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1.  Crystal Structures of Influenza A Virus Matrix Protein M1: Variations on a Theme 
PLoS ONE  2014;9(10):e109510.
Matrix protein 1 (M1) of the influenza A virus plays multiple roles in virion assembly and infection. Interest in the pH dependence of M1's multiple functions led us to study the effect of subtle pH changes on M1 structure, resulting in the elucidation of a unique low-pH crystal structure of the N1-165-domain of A/WSN/33 (H1N1) M1 that has never been reported. Although the 2.2 Å crystal structure of M1 N-terminus shows a dimer with the two monomers interacting in a face-to-face fashion at low pH as observed earlier, a 44° rotation of the second monomer has led to a significantly different dimer interface that possibly affects dimer stability. More importantly, while one of the monomers is fully defined, the N-terminal half of the second monomer shows considerable disorder that appears inherent in the protein and is potentially physiologically relevant. Such disorder has not been observed in any other previously reported structure at either low or high pH conditions, despite similar crystallization pH conditions. By comparing our novel N1-165-domain structure with other low-pH or neutral-pH M1 structures, it appears that M1 can energetically access different monomer and dimer conformations, as well as oligomeric states, with varying degree of similarities. The study reported here provides further insights into M1 oligomerization that may be essential for viral propagation and infectivity.
PMCID: PMC4190115  PMID: 25295515
2.  Crystallographic analysis of human hemoglobin elucidates the structural basis of the potent and dual antisickling activity of pyridyl derivatives of vanillin. Corrigendum 
A correction to the paper by Abdulmalik et al. [(2011), Acta Cryst. D67, 920–928].
The affiliation of one of the authors of Abdulmalik et al. (2011) [Acta Cryst. D67, 920–928] is corrected.
PMCID: PMC3337008
hemoglobin; oxygen affinity; sickle-cell disease; polymerization; T state; R2 state; corrigendum
3.  Crystallographic analysis of human hemoglobin elucidates the structural basis of the potent and dual antisickling activity of pyridyl derivatives of vanillin 
Pyridyl derivatives of vanillin increase the fraction of the more soluble oxygenated sickle hemoglobin and/or directly increase the solubility of deoxygenated sickle hemoglobin. Crystallographic analysis reveals the structural basis of the potent and dual antisickling activity of these derivatives.
Vanillin has previously been studied clinically as an antisickling agent to treat sickle-cell disease. In vitro investigations with pyridyl derivatives of vanillin, including INN-312 and INN-298, showed as much as a 90-fold increase in antisickling activity compared with vanillin. The compounds preferentially bind to and modify sickle hemoglobin (Hb S) to increase the affinity of Hb for oxygen. INN-312 also led to a considerable increase in the solubility of deoxygenated Hb S under completely deoxygenated conditions. Crystallographic studies of normal human Hb with INN-312 and INN-298 showed that the compounds form Schiff-base adducts with the N-terminus of the α-subunits to constrain the liganded (or relaxed-state) Hb conformation relative to the unliganded (or tense-state) Hb conformation. Interestingly, while INN-298 binds and directs its meta-positioned pyridine-methoxy moiety (relative to the aldehyde moiety) further down the central water cavity of the protein, that of INN-312, which is ortho to the aldehyde, extends towards the surface of the protein. These studies suggest that these compounds may act to prevent sickling of SS cells by increasing the fraction of the soluble high-affinity Hb S and/or by stereospecific inhibition of deoxygenated Hb S polymerization.
PMCID: PMC3211971  PMID: 22101818
hemoglobin; oxygen affinity; sickle-cell disease; polymerization; T state; R2 state
4.  Crystallographic Trapping of Heme Loss Intermediates During the Nitrite-Induced Degradation of Human Hemoglobin 
Biochemistry  2011;50(39):8323-8332.
Heme is an important cofactor in a large number of essential proteins and is often involved in small molecule binding and activation. Heme loss from proteins thus negatively affects the function of these proteins, but is also an important component of iron recycling. The characterization of intermediates that form during the loss of heme from proteins has been problematic due, in a large part, to the instability of such intermediates. We have characterized, by X-ray crystallography, three compounds that form during the nitrite-induced degradation of human α2β2 hemoglobin (Hb). The first is an unprecedented complex that exhibits a large β heme displacement of 4.8 Å towards the protein exterior; the heme displacement is stabilized by the binding of the distal His residue to the heme Fe, which in turn allows for the unusual binding of an exogenous ligand at the proximal face of the heme. We have also structurally characterized complexes that display regiospecific nitration of the heme at the 2-vinyl position; we show that heme nitration is not a prerequisite for heme loss. Our results provide structural insight into a possible pathway for nitrite-induced heme loss from human Hb.
PMCID: PMC3209482  PMID: 21863786
iron; heme; binding; heme loss; nitrogen oxides; blood; hemoglobin
5.  Crystal Structures of Human Pyridoxal Kinase in Complex with the Neurotoxins, Ginkgotoxin and Theophylline: Insights into Pyridoxal Kinase Inhibition 
PLoS ONE  2012;7(7):e40954.
Several drugs and natural compounds are known to be highly neurotoxic, triggering epileptic convulsions or seizures, and causing headaches, agitations, as well as other neuronal symptoms. The neurotoxic effects of some of these compounds, including theophylline and ginkgotoxin, have been traced to their inhibitory activity against human pyridoxal kinase (hPL kinase), resulting in deficiency of the active cofactor form of vitamin B6, pyridoxal 5′-phosphate (PLP). Pyridoxal (PL), an inactive form of vitamin B6 is converted to PLP by PL kinase. PLP is the B6 vitamer required as a cofactor for over 160 enzymatic activities essential in primary and secondary metabolism. We have performed structural and kinetic studies on hPL kinase with several potential inhibitors, including ginkgotoxin and theophylline. The structural studies show ginkgotoxin and theophylline bound at the substrate site, and are involved in similar protein interactions as the natural substrate, PL. Interestingly, the phosphorylated product of ginkgotoxin is also observed bound at the active site. This work provides insights into the molecular basis of hPL kinase inhibition and may provide a working hypothesis to quickly screen or identify neurotoxic drugs as potential hPL kinase inhibitors. Such adverse effects may be prevented by administration of an appropriate form of vitamin B6, or provide clues of how to modify these drugs to help reduce their hPL kinase inhibitory effects.
PMCID: PMC3412620  PMID: 22879864
6.  Structural and functional divergence within the Dim1/KsgA family of rRNA methyltransferases 
Journal of molecular biology  2009;391(5):884-893.
The enzymes of the KsgA/Dim1 family are universally distributed throughout all phylogeny; however, structural and functional differences are known to exist. The well-characterized function of these enzymes is to dimethylate two adjacent adenosines of the small ribosomal subunit in the normal course of ribosome maturation and the structures of KsgA from Escherichia coli and Dim1 from Homo sapiens and Plasmodium falciparum have been determined. To this point no examples of archaeal structures have been reported. Here we report the structure of Dim1 from the thermophilic archaeon Methanocaldococcus jannaschii. While it shares obvious similarities with the bacterial and eukaryotic orthologs, notable structural differences exist among the three members, particularly in the C-terminal domain. Previous work showed that eukaryotic and archaeal Dim1 were able to robustly complement for KsgA in E. coli. Here we repeated similar experiments to test for complementarity of archaeal Dim1 and bacterial KsgA in Saccharomyces cerevisiae. However, neither the bacterial nor the archaeal ortholog could complement for the eukaryotic Dim1. This might be related to the secondary, non-methyltransferase function that Dim1 is known to play in eukaryotic ribosomal maturation. To further delineate regions of the eukaryotic Dim1 critical to its function, we created and tested KsgA/Dim1 chimeras. Of the chimeras, only one constructed with the N-terminal domain from eukaryotic Dim1 and the C-terminal domain from archaeal Dim1 was able to complement, suggesting that eukaryotic-specific Dim1 function resides in the N-terminal domain also, where few structural differences are observed between members of the KsgA/Dim1 family. Future work is required to identify those determinants directly responsible for Dim1 function in ribosome biogenesis. Finally, we have conclusively established that none of the methyl groups are critically important to growth in yeast under standard conditions at a variety of temperatures.
PMCID: PMC2753216  PMID: 19520088
Dim1; KsgA; Archaea; dimethyltransferase; rRNA methylation
7.  Structure of the MecI repressor from Staphylococcus aureus in complex with the cognate DNA operator of mec  
The up-and-down binding of dimeric MecI to mecA dyad DNA may account for the cooperative effect of the repressor.
The dimeric repressor MecI regulates the mecA gene that encodes the penicillin-binding protein PBP-2a in methicillin-resistant Staphylococcus aureus (MRSA). MecI is similar to BlaI, the repressor for the blaZ gene of β-lactamase. MecI and BlaI can bind to both operator DNA sequences. The crystal structure of MecI in complex with the 32 base-pair cognate DNA of mec was determined to 3.8 Å resolution. MecI is a homodimer and each monomer consists of a compact N-­terminal winged-helix domain, which binds to DNA, and a loosely packed C-­terminal helical domain, which intertwines with its counter-monomer. The crystal contains horizontal layers of virtual DNA double helices extending in three directions, which are separated by perpendicular DNA segments. Each DNA segment is bound to two MecI dimers. Similar to the BlaI–mec complex, but unlike the MecI–bla complex, the MecI repressors bind to both sides of the mec DNA dyad that contains four conserved sequences of TACA/TGTA. The results confirm the up-and-down binding to the mec operator, which may account for cooperative effect of the repressor.
PMCID: PMC2222568  PMID: 16582476
MecI repressor
8.  Crystal Structure of Pyridoxal Kinase from the Escherichia coli pdxK Gene: Implications for the Classification of Pyridoxal Kinases 
Journal of Bacteriology  2006;188(12):4542-4552.
The pdxK and pdxY genes have been found to code for pyridoxal kinases, enzymes involved in the pyridoxal phosphate salvage pathway. Two pyridoxal kinase structures have recently been published, including Escherichia coli pyridoxal kinase 2 (ePL kinase 2) and sheep pyridoxal kinase, products of the pdxY and pdxK genes, respectively. We now report the crystal structure of E. coli pyridoxal kinase 1 (ePL kinase 1), encoded by a pdxK gene, and an isoform of ePL kinase 2. The structures were determined in the unliganded and binary complexes with either MgATP or pyridoxal to 2.1-, 2.6-, and 3.2-Å resolutions, respectively. The active site of ePL kinase 1 does not show significant conformational change upon binding of either pyridoxal or MgATP. Like sheep PL kinase, ePL kinase 1 exhibits a sequential random mechanism. Unlike sheep pyridoxal kinase, ePL kinase 1 may not tolerate wide variation in the size and chemical nature of the 4′ substituent on the substrate. This is the result of differences in a key residue at position 59 on a loop (loop II) that partially forms the active site. Residue 59, which is His in ePL kinase 1, interacts with the formyl group at C-4′ of pyridoxal and may also determine if residues from another loop (loop I) can fill the active site in the absence of the substrate. Both loop I and loop II are suggested to play significant roles in the functions of PL kinases.
PMCID: PMC1482971  PMID: 16740960
9.  Crystal Structures of the BlaI Repressor from Staphylococcus aureus and Its Complex with DNA: Insights into Transcriptional Regulation of the bla and mec Operons 
Journal of Bacteriology  2005;187(5):1833-1844.
The 14-kDa BlaI protein represses the transcription of blaZ, the gene encoding β-lactamase. It is homologous to MecI, which regulates the expression of mecA, the gene encoding the penicillin binding protein PBP2a. These genes mediate resistance to β-lactam antibiotics in staphylococci. Both repressors can bind either bla or mec DNA promoter-operator sequences. Regulated resistance genes are activated via receptor-mediated cleavage of the repressors. Cleavage is induced when β-lactam antibiotics bind the extramembrane sensor of the sensor-transducer signaling molecules, BlaR1 or MecR1. The crystal structures of BlaI from Staphylococcus aureus, both in free form and in complex with 32 bp of DNA of the mec operator, have been determined to 2.0- and 2.7-Å resolutions, respectively. The structure of MecI, also in free form and in complex with the bla operator, has been previously reported. Both repressors form homodimers, with each monomer composed of an N-terminal DNA binding domain of winged helix-turn-helix topology and a C-terminal dimerization domain. The structure of BlaI in complex with the mec operator shows a protein-DNA interface that is conserved between both mec and bla targets. The recognition helix α3 interacts specifically with the conserved TACA/TGTA DNA binding motif. BlaI and, probably, MecI dimers bind to opposite faces of the mec DNA double helix in an up-and-down arrangement, whereas MecI and, probably, BlaI dimers bind to the same DNA face of bla promoter-operator DNA. This is due to the different spacing of mec and bla DNA binding sites. Furthermore, the flexibility of the dimeric proteins may make the C-terminal proteolytic cleavage site more accessible when the repressors are bound to DNA than when they are in solution, suggesting that the induction cascade involves bound rather than free repressor.
PMCID: PMC1064009  PMID: 15716455
10.  Crystal Structure of the PdxY Protein from Escherichia coli 
Journal of Bacteriology  2004;186(23):8074-8082.
The crystal structure of Escherichia coli PdxY, the protein product of the pdxY gene, has been determined to a 2.2-Å resolution. PdxY is a member of the ribokinase superfamily of enzymes and has sequence homology with pyridoxal kinases that phosphorylate pyridoxal at the C-5′ hydroxyl. The protein is a homodimer with an active site on each monomer composed of residues that come exclusively from each respective subunit. The active site is filled with a density that fits that of pyridoxal. In monomer A, the ligand appears to be covalently attached to Cys122 as a thiohemiacetal, while in monomer B it is not covalently attached but appears to be partially present as pyridoxal 5′-phosphate. The presence of pyridoxal phosphate and pyridoxal as ligands was confirmed by the activation of aposerine hydroxymethyltransferase after release of the ligand by the denaturation of PdxY. The ligand, which appears to be covalently attached to Cys122, does not dissociate after denaturation of the protein. A detailed comparison (of functional properties, sequence homology, active site and ATP-binding-site residues, and active site flap types) of PdxY with other pyridoxal kinases as well as the ribokinase superfamily in general suggested that PdxY is a member of a new subclass of the ribokinase superfamily. The structure of PdxY also permitted an interpretation of work that was previously published about this enzyme.
PMCID: PMC529075  PMID: 15547280

Results 1-10 (10)