivity to R, which included different mutations in their polymerase (3D): G64S located in the finger subdomain in the case of PV and M296I located within loop β9-α11 at the active site in the case of FMDV. To investigate why disparate substitutions were selected in two closely related 3Ds, we constructed FMDVs with a 3D that included either G62S (the equivalent replacement in FMDV of PV G64S), M296I, or both substitutions. G62S, but not M296I, inflicts upon FMDV a strong selective disadvantage which is partially compensated for by the substitution M296I. The corresponding mutant polymerases, 3D(G62S), 3D(M296I), and 3D(G62S-M296I), were analyzed functionally and structurally. G62S in 3D impairs RNA-binding, polymerization, and R monophosphate incorporation activities. The X-ray structures of the 3D(G62S)-RNA, 3D(M296I)-RNA, and 3D(G62S-M296I)-RNA complexes show that although the two positions are separated by 13.1 Å, the loops where the replacements reside are tightly connected through an extensive network of interactions that reach the polymerase active site. In particular, G62S seems to restrict the flexibility of loop β9-α11 and, as a consequence, the flexibility of the active site and its ability to bind the RNA template. Thus, a localized change in the finger subdomain of 3D may affect the catalytic domain. The results provide a structural interpretation of why different amino acid substitutions were selected to confer R resistance in closely related viruses and reveal a complex network of intra-3D interactions that can affect the recognition of both the RNA template and incoming nucleotide. Copyright © 2010, American Society for Microbiology. All Rights Reserved., Work in Barcelona was supported by grant BIO2008-02556, and work in Madrid was supported by grant BFU2008-02816/BMC from MCINN and by Fundación R. Areces. CIBERehd (Centro de Investigación Biomédica en Red de Enfermedades Hepáticas y Digestivas) is funded by Instituto de Salud Carlos III. Work in Barcelona and Madrid was further supported by Proyecto Intramural de Frontera 2000820FO191 (CSIC). X-ray data were collected at ESRF beamlines ID14.1 and ID14.2 (Grenoble, France) within a Block Allocation Group (BAG Barcelona). Financial support was provided by the ESRF. A.A. and C.F.-O. are recipients of I3P and Juan de la Cierva postdoctoral contracts, respectively.

Resistance of viruses to mutagenic agents is an important problem for the development of lethal mutagenesis as an antiviral strategy. Previous studies with RNA viruses have documented that resistance to the mutagenic nucleoside analogue ribavirin (1-b-D-ribofuranosyl-1-H-1,2,4-triazole-3-carboxamide) is mediated by amino acid substitutions in the viral polymerase that either increase the general template copying fidelity of the enzyme or decrease the incorporation of ribavirin into RNA. Here we describe experiments that show that replication of the important picornavirus pathogen foot- and-mouth disease virus (FMDV) in the presence of increasing concentrations of ribavirin results in the sequential incorporation of three amino acid substitutions (M296I, P44S and P169S) in the viral polymerase (3D). The main biological effect of these substitutions is to attenuate the consequences of the mutagenic activity of ribavirin -by avoiding the biased repertoire of transition mutations produced by this purine analogue-and to maintain the replicative fitness of the virus which is able to escape extinction by ribavirin. This is achieved through alteration of the pairing behavior of ribavirin- triphosphate (RTP), as evidenced by in vitro polymerization assays with purified mutant 3Ds. Comparison of the three- dimensional structure of wild type and mutant polymerases suggests that the amino acid substitutions alter the position of the template RNA in the entry channel of the enzyme, thereby affecting nucleotide recognition. The results provide evidence of a new mechanism of resistance to a mutagenic nucleoside analogue which allows the virus to maintain a balance among mutation types introduced into progeny genomes during replication under strong mutagenic pressure. © 2010 Agudo et al.

RNA-dependent RNA polymerases (RdRPs) play central roles in both transcription and viral genome replication. In picornaviruses, these functions are catalyzed by the virally encoded RdRP, termed 3D. Polymerase 3D also catalyzes the covalent linkage of UMP to a tyrosine on the small protein VPg. Uridylylated VPg then serves as a protein primer for the initiation of RNA synthesis. Seven different crystal structures of foot-and-mouth disease virus (FMDV) 3D catalytic complexes have enhanced our understanding of template and primer recognition, VPg uridylylation, and rNTP binding and catalysis. Such structural information is providing new insights into the fidelity of RNA replication, and for the design of antiviral compounds. © 2009 Elsevier Ltd. All rights reserved., Primary research from Barcelona laboratory described in this review was supported by grant BFU2005-02376/BMC BIO2008-02556. Work in Madrid was supported by grants BFU2008-028/6BMC and Fundación R. Areces. Work in Barcelona and Madrid was further supported by Proyecto Intramural de Frontera 200820FO191(CSIC)

24 pages, 2 figures, 3 tables.-- 17182055 [PubMed].-- PMCID: PMC1855202.-- NIHMSID: NIHMS17626.-- Available online No. 14, 2006, The PhoR/PhoB two-component system is a key regulatory protein network enabling Escherichia coli to respond to inorganic phosphate (Pi) starvation conditions by turning on Pho regulon genes for more efficient Pi uptake and use of alternative phosphorus sources. Under environmental Pi depletion, the response regulator (RR) component, PhoB, is phosphorylated at the receiver domain (RD), a process that requires Mg2+ bound at the active site. Phosphorylation of the RD relieves the inhibition of the PhoB effector domain (ED), a DNA-binding region that binds to Pho regulon promoters to activate transcription. The molecular details of the activation are proposed to involve dimerisation of the RD and a conformational change in the RD detected by the ED. The structure of the PhoB RD shows a symmetrical interaction involving α1, loop β5α5 and N-terminus of α5 elements, also seen in the complex of PhoB RD with Mg2+, in which helix α4 highly increases its flexibility. PhoB RD in complex with Mg2+ and BeF3− (an emulator of the phosphate moiety) undergoes a dramatic conformational change on helix α4 and shows another interaction involving α4, β5 and α5 segments. We have selected a series of constitutively active PhoB mutants (PhoBCA) that are able to turn on the Pho regulon promoters in the absence phosphorylation and, as they cannot be inactivated, should therefore mimic the active RD state of PhoB and its functional oligomerisation. We have analysed the PhoBCA RD crystal structures of two such mutants, Asp53Ala/Tyr102Cys and Asp10Ala/Asp53Glu. Interestingly, both mutants reproduce the homodimeric arrangement through the symmetric interface encountered in the unbound and magnesium-bound wild-type PhoB RD structures. Besides, the mutant RD structures show a modified active site organization as well as changes at helix α4 that correlate with repositioning of surrounding residues, like the active-site events indicator Trp54, putatively redifining the interaction with the ED in the full-length protein., This work was supported by the Spanish Ministerio de Educación y Ciencia (grants BFU2005-512028, GEN2003-20642 and BIO2003-132) and the Generalitat de Catalunya (Centre de Referència en Biotecnologia and grant 2005SGR-00280).

Joana Fort et al., 4F2hc (CD98hc) is a multifunctional type II membrane glycoprotein involved in amino acid transport and cell fusion, adhesion, and transformation. The structure of the ectodomain of human 4F2hc has been solved using monoclinic (Protein Data Bank code 2DH2) and orthorhombic (Protein Data Bank code 2DH3) crystal forms at 2.1 and 2.8 Å, respectively. It is composed of a (βα)8 barrel and an antiparallel β8 sandwich related to bacterial α-glycosidases, although lacking key catalytic residues and consequently catalytic activity. 2DH3 is a dimer with Zn2+ coordination at the interface. Human 4F2hc expressed in several cell types resulted in cell surface and Cys109 disulfide bridge-linked homodimers with major architectural features of the crystal dimer, as demonstrated by cross-linking experiments. 4F2hc has no significant hydrophobic patches at the surface. Monomer and homodimer have a polarized charged surface. The N terminus of the solved structure, including the position of Cys109 residue located four residues apart from the transmembrane domain, is adjacent to the positive face of the ectodomain. This location of theNterminus and the Cys109-intervening disulfide bridge imposes space restrictions sufficient to support a model for electrostatic interaction of the 4F2hc ectodomain with membrane phospholipids. These results provide the first crystal structure of heteromeric amino acid transporters and suggest a dynamic interaction of the 4F2hc ectodomain with the plasma membrane. © 2007 by The American Society for Biochemistry and Molecular Biology, Inc., This work was supported in part by the Spanish Ministry of Science and Education Grant SAF2003-08940, BFU2006-14600, and BIO2005-06753, by the EC Project Grant 502802 EUGINDAT, and by La Marató-TV3

A foot-and-mouth disease virus (FMDV) polymerase (3D) with amino acid replacements G118D, V239M and G373D (triple DMD mutant) was obtained from a molecular clone derived from a virus population treated with ribavirin, in the transition to error catastrophe (virus extinction through lethal mutagenesis). DMD 3D was expressed in Escherichia coli, purified, and its activity compared with that of wild-type enzyme and mutant enzymes with either replacement G118D, G118A or D338A (the latter affecting the catalytic motif YGDD), generated by site-directed mutagenesis. No differences among the enzymes were noted in their interaction with monoclonal antibodies specific for the FMDV polymerase. Mutant enzymes with G118D or G118A showed a 100-fold decrease in polymerization activity relative to wild-type 3D, using poly(A)/oligo(dT)15 and poly(A)/VPg as template-primers, under several reaction conditions. As expected, the activity of 3D with D338A was undetectable (, This work was supported by Grants BMC2001-1823-C02-01 of Comisión Interministerial de Ciencia y Tecnología, 08.2/0015/2001 of Comunidad Autónoma de Madrid and Fundación R. Areces (to E.D.) and Grant BIO2002-00517 of Comisión Interministerial de Ciencia y Tecnología (to N.V.). A.A. is supported by a predoctoral I3P fellowship from Ministerio de Educación y Ciencia. R.A. is supported by a predoctoral fellowship from the Comunidad Autónoma de Madrid. C.F.-O. is supported by a predoctoral fellowship from the Ministerio de Ciencia y Tecnología.

Genome replication in picornaviruses is catalyzed by a virally encoded RNA-dependent RNA polymerase, termed 3D. These viruses also use a small protein primer, named VPg, to initiate RNA replication. The recent explosion of structural information on picornaviral 3D polymerases has provided insights into the initiation of RNA synthesis and chain elongation. Comparing these data with results from previous structural analyses of viral RNA-dependent RNA polymerases that catalyze de novo RNA synthesis sheds light on the different strategies that these viruses use to initiate replication. © 2006 Elsevier Ltd. All rights reserved., Primary research from the Barcelona laboratory described in this review was supported by grant BIO2002-00517. Work in Madrid was supported by grants BMC2001-1823-C02-01, BFU2005-00863/BMC and Fundación R Areces

Genome replication in picornaviruses is catalyzed by a virally encoded RNA-dependent RNA polymerase, termed 3D. The enzyme performs this operation, together with other viral and probably host proteins, in the cytoplasm of their host cells. The crystal structure of the 3D polymerase of foot-and-mouth disease virus, one of the most important animal pathogens, has been determined unliganded and bound to a template-primer RNA decanucleotide. The enzyme folds in the characteristic fingers, palm and thumb subdomains, with the presence of an NH2-terminal segment that encircles the active site. In the complex, several conserved amino acid side chains bind to the template-primer, likely mediating the initiation of RNA synthesis. The structure provides essential information for studies on RNA replication and the design of antiviral compounds., This work was supported in part by Grants BMC2001-1823-C02-01 of Comisión Interministerial de Ciencia y Tecnología, Comunidad Autónoma de Madrid 08.2/0015/2001 and Fundación R. Areces (to E. D.) and Grant BIO2002-00517 of the Comisión Interministerial de Ciencia y Tecnología (to N. V.)

Besides their classical role in alimentary protein degradation, zinc-dependant carboxypeptidases also participate in more selective regulatory processes like prohormone and neuropeptide processing or fibrinolysis inhibition in blood plasma. Human pancreatic procarboxypeptidase B (PCPB) is the prototype for those human exopeptidases that cleave off basic C-terminal residues and are secreted as inactive zymogens. One such protein is thrombin-activatable fibrinolysis inhibitor (TAFI), also known as plasma PCPB, which circulates in human plasma as a zymogen bound to plasminogen. The structure of human pancreatic PCPB displays a 95-residue pro-segment consisting of a globular region with an open-sandwich antiparallel-alpha antiparallel-beta topology and a C-terminal alpha-helix, which connects to the enzyme moiety. The latter is a 309-amino acid residue catalytic domain with alpha/beta hydrolase topology and a preformed active site, which is shielded by the globular domain of the pro-segment. The fold of the proenzyme is similar to previously reported procarboxypeptidase structures, also in that the most variable region is the connecting segment that links both globular moieties. However, the empty active site of human procarboxypeptidase B has two alternate conformations in one of the zinc-binding residues, which account for subtle differences in some of the key residues for substrate binding. The reported crystal structure, refined with data to 1.6A resolution, permits in the absence of an experimental structure, accurate homology modelling of TAFI, which may help to explain its properties.

Infectious bursal disease virus (IBDV), a member of the Birnaviridae family, is the causative agent of one of the most harmful poultry diseases. The IBDV genome encodes five mature proteins; of these, the multifunctional protein VP3 plays an essential role in virus morphogenesis. This protein, which interacts with the structural protein VP2, with the double-stranded RNA genome, and with the virus-encoded, RNA-dependent RNA polymerase, VP1, is involved not only in the formation of the viral capsid, but also in the recruitment of VP1 into the capsid and in the encapsidation of the viral genome. Here, we report the X-ray structure of the central region of VP3, residues 92-220, consisting of two α-helical domains connected by a long and flexible hinge that are organized as a dimer. Unexpectedly, the overall fold of the second VP3 domain shows significant structural similarities with different transcription regulation factors. © 2008 Elsevier Ltd. All rights reserved., This work was supported by grants AGL2003-07189/BIO2006-09407 and BFU2005-02376/BMC from The Ministerio de Educacion y Ciencia to J.F.R. and to N.V., respectively. A.C. was supported by an I3P fellowship from Consejo Superior de Investigaciones Cientificas (CSIC)-Ministerio de Educación y Ciencia, and C.F.-O. was supported by a Proyecto Intramural de Frontera fellowship from CSIC