Structural insights into mechanisms of non-nucleoside drug resistance for HIV-1 reverse transcriptases mutated at codons 101 or 138 Jingshan Ren1, Charles E Nichols1, Anna Stamp1, Phillip P Chamberlain1, Robert Ferris2, Kurt L Weaver2, Steven A Short2 and David K Stammers1 Division of Structural Biology, The Wellcome Trust Centre for Human Genetics, Henry Wellcome Building for Genomic Medicine, University of Oxford, UK Glaxo Smith Kline Inc., Research Triangle Park, NC, USA Keywords drug resistance; HIV-1 reverse transcriptase mutants; Lys101Glu, Glu138Lys; nonnucleoside inhibitors; X-ray crystallography Correspondence D.K Stammers, Division of Structural Biology, The Wellcome Trust Centre for Human Genetics, University of Oxford, Roosevelt Drive, Oxford OX3 7BN, UK Fax: +44 1865 287 547 Tel: +44 1865 287 565 E-mail: daves@strubi.ox.ac.uk (Received 23 March 2006, revised 20 June 2006, accepted 22 June 2006) doi:10.1111/j.1742-4658.2006.05392.x Lys101Glu is a drug resistance mutation in reverse transcriptase clinically observed in HIV-1 from infected patients treated with the non-nucleoside inhibitor (NNRTI) drugs nevirapine and efavirenz In contrast to many NNRTI resistance mutations, Lys101(p66 subunit) is positioned at the surface of the NNRTI pocket where it interacts across the reverse transcriptase (RT) subunit interface with Glu138(p51 subunit) However, nevirapine contacts Lys101 and Glu138 only indirectly, via water molecules, thus the structural basis of drug resistance induced by Lys101Glu is unclear We have determined crystal structures of RT(Glu138Lys) and RT(Lys101Glu) ˚ in complexes with nevirapine to 2.5 A, allowing the determination of water structure within the NNRTI-binding pocket, essential for an understanding of nevirapine binding Both RT(Glu138Lys) and RT(Lys101Glu) have remarkably similar protein conformations to wild-type RT, except for significant movement of the mutated side-chains away from the NNRTI pocket induced by charge inversion There are also small shifts in the position of nevirapine for both mutant structures which may influence ring stacking interactions with Tyr181 However, the reduction in hydrogen bonds in the drug-water-side-chain network resulting from the mutated side-chain movement appears to be the most significant contribution to nevirapine resistance for RT(Lys101Glu) The movement of Glu101 away from the NNRTI pocket can also explain the resistance of RT(Lys101Glu) to efavirenz but in this case is due to a loss of side-chain contacts with the drug RT(Lys101Glu) is thus a distinctive NNRTI resistance mutant in that it can give rise to both direct and indirect mechanisms of drug resistance, which are inhibitor-dependent The emergence of resistant viruses resulting from drug treatment of HIV-infected patients poses one of the most significant problems in countering AIDS in Western countries [1] Two virus specific proteins, reverse transcriptase (RT) and protease, have been the main targets for the development of anti-HIV drugs used in multidrug combination therapy regimens [2] HIV-1 RT contains two distinct inhibitor binding sites for Abbreviations NRTI, nucleoside analogue inhibitors of RT; NNRTI, non-nucleoside reverse transcriptase inhibitor; NtRTI, nucleotide analogue; PETT, phenethylthiazolylthiourea; RT, reverse transcriptase; TSAO, (2¢,5¢-bis-O-(tert-butyldimethylsilyl)-b-d-ribofuranosyl]-3¢-spiro-5¢¢-(4¢¢-amino1¢¢,2¢¢-oxathiole-2¢¢,2¢¢-dioxide) 3850 FEBS Journal 273 (2006) 3850–3860 ª 2006 The Authors Journal compilation ª 2006 FEBS J Ren et al HIV-1 RT drug resistance mutations at 101 and 138 N N N 10 O N S NH O Nevirapine N Cl N H F N H PETT-2 Scheme Structures of nevirapine and PETT-2 are shown different classes of drugs within the larger subunit of the p66 ⁄ p51 heterodimer [3,4] Nucleoside analogue inhibitors of RT (NRTIs), such as zidovudine, lamivudine, stavudine and the nucleotide analogue (NtRTI) tenofovir, bind, respectively, as activated tri- or diphosphate forms at the RT polymerase active site NRTIs and NtRTIs, as well as competing with cognate nucleotide substrates, are also incorporated into the primer strand causing termination of the growing DNA chain [5] Non-nucleoside inhibitors (NNRTIs) form a second class of compounds that target HIV-1 ˚ RT [6] NNRTIs bind at an allosteric site about 10 A from the polymerase active site The structural mechanism of NNRTI inhibition involves distortion of the catalytically essential triad of aspartic acid residues [7,8] The non-nucleoside site, although almost entirely contained within the p66 subunit, also has Glu138 from the p51 subunit located at the edge of the inhibitor pocket [3,4] First-generation NNRTI drugs such as nevirapine (Scheme 1) and delavirdine generally show significant reduction in potency in the presence of a wide range of single point mutations [9] Second generation compounds, including efavirenz, have greater resilience to many such mutations [10,11] Whilst the majority of the mutation sites encoding resistance to NRTIs are distal to the dNTP site within RT [12,13], those which result in NNRTI drug resistance are generally proximal to the inhibitor binding pocket [3,4] Many of the most commonly observed NNRTI resistance mutations are located at hydrophobic residues at the centre of the drug binding pocket (e.g Leu100, Val106, Val108, Tyr181 and Tyr188) However, charged residues positioned towards the surface of the drug pocket are also known sites of drug resistance mutations Indeed, Lys103Asn is the mutation most frequently observed in clinical use of NNRTIs Other NNRTI resistance mutations at surface residues in RT include Lys101 and Glu138 (the latter residue from the p51 subunit) Mutations at residues 101 and 138 that give resistance to NNRTIs usually involve a change in the sign of the side-chain charge, e.g Lys101Glu or Glu138Lys ⁄ Arg Pyridinone NNRTIs can select for the Lys101Glu mutation in HIV-1 RT, giving approximately eight-fold resistance to compounds in this series [14] The Lys101Glu mutation in RT also gives eight-fold resistance to nevirapine, based on the mean of four published values [15–18] Significantly, Lys101Glu in RT is observed in the clinic for drug regimens which include nevirapine [19,20] or efavirenz [21] as the NNRTI component The Lys101Glu mutation in RT is also selected by passaging HIV in tissue culture in the presence of carboxanilide NNRTIs such as UC-38 [22], giving greater than 100-fold resistance [23] In the crystal structure with wild-type HIV-1 RT, UC-38 has a van der Waals contact with the side-chain of Lys101 [24] Consistent with the reports of the selection of Lys101Glu in clinical use of efavirenz, the crystal structure of RT with this NNRTI shows a contact of the inhibitor with the CD atom of Lys101 [25] In the case of nevirapine, however, there are only indirect contacts of Lys101 in the complex with wild-type RT, via a series of three water molecules that link the drug, the side-chain of Glu138 and the main-chain carbonyl of Lys101 [4] The molecular mechanism of drug resistance for nevirapine induced by Lys101Glu mutant is thus unclear, although an indirect mechanism is present The RT(Glu138Lys) mutation in HIV is selected in tissue culture by inhibitors of the TSAO (2¢,5¢-bisO-(tert-butyldimethylsilyl)-b-d-ribofuranosyl]-3¢-spiro5¢¢-(4¢¢-amino-1¢¢,2¢¢-oxathiole-2¢¢,2¢¢-dioxide) series [26] TSAO compounds have been shown to be capable of disrupting the p66 ⁄ p51 heterodimer [27], indeed modelling studies suggest that certain TSAO NNRTIs may bind at a novel site between the subunits of the RT heterodimer [28] It is proposed that mutation of Glu138Lys causes loss of a key interaction of the glutamic acid carboxyl group to an amino group of TSAO inhibitors Mutations in RT at codon 138 (to Lys ⁄ Arg) have been shown to give cross resistance to a number of NNRTIs including members of the PETT series [29] as well as to emivirine [17] In contrast, RT(Glu138Lys) is reported as having minimal cross resistance to nevirapine [18,30] PETT-2 (Scheme 1) forms a van der Waals contact with the carboxyl group of Glu138 in the complex with wild-type RT [31] To date, crystal structure determinations of NNRTI resistant HIV-1 RTs have mainly focussed on mutations at positions 103, 181 and 188 [25,32– 36] More recently structures of RT with drug resistance mutations Leu100Ile, Val106Ala and Val108Ile have been reported, which indicate an indirect component in the mechanism of resistance via modulation FEBS Journal 273 (2006) 3850–3860 ª 2006 The Authors Journal compilation ª 2006 FEBS 3851 HIV-1 RT drug resistance mutations at 101 and 138 J Ren et al of interactions with the key aromatic side-chains of Tyr181 and Tyr188 [37] For RT(Lys103Asn) it has been shown that the second generation NNRTI efavirenz can bind to the enzyme in both a wild-type conformation as well as with Tyr181 in a rearranged position [25,36] The side-chain of Asn103 can form a hydrogen bond to the Tyr188 hydroxyl thereby stabilizing the unliganded RT structure [7,35] In work reported here, crystal structures of HIV-1 RTs either containing the mutation Lys101Glu or Glu138Lys in complexes with nevirapine and PETT-2 have been determined in order to investigate structural effects of such mutants on the drug binding pocket and how these might relate to resistance mechanisms RT(Glu138Lys) and RT(Lys101Glu) mutants are logical to study as a pair as they are both charged surface residues interacting across the p66 ⁄ p51 interface The availability of a range of structures of NNRTIresistant RTs drugs will provide greater understanding of the molecular basis of drug resistance mechanisms Such data will contribute to the design of novel inhibitors, which are still needed for containing the effects of HIV infection Results Overall RT structure Details of the X-ray data collection and the refinement statistics are shown in Table Of the wide range of NNRTIs tested for co-crystallization with RT(Glu138Lys) and RT(Lys101Glu), three complexes gave crystals suitable for structure ˚ determination: RT(Lys101Glu)-nevirapine (to 2.5-A ˚ resolution), RT(Glu138Lys)-nevirapine (2.5 A) and ˚ RT(Glu138Lys)-PETT2 (3.0 A) The relatively high resolution of the mutant RT-nevirapine structures allowed details of the water structure associated with the protein relevant to NNRTI binding to be determined Water molecules are important for the interactions of nevirapine with RT, particularly via Lys101 and Glu138, as there are no direct contacts of the inhibitor with these side-chains Fo-Fc omit maps show clear electron density for the bound NNRTI, some water molecules, as well as for the mutated sidechains (Fig 1) in each case Side-chain electron density relevant to NNRTI drug resistance is also shown in Table Statistics for crystallographic structure determinations Data collection details Data set Data collection site Detector ˚ Wavelength (A) ˚ Unit cell dimensions (a,b,c in A)a ˚ Resolution range (A) Observations Unique reflections Completeness (%) Average I ⁄ r(I) Rmergeb Outer resolution shell ˚ Resolution range (A) Unique reflections Completeness (%) Average I ⁄ r(I) Refinement statistics: ˚ Resolution range (A) Number of reflections(working ⁄ test) R-factorc (Rwork ⁄ Rfree) R-factorc (all data) Number of atoms (protein ⁄ inhibitor ⁄ water) ˚ Rms bond length deviation (A) Rms bond angle deviation (°) ˚ Mean B-factor (A2)d ˚ Rms backbone B-factor deviation (A2) K101E-nevirapine PF BL-6 A Fuji BAS III 1.000 138.9, 114.9, 65.6 30.0–2.5 149821 35057 94.4 9.8 0.100 E138K-nevirapine APS SBC-2 CCD SBC 0.82657 139.6, 115.0, 65.6 30.0–2.5 251696 37104 99.5 12.6 0.089 E138K-PETT-2 SRS PX14.2 CCD ADSC-Q4 0.979 138.8, 114.8, 65.9 30.0–3.0 57571 19095 88.0 11.8 0.083 2.59–2.50 3371 92.5 1.0 2.59–2.50 3627 99.5 1.1 3.11–3.00 1537 72.6 1.4 30.0–2.5 33264 ⁄ 1740 0.198 ⁄ 0.270 0.194 7570 ⁄ 20 ⁄ 80 0.008 1.4 59 ⁄ 65 ⁄ 37 ⁄ 44 4.4 30.0–2.5 35199 ⁄ 1856 0.216 ⁄ 0.288 0.203 7614 ⁄ 20 ⁄ 79 0.008 1.4 62 ⁄ 68 ⁄ 39 ⁄ 48 4.2 20.0–3.0 18153 ⁄ 929 0.226 ⁄ 0.277 0.226 7554 ⁄ 23 ⁄ – 0.009 1.5 73 ⁄ 81 ⁄ 60 ⁄ – 3.4 a All crystals belong to space group P212121 [40,41]; bRmerge ¼ S|I – < I > | ⁄ S < I > ; cR factor ¼ S|Fo- Fc| ⁄ SFo; dmean B factor for main-chain, side-chain, inhibitor and water molecules 3852 FEBS Journal 273 (2006) 3850–3860 ª 2006 The Authors Journal compilation ª 2006 FEBS J Ren et al HIV-1 RT drug resistance mutations at 101 and 138 mic acid side-chain is well defined (Fig 1A) The structure of the NNRTI pocket is overall remarkably similar for mutant and wild-type, with the exception of the mutated side-chain and adjacent interacting residue There is a significant movement of Glu101 relative to the position of the wild-type Lys101, such that the side-chain carboxyl group points away from the NNRTI site (distance of the carboxyl group to the equivalent wild-type lysine side-chain amino group of ˚ 3.4 A) (Fig 2A) There is also a smaller displacement ˚ of the Glu138 side-chain with a shift of 0.5 A relative to wild-type The result of these changes is the loss of the salt bridge that links the protonated amino group of Lys101 in the p66 subunit to the negatively charged carboxyl of the Glu138 side-chain from the p51 subunit in the wild-type RT nevirapine complex [4] Accompanying the side-chain movement related to the Lys101Glu mutation there is also a shift in the position of nevirapine, with an average atom displacement ˚ ˚ of 0.3 A and a maximum movement of 0.5 A The side-chains of Tyr181 and Tyr188 are however, maintained in a wild-type conformation In spite of the observed rearrangements of inhibitor and the sidechains of residues 101 and 138, the three water molecules that link nevirapine to Glu138 in the wild-type complex are retained in the mutant structure albeit with shifts in position in the same direction as the nevirapine, i.e away from the binding pocket Glu138Lys Fig Simulated annealing omit electron density maps contoured at 3.5r showing bound inhibitors, waters and mutated residues within one of the HIV-1 RT subunits as indicated (A) Lys101Glu in p66 and nevirapine (B) Glu138Lys in p51 and nevirapine (C) Glu138Lys in p51 and PETT-2 Fig 1, i.e for Lys101Glu in the p66 subunit (Fig 1A) and Glu138Lys in the p51 subunit (Fig 1B,C) Comparison of wild-type and mutant RT structures Lys101Glu In the crystal structure of RT(Lys101Glu) in complex with nevirapine, electron density for the mutant gluta- The structures of HIV-1 RT(Glu138Lys) in complexes with nevirapine and PETT-2 complex have well defined side-chain density for Lys138 (p51) (Fig 1B,C) As with the Lys101Glu mutant, the overall structure of the NNRTI pocket in RT(Glu138Lys) closely matches that of the equivalent nevirapine complex with wildtype RT, with the exception of the mutated side-chain itself, which together with Lys101 undergoes large positional changes Lys138 moves away from the NNRTI pocket in the complex with nevirapine as indi˚ cated by the shift in position of the CD atom of 2.8 A (Fig 2B), whilst Lys101 becomes much less ordered than in the wild-type structure The best fit of the sidechain to electron density indicates a rotation of close to 150° about the Lys101 CA–CB bond, positioning it away from the NNRTI pocket (Fig 2B) and resulting in the loss of interaction of residue 138 in p51 with Lys101 in the p66 subunit There is also a shift in the position of nevirapine outwards from the binding ˚ pocket with an average atom displacement of 0.4 A ˚ and a maximum displacement of 0.6 A The three water molecules that bridge between Glu138 and FEBS Journal 273 (2006) 3850–3860 ª 2006 The Authors Journal compilation ª 2006 FEBS 3853 HIV-1 RT drug resistance mutations at 101 and 138 J Ren et al Fig Stereo-diagrams comparing the NNRTI binding sites of wild-type and mutant RTs for the following complexes: (A) Lys101Glu and nevirapine, (B) Glu138Lys and nevirapine, and (C) Glu138Lys and PETT-2 The thinner and thicker bonds show the CA backbone and sidechains with wild-type RT coloured orange and the mutant RTs coloured blue, respectively Inhibitors and water molecules are shown in balland-stick representation and coloured red for wild-type RT and cyan for mutant RTs For clarity, the side-chains at the site of mutation are shown in magenta for wild-type and green for the mutants Hydrogen bonds linking drug to protein and drug to water molecules are marked in dashed lines, coloured yellow for wild-type and blue for mutant RTs nevirapine in the wild-type complex remain in the Glu138Lys mutant but are displaced in position and the network of hydrogen bonds is altered The remain3854 ing side-chains within the mutant NNRTI binding site are essentially positioned the same as for the wild-type enzyme FEBS Journal 273 (2006) 3850–3860 ª 2006 The Authors Journal compilation ª 2006 FEBS J Ren et al HIV-1 RT drug resistance mutations at 101 and 138 Comparison of RT wild-type and RT(Glu138Lys) complexes with PETT-2 shows that there appears to be a perturbation in the positions of a number of sidechains, with the largest changes at the mutation site The wild-type Glu138 and mutant Lys138 residues overlap as far as the CB atoms, the remainder of the side chain of Lys138 is swung away from the NNRTI pocket by a rotation of 170° about the CA–CB bond giving a difference in position of amino and carboxyl ˚ groups of 6.3 A (Fig 2C) The difference in conformation of Lys138 relative to Glu138 results in the loss of the van der Waals contact between the carboxyl and the pyridine nitrogen ring of PETT-2 in the wild-type complex Additionally, there is a movement in the Lys101 side chain which involves a rotation about the CD–CE bond resulting in the amino group moving ˚ 3.4 A relative to the wild-type structure, positioning it further away from the NNRTI pocket The changes in side-chain conformations are also accompanied by a shift in the position of PETT-2 within the binding site ˚ with an average atom displacement of 0.4 A and a ˚ (Fig 2C) maximum displacement of 0.7 A Potency of PETT-2 against RT(Glu138Lys) in viral and enzyme assays The results of EC50 determinations for PETT-2 in the HIV assay together with fold-resistance for the Glu138Lys mutant are shown in Table IC50 values determined from inhibition assays of recombinant HIV-1 RT by PETT-2 for wild-type and RT(Glu138Lys) are also shown in Table In both antiviral and enzyme assays, no significant difference in potency was observed for PETT2 between wild-type and the Glu138Lys mutant, hence no cross-resistance was detectable Discussion The effect of many of the drug resistance mutations reported for NNRTIs can be rationalized at the molecular level in terms of direct alterations in the stereochemistry of inhibitor–RT interactions Thus, in the case of both RT(Tyr181Cys) and RT(Tyr188Cys), the loss of aromatic ring stacking with first generation NNRTIs, such as nevirapine, explains the observed drug resistance [32,34] The mechanism whereby RT(Lys101Glu) gives resistance to nevirapine is more complex as no direct contacts between the drug and Lys101 exist, rather a network of three water molecules links the inhibitor to the side-chains of Lys101 and Glu138 in wild-type RT [4] We were fortunate in being able to solve both RT(Glu138Lys) and RT(Lys101Glu) nevirapine complexes to the relatively ˚ high resolution for HIV-1 RT crystals of 2.5 A, thereby allowing full structural refinement and determination of the associated water structure, of key importance in understanding the interaction of the drug with these residues There are some similarities in the structural changes seen in the nevirapine complexes for both the RT(Glu138Lys) and RT(Lys101Glu) mutants Thus significant movements of side-chains away from the NNRTI pocket are observed in both structures due to the juxtaposition of like charges caused by the charge inversion resulting from these mutations Although three water molecules, which link Glu138, Lys101 and nevirapine in wild-type RT are also present in both RT(Glu138Lys) and RT(Lys101Glu) structures, they are shifted in position and hydrogen bonding patterns are perturbed A set of H-bonds linking waters to the main-chain carbonyl and amide groups of residue 101 to one of the nevirapine pyridine rings is retained in each case However the side-chain movements away from the NNRTI pocket observed for RT(Lys101Glu) and RT(Glu138Lys) nevirapine complexes in both cases abolish the H-bond and salt bridge linking Lys101 to Glu138 in wild-type RT Such movement leads to a reduction in the extent of the H-bond network so that there is no longer a water molecule H-bonded to residue 138 in the Glu138Lys mutant Paradoxically the greater loss of H-bonding seen in RT(Glu138Lys) does not lead to a bigger reduction in binding potency for nevirapine, clearly indicating that there must be some compensatory changes Detailed inspection of overlaps of the RT(Lys101Glu) and RT(Glu138Lys) structures with Table Potency of PETT-2 against HIV-1 RT(Glu138Lys) EC50 virus assay (nM) IC50 enzyme assay (nM) WT PETT-2 Nevirapine E138K Fold resistance WT E138K Fold resistance 3.2 ± 2.0 81 ± 12 2.8 ± 0.5 97 ± 21 0.9 1.2 2.8 ± 0.3 1955 ± 376a 2.7 ± 0.4 1165 ± 375a 1.0 0.6 a Data from reference [30] FEBS Journal 273 (2006) 3850–3860 ª 2006 The Authors Journal compilation ª 2006 FEBS 3855 HIV-1 RT drug resistance mutations at 101 and 138 J Ren et al the equivalent nevirapine complex of wild-type RT indicates the NNRTI site protein conformation is remarkably similar in both mutants and wild-type RT, thus limiting possible options for compensatory changes Indeed, apart from movements of the mutated and immediately interacting side-chains and associated water molecules, the only other changes visible are shifts in the position of nevirapine for both RT(Lys101Glu) and RT(Glu138Lys), with the biggest movement observed for the drug in the latter mutant Previous structural work has shown that a shift of nevirapine relative to Tyr181 can compromise drug to side-chain aromatic ring stacking interactions, thereby contributing to weaker drug binding [37] There are, however, distinctive features in the case of RT(Glu138Lys), in which the movement of nevirapine relative to the Tyr181 side-chain is by a translation in one direction rather than a rotation, thus potentially optimizing the favourable parallel offset ring stacking interactions with Tyr181 Normally, where changes of the interactions of nevirapine with the tyrosine sidechain leads to reduced binding, e.g Val106Ala, there is a rotation of the nevirapine and significant movement of the side-chain [37] Thus the nevirapine movement in RT(Glu138Lys) appears in this case to lead to an optimization of interactions rather than the reverse A factor that is different in RT(Glu138Lys) compared with RT(Val106Ala) is that the former mutation slightly expands the NNRTI pocket in a way that is not possible for resistance mutations at the hydrophobic core of the pocket which are more structurally constrained For RT(Lys101Glu), where the H-bonding network involving nevirapine is less disrupted (the link to Glu138 being maintained) there is a smaller shift in the nevirapine position and therefore suggesting there is less opportunity to optimize interactions, resulting in a greater overall loss of binding affinity A corollary to this is that the interactions of nevirapine with wildtype RT would seem non-optimal Indeed, there is evidence of this for the mutation Pro236Leu which, although it gives resistance to delavirdine [38], causes hypersensitivity to nevirapine There thus appears to be an incompatibility between the competing requirements of nevirapine hydrogen bonding via water molecules to side-chains at the outer edge of the pocket (Lys101, Glu138) and aromatic ring stacking to tyrosines in the inner region This would also explain why nevirapine, although a much bulkier molecule than efavirenz, and capable of making more interactions with RT, is actually of much lower potency (30-fold) than the latter drug A further implication is that, as nevirapine does not appear to optimally fit the NNRTI pocket, there are possibilities for designing analogues 3856 that better meet both hydrophobic and hydrogen bonding requirements which might therefore have greater potency As an example, it may be possible to introduce additional substituents into a nevirapine-like inhibitor in order to displace the bound waters, allowing direct interactions with the protein Comparison of the crystal structures of wild-type RT and RT(Glu138Lys) with bound PETT-2 shows that there is a loss of a van der Waals contact between the carboxyl of Glu138 and the inhibitor It would thus be expected that there is an accompanying reduction in binding affinity for PETT-2 for the Glu138Lys mutation In fact the EC50 and IC50 data both from the mutant HIV in tissue culture and recombinant RT enzyme assays clearly indicate essentially equal potency for PETT-2 against WT and mutant forms It is possible that the observed PETT-2 contact with the Glu138 carboxyl may be induced as a result of the crystals being grown at pH 5.0, which approaches the range of pKa values for glutamic acid side-chains (4.4) Glu138 may therefore be protonated and as a consequence induce a contact which would not be found under conditions for assay of RT inhibition (pH 7.4) It should also be born in mind that since the resolution of the RT complexes with PETT-2 (both wild-type and ˚ for RT(Glu138Lys)) are limited to 3.0 A, it is not possible to fully define all aspects of the structure, particularly bound water molecules It is thus not known whether, for example, changes in hydrogen bonding and water networks may occur as is the case in the work reported here on the RT(Lys101Glu) and RT(Glu138Lys) mutants in complexes with nevirapine Such changes might compensate for the loss of the van der Waals interactions between PETT-2 and RT(Glu138Lys) compared with wild-type This limitation in structural detail emphasizes the need to exercise caution in the interpretation of molecular details of inhibitor binding when using lower resolution X-ray structures of HIV-1 RT For the Glu138Lys mutation, resistance to the TSAO series of NNRTIs is thought to be due to the loss of interaction of the side-chain carboxyl group with a key amino group of the compounds [28] Unfortunately it has not been possible to obtain crystal structures of TSAO bound to RT as inhibitors of this series cause the protein to precipitate prior to cocrystallization set-ups or induce disorder in pre-grown crystals following soaking attempts Such properties may relate to the known disrupting effects of TSAO on the RT dimer structure The work described here shows how the charge inversion introduced by the Lys101Glu and Glu138Lys mutations in RT results in a common structural FEBS Journal 273 (2006) 3850–3860 ª 2006 The Authors Journal compilation ª 2006 FEBS J Ren et al feature, the significant movement of mutated sidechains away from the NNRTI pocket, due to the repulsion of like charges The structural consequences of the mutant side-chain movement include partially disrupting the hydrogen bond network involving nevirapine, water and side-chain ⁄ main-chain interactions in wild-type RT Compensatory changes must occur for the Glu138Lys mutation as this has little effect on nevirapine binding which we suggest to be optimization of inhibitor ring stacking interactions with Tyr181 Lys101Glu provides an example of an indirect mechanism of inducing drug resistance in RT but via a different means to other reported examples viz Lys103Asn, which apparently stabilizes the unliganded RT conformation [7,35] whilst Val108Ile perturbs the interaction of Tyr188 and Tyr181 with nevirapine [37] Interestingly, the Lys101Glu mutation in RT also appears to be capable of a direct mechanism of drug resistance For the carboxanilide, UC-38, and for efavirenz there are van der Waals contacts between the side-chain of Lys101 and the inhibitors [24] Movement of the Lys101 side-chain away from the NNRTI pocket on mutation to glutamic acid, as observed for the nevirapine complex, would thus cause loss of contacts and a consequent weakening in binding for UC-38 and efavirenz The selection of drug-resistant virus remains a significant problem in controlling HIV infection and AIDS The work reported here is part of ongoing studies aimed at understanding mechanisms of drug resistance in HIV RT by the use of X-ray crystal structure analysis Combining such data with biochemical and virological studies will be of value in contributing to the development of further generations of NNRTIs with improved resilience to existing drug resistance mutations present in HIV Such drugs are needed as an important priority for continued efforts to treat HIV Experimental procedures Crystallization and data collection Expression vectors for HIV-1 RT (HXB-2 isolate) containing either Lys101Glu or Glu138Lys mutations were made using site-directed mutagenesis as previously reported [34] Purification of mutant RTs from recombinant Escherichia coli was based on the ion-exchange procedures as described in an earlier report [39] A wide range of NNRTIs were screened for crystallization with the mutant RTs, producing crystals of three complexes suitable for structure determination Crystals of complexes of RTs with NNRTIs were grown and soaked in 50% PEG 3400 prior to data collection as described previously [40,41] All the crystals of HIV-1 RT drug resistance mutations at 101 and 138 mutant RT inhibitor complexes used in this work were obtained by co-crystallization X-ray data were either collected at SRS, Daresbury, UK and APS, Chicago, IL, USA, using the oscillation method or at the Photon Factory Tsukuba, Japan, using a Weissenberg camera [42] Data collected for each mutant–inhibitor complex were from crystals flash-cooled in liquid propane and maintained at 100 K in a nitrogen gas stream Indexing and integration of data images were carried out with denzo, and data were merged with scalepack [43] Details of the X-ray data statistics are given in Table Structure solution and refinement The molecular orientation and position of HIV-1 RT heterodimers in each unit cell were determined using rigidbody refinement with cns [44] The initial model used as the starting point for the current structure determinations was chosen from our collection of RT–NNRTI complexes on the basis of closeness of unit cell parameters [4,24,25,34,45–48] All structures were refined with cns [44] using positional, simulated annealing and individual B-factor refinement with bulk solvent correction and anisotropic B-factor scaling Model rebuilding was carried out using ‘o’ [49] Table gives the refinement statistics for the three structures Structures of wild-type and mutant RTs were overlapped using the program SHP [50] The wild-type RT complexes used for these comparisons were 1rtd (for nevirapine) [4] and 1dtt (for PETT-2) [31] Data deposition Coordinates and structure factors for all of the HIV-1 RT mutant structures reported here have been deposited in the Protein Data Bank for immediate release on publication The codes are as follows: RT(Lys101Glu)-nevirapine, 2HND; RT(Glu138Lys)-nevirapine, 2HNY; RT(Glu138Lys)PETT-2, 2HNZ Potency of PETT-2 against HIV RT(Glu138Lys) in tissue culture and enzyme assays The assay to determine EC50 values for PETT-2 used a modified HeLa MAGI system in which HIV-1 infection is detected due to the activation of the LTR-driven b-galactosidase reporter in HeLa CD4-LTR-b-gal cells as described previously [51] Quantitation of the b-galactosidase was carried out by measurement of the activation of the chemoluminescent substrate, tropix, after days infection The value of the EC50 was the mean of three separate experiments PETT-2 inhibition of recombinant wild-type HIV-1 RT and RT(Glu138Lys) were determined in an enzyme assay using rC-dG as template-primer and measur- FEBS Journal 273 (2006) 3850–3860 ª 2006 The Authors Journal compilation ª 2006 FEBS 3857 HIV-1 RT drug resistance mutations at 101 and 138 J Ren et al ing the incorporation of radiolabelled dGTP [39] Curve fitting of enzyme inhibition data and IC50 determination were carried out using origin (Microcal) 11 Acknowledgements 12 We thank the staff of the following synchrotrons for their assistance in data collection: SRS, Daresbury, UK; The Photon Factory, Tsukuba, Japan and Advanced Photon Source, Chicago, IL, USA Supporting grants from the Medical Research Council, the Biotechnology and Biological Sciences Research Council and the EC (QLKT-2000–00291, QLKT2-CT-2002– 01311) to DKS are acknowledged References 14 Richman DD (2001) HIV chemotherapy Nature 410, 995–1001 Brettle RP, Wilson A, Povey S, Morris S, Morgan R, Leen CL, Hutchinson S, Lewis S & Gore S (1998) Combination therapy for HIV: the effect on inpatient activity, morbidity and mortality of 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Authors Journal compilation ª 2006 FEBS 3857 HIV-1 RT drug resistance mutations at 101 and 138 J Ren et al ing the incorporation of radiolabelled dGTP [39] Curve fitting of enzyme inhibition data... Short S, Stuart DI & Stammers DK (2001) Structural mechanisms of drug resistance for mutations at codons 181 and 188 in HIV-1 reverse transcriptase and the improved resilience of second generation