Structural basis of pyrimidine specificity in the MS2 RNA hairpin-coat-protein complex.

We have determined the X-ray structures of six MS2 RNA hairpin-coat-protein complexes having five different substitutions at the hairpin loop base -5. This is a uracil in the wild-type hairpin and contacts the coat protein both by stacking on to a tyrosine side chain and by hydrogen bonding to an asparagine side chain. The RNA consensus sequence derived from coat protein binding studies with natural sequence variants suggested that the -5 base needs to be a pyrimidine for strong binding. The five -5 substituents used in this study were 5-bromouracil, pyrimidin-2-one, 2-thiouracil, adenine, and guanine. The structure of the 5-bromouracil complex was determined to 2.2 A resolution, which is the highest to date for any MS2 RNA-protein complex. All the complexes presented here show very similar conformations, despite variation in affinity in solution. The results suggest that the stacking of the -5 base on to the tyrosine side chain is the most important driving force for complex formation. A number of hydrogen bonds that are present in the wild-type complex are not crucial for binding, as they are missing in one or more of the complexes. The results also reveal the flexibility of this RNA-protein interface, with respect to functional group variation, and may be generally applicable to other RNA-protein complexes.

Deletion of a single hydrogen bonding atom from the MS2 RNA operator leads to dramatic rearrangements at the RNA-coat protein interface.

The MS2 coat protein binds specifically to an RNA hairpin formed within the viral genome. By soaking different RNA fragments into crystals of MS2 coat protein capsids it is possible to determine the X-ray structure of the RNA-protein complexes formed. Here we present the structure to 2.85 A resolution of a complex between a chemically modified RNA hairpin variant and the MS2 coat protein. This RNA variant has a substitution at the -5 base position, which has been shown previously to be pyrimidine-specific and is a uracil in the wild-type RNA. The modified RNA hairpin contains a pyridin-4-one base (4one) at this position that lacks the exocyclic 2-oxygen eliminating the possibility of forming a hydrogen bond to asparagine A87 in the protein. The 4one complex structure shows an unprecedented major conformational change in the loop region of the RNA, whereas there is almost no change in the conformation of the protein.

The three-dimensional structure of bacteriophage PP7 from Pseudomonas aeruginosa at 3.7-A resolution.

The three-dimensional structure of phage PP7 from Pseudomonas aeruginosa has been determined to 3.7-A resolution. A comparison with distantly related small RNA phages showed that the biggest differences were found in the FG loops, forming the contacts around the fivefold and threefold axes. In contrast to the situation in other phages, the FG loops of phage PP7 are very similar in all three subunits. This supports the hypothesis that no switches are needed for the assembly control in these viruses. Some of the most conserved residues lie within the region involved in RNA binding in the related phages MS2 and seem to have the same function.

Structure determination of bacteriophage PP7 from Pseudomonas aeruginosa: from poor data to a good map.

The structure of bacteriophage PP7 from Pseudomomas aeruginosa was determined to 3.7 A resolution. Triclinic crystals of three forms were obtained, diffracting to between 4.5 and 3.4 A resolution. The quality of the crystals was exceptionally poor, leading to problems in the evaluation of the recorded images and to a final data set which would appear to be useless with standard criteria for protein crystals. In all crystal forms, the unit cell contains two icosahedral particles, providing 120-fold non-crystallographic symmetry. For two of the crystal forms, the particle orientations were calculated using the self-rotation function. The two particles in the asymmetric unit had very similar but distinct orientations. The position of the second particle was found using the Patterson function. Initial phases to 15 A resolution were calculated using the related phage MS2 as a model. Real-space averaging was performed and phases were extended from 15 A resolution to the limit of the data. The map was improved significantly by using only the 'high' resolution data in the resolution range 7-3.7 A, allowing the positions of most side chains to be determined. The better quality of the 7-3.7 A resolution map is presumably a consequence of the presence of satellite crystals. The position of the second particle was improved using the correlation coefficient in the averaging process to monitor the refinement by moving the particle around in small steps.

Urea-PETT compounds as a new class of HIV-1 reverse transcriptase inhibitors. 3. Synthesis and further structure-activity relationship studies of PETT analogues.

The further development of allosteric HIV-1 RT inhibitors in the urea analogue series of PETT (phenylethylthiazolylthiourea) derivatives is described here. The series includes derivatives with an ethyl linker (1-5) and racemic (6-16) and enantiomeric (17-20) cis-cyclopropane compounds. The antiviral activity was determined both at the RT level and in cell culture on both wild-type and mutant forms of HIV-1. Most compounds have anti-HIV-1 activity on the wt in the nanomolar range. Resistant HIV-1 was selected in vitro for some of the compounds, and the time for resistant HIV-1 to develop was longer for urea-PETT compounds than it was for reference compounds. Preliminary pharmacokinetics in rats showed that compound 18 is orally bioavailable and penetrates well into the brain. The three-dimensional structure of complexes between HIV-1 RT and two enantiomeric compounds (17 and 18) have been determined. The structures show similar binding in the NNI binding pocket. The propionylphenyl moieties of both inhibitors show perfect stacking to tyrosine residues 181 and 188. The cyclopropyl moiety of the (+)-enantiomer 18 exhibits optimal packing distances for the interactions with leucine residue 100 and valine residue 179.

Crystallographic studies of RNA hairpins in complexes with recombinant MS2 capsids: implications for binding requirements.

The coat protein of bacteriophage MS2 is known to bind specifically to an RNA hairpin formed within the MS2 genome. Structurally this hairpin is built up by an RNA double helix interrupted by one unpaired nucleotide and closed by a four-nucleotide loop. We have performed crystallographic studies of complexes between MS2 coat protein capsids and four RNA hairpin variants in order to evaluate the minimal requirements for tight binding to the coat protein and to obtain more information about the three-dimensional structure of these hairpins. An RNA fragment including the four loop nucleotides and a two-base-pair stem but without the unpaired nucleotide is sufficient for binding to the coat protein shell under the conditions used in this study. In contrast, an RNA fragment containing a stem with the unpaired nucleotide but missing the loop nucleotides does not bind to the protein shell.

Structure of phage fr capsids with a deletion in the FG loop: implications for viral assembly.

The loop between beta-strands F and G in the coat protein of small RNA bacteriophages forms the interactions at the fivefold and threefold (quasi-sixfold) icosahedral axes. In many cases, mutations in this region renders the coat protein unable to form capsids. This FG loop has therefore been suggested to be of major importance for the virus assembly process by guiding the assembly and helping to define the correct curvature of the virus shell. We have determined the crystal structure of a phage fr capsid where the coat protein has a four-residue deletion in the FG loop. This mutant retains the ability to form virus capsids of normal size but has a significantly lower temperature stability than the wild type. The structure reveals that the mutated loops are flexible and too short to interact with each other. This seems incompatible with a role of the FG loop in the regulation of capsid size.

Crystal structures of MS2 coat protein mutants in complex with wild-type RNA operator fragments.

In MS2 assembly of phage particles results from an interaction between a coat protein dimer and a stem-loop of the RNA genome (the operator hairpin). Amino acid residues Thr45, which is universally conserved among the small RNA phages, and Thr59 are part of the specific RNA binding pocket and interact directly with the RNA; the former through a hydrogen bond, the latter through hydrophobic contacts. The crystal structures of MS2 protein capsids formed by mutants Thr45Ala and Thr59Ser, both with and without the 19 nt wild-type operator hairpin bound, are reported here. The RNA hairpin binds to these mutants in a similar way to its binding to wild-type protein. In a companion paper both mutants are shown to be deficient in RNA binding in an in vivo assay, but in vitro the equilibrium dissociation constant is significantly higher than wild-type for the Thr45Ala mutant. The change in binding affinity of the Thr45Ala mutant is probably a direct consequence of removal of direct hydrogen bonds between the protein and the RNA. The properties of the Thr59Ser mutant are more difficult to explain, but are consistent with a loss of non-polar contact.

The crystal structure of bacteriophage GA and a comparison of bacteriophages belonging to the major groups of Escherichia coli leviviruses.

The three-dimensional structure of the small T=3 RNA bacteriophage GA has been determined at 3.4 A resolution. The structure was solved by molecular replacement, using the phage MS2 as an initial model. A comparison of the protein shells of the four related phages GA, MS2, fr and Qbeta was carried out in order to define structural features of particular importance for their assembly and specific RNA interaction. A high degree of similarity was found in the RNA binding sites, whereas larger structural differences are located in the loop regions of the coat proteins, especially in the FG loops forming 5-fold and quasi-6-fold contacts. The overall arrangement of the protein subunits in the shells of these phages is very similar, although the details of the interactions differ. The few conserved interactions are suggested to govern the subunit packing during assembly.

The crystal structure of bacteriophage Q beta at 3.5 A resolution.

BACKGROUND: The capsid protein subunits of small RNA bacteriophages form a T = 3 particle upon assembly and RNA encapsidation. Dimers of the capsid protein repress translation of the replicase gene product by binding to the ribosome binding site and this interaction is believed to initiate RNA encapsidation. We have determined the crystal structure of phage Q beta with the aim of clarifying which factors are the most important for particle assembly and RNA interaction in the small phages. RESULTS: The crystal structure of bacteriophage Q beta determined at 3.5 A resolution shows that the capsid is stabilized by disulfide bonds on each side of the flexible loops that are situated around the fivefold and quasi-sixfold axes. As in other small RNA phages, the protein capsid is constructed from subunits which associate into dimers. A contiguous ten-stranded antiparallel beta sheet facing the RNA is formed in the dimer. The disulfide bonds lock the constituent dimers of the capsid covalently in the T = 3 lattice. CONCLUSIONS: The unusual stability of the Q beta particle is due to the tight dimer interactions and the disulfide bonds linking each dimer covalently to the rest of the capsid. A comparison with the structure of the related phage MS2 shows that although the fold of the Q beta coat protein is very similar, the details of the protein-protein interactions are completely different. The most conserved region of the protein is at the surface, which, in MS2, is involved in RNA binding.

Crystal structure of bacteriophage fr capsids at 3.5 A resolution.

The structure of recombinant capsids of the bacterial virus fr has been determined by X-ray crystallography at 3.5 A resolution. The capsids were produced by expressing the fr coat protein in Escherichia coli, the natural host of the virus, and are probably essentially identical to the protein shell of the native virus. The structure was determined using molecular replacement with the protein shell of the related MS2 virus, and refined to a crystallographic R-factor of 0.228. A comparison of the protein shells of the viruses shows that they are very similar, and indicates that they may have a similar regulation of the assembly of the quasi-symmetrical protein shell.

Crystallization and preliminary X-ray diffraction studies of the bacteriophage Qbeta.

Crystals of bacteriophage Qbeta have been obtained by the vapor-diffusion technique. The crystals diffract to at least 3.5 A resolution. The crystal space group is C222(1) with the unit-cell parameters a = 478, b = 296, c = 477 A, alpha = beta = gamma = 90 degrees. The unit cell contains four virus particles. A pattern of systematic extinctions has been used to deduce the packing of the particles in the cell. A limited data set to 3.9 A resolution has been collected, and the predicted position has been confirmed by the self-rotation and the Patterson functions.

The refined structure of bacteriophage MS2 at 2.8 A resolution.

Bacteriophage MS2 is an icosahedral virus with 180 copies of a coat protein forming a shell around a single-stranded RNA molecule. The coat protein subunits form a lattice with the triangulation number T = 3. The coat protein has a fold which is different from the fold of all other viral coat proteins so far known. It consists of a five-stranded beta sheet facing the inside of the particle, and a hairpin and two helices on the outside. The crystal structure has been refined at 2.8 A resolution. The final R-factor was 0.189 for reflections with F > 2 sigma, and the root-mean-square deviation from idealized bond lengths and bond angles was 0.015 A and 2.9 degrees, respectively. The three chemically identical conformers A, B and C are largely similar. The B conformer has a unique conformation in one loop, which is involved in 5-fold interactions, while the A and C conformers, which are involved in the quasi-6-fold contacts, are similar throughout the structure. One cis-proline has been identified in the B conformer but the corresponding prolines in A and C are of the trans isomer. This residue is conserved within small RNA coliphages and it is proposed that this isomerization enables a less elongated loop (FG) around the 5-fold axis, thus creating a channel. The extensive dimer contact supports the idea of dimers as initial building blocks. An assembly pathway is proposed where five dimers converge into a pentamer and 12 pentamers are linked together with free dimers creating a complete particle.

Structure determination of the bacteriophage MS2.

The structure of the bacterial virus MS2 has been solved at 3.3 A resolution. Initial phases to 13 A resolution were obtained from a model based on the known coordinates of the plant virus southern bean mosaic virus. These phases were extended in small steps to a resolution of 3.4 A. The phases obtained represented essentially the Babinet opposite of the true structure and were not of a sufficiently good quality to allow an interpretation of the electron density contoured at negative levels. Difference Fourier maps of two heavy-atom derivatives based on these phases were interpretable, and these derivatives were used to calculate isomorphous replacement phases at 8.8 A resolution. Phase extension to 3.3 A resolution led to maps which could be easily interpreted.

The three-dimensional structure of the bacterial virus MS2.

The structure of the icosahedral bacteriophage MS2 has been determined to 3.3 A resolution by X-ray crystallography. The phase determination involved both molecular replacement at low resolution using a known structure and heavy-atom substitution. The coat protein has no structural similarity to that of any other known RNA virus.

Crystallization of P2 myelin protein.

Single crystals of bovine P2 myelin protein have been grown in polyethylene glycol 4000 by the hanging-drop vapor diffusion method. Crystals belonging to space group P2(1)2(1)2(1) with cell dimensions a = 91.8 A, b = 99.5 A, c = 56.5 A (1 A = 0.1 nm). The diffraction pattern extends to better than 2.3 A resolution.

Crystal structure of human erythrocyte carbonic anhydrase B. Three-dimensional structure at a nominal 2.2-A resolution.

The three-dimensional structure of carbonic anhydrase B (EC 4,2,1,1; carbonate hydro-lyase) from human erythrocytes has been determined to high resolution. Parallel and antiparallel pleated sheet makes up the predominant secondary structure of the enzyme. The tertiary structure is unique for its folding and is very similar to the structure is unique for its folding and is very similar to the structure of the isoenzyme, human erythrocyte carbonic anhydrase C. The essential metal ion, zinc, is firmly bound to the enzyme through three histidyl ligands and located at the bottom of a 12-A deep conical cavity. The zinc ligands are involved in a number of hydrogen bond formations with residues in the immediate vicinity of the active site cavity. Some of the similarities and differences in the sidechain orientation and active site topography of the two isoenzymes are also discussed.