Báo cáo khoa học: Structural insights into mechanisms of non-nucleoside drug resistance for HIV-1 reverse transcriptases mutated at codons 101 or 138 pot

HIV-1 RT contains two distinct inhibitor binding sites for Keywords drug resistance; HIV-1 reverse transcriptase mutants; Lys101Glu, Glu138Lys; non-nucleoside inhibitors; X-ray crystall

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 Short2and David K Stammers1

1 Division of Structural Biology, The Wellcome Trust Centre for Human Genetics, Henry Wellcome Building for Genomic Medicine,

University of Oxford, UK

2 Glaxo Smith Kline Inc., Research Triangle Park, NC, USA

The emergence of resistant viruses resulting from drug

treatment of HIV-infected patients poses one of the

most significant problems in countering AIDS in

West-ern 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

Keywords

drug resistance; HIV-1 reverse transcriptase

mutants; Lys101Glu, Glu138Lys;

non-nucleoside 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 sur-face of the NNRTI pocket where it interacts across the reverse transcrip-tase (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 sig-nificant movement of the mutated side-chains away from the NNRTI pocket induced by charge inversion There are also small shifts in the posi-tion 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 resist-ance, which are inhibitor-dependent

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¢¢-amino-1¢¢,2¢¢-oxathiole-2¢¢,2¢¢-dioxide).

different classes of drugs within the larger subunit of

the p66⁄ p51 heterodimer [3,4] Nucleoside analogue

inhibitors of RT (NRTIs), such as zidovudine,

lamivu-dine, stavudine and the nucleotide analogue (NtRTI)

tenofovir, bind, respectively, as activated tri- or

di-phosphate 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

mech-anism 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

inhib-itor 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

resist-ance are generally proximal to the inhibitor binding

pocket [3,4]

Many of the most commonly observed NNRTI

resistance mutations are located at hydrophobic

resi-dues at the centre of the drug binding pocket (e.g

Leu100, Val106, Val108, Tyr181 and Tyr188)

How-ever, charged residues positioned towards the surface

of the drug pocket are also known sites of drug

resistance mutations Indeed, Lys103Asn is the

muta-tion most frequently observed in clinical use of

NNRTIs Other NNRTI resistance mutations at

sur-face 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 selec-tion 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¢-bis- O-(tert-butyldimethylsilyl)-b-d-ribofuranosyl]-3¢-spiro-5¢¢-(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 model-ling 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 gluta-mic 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 nevi-rapine [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 resist-ance mutations Leu100Ile, Val106Ala and Val108Ile have been reported, which indicate an indirect com-ponent in the mechanism of resistance via modulation

N N

O

N

N

Nevirapine

N

N N N Cl

O S

F

PETT-2

H

1

10

Scheme 1 Structures of nevirapine and PETT-2 are shown.

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

efavi-renz can bind to the enzyme in both a wild-type

con-formation 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

stabil-izing 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

logi-cal to study as a pair as they are both charged

sur-face residues interacting across the p66⁄ p51 interface

The availability of a range of structures of

NNRTI-resistant RTs drugs will provide greater understanding

of the molecular basis of drug resistance mechanisms

Such data will contribute to the design of novel

inhibi-tors, 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 1 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 deter-mined Water molecules are important for the interac-tions 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 side-chains (Fig 1) in each case Side-chain electron density relevant to NNRTI drug resistance is also shown in

Table 1 Statistics for crystallographic structure determinations.

Data collection details

Outer resolution shell

Refinement statistics:

a

All crystals belong to space group P2 1 2 1 2 1 [40,41];bR merge ¼ S|I – < I > | ⁄ S < I > ; c

R factor ¼ S|F o - F c | ⁄ SF o ;dmean B factor for main-chain, side-chain, inhibitor and water molecules.

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-mic acid side-chain is well defined (Fig 1A) The struc-ture of the NNRTI pocket is overall remarkably similar for mutant and wild-type, with the exception of the mutated side-chain and adjacent interacting resi-due There is a significant movement of Glu101 relat-ive 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 of0.5 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 sub-unit 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 posi-tion 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, main-tained in a wild-type conformation In spite of the observed rearrangements of inhibitor and the side-chains of residues 101 and 138, the three water mole-cules 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 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 wild-type 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 side-chain 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

Fig 1 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.

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

remain-ing side-chains within the mutant NNRTI bindremain-ing site are essentially positioned the same as for the wild-type enzyme

Fig 2 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 side-chains with wild-type RT coloured orange and the mutant RTs coloured blue, respectively Inhibitors and water molecules are shown in ball-and-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.

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

side-chains, 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 of170 about the CA–CB bond

giving a difference in position of amino and carboxyl

groups of 6.3 A˚ (Fig 2C) The difference in

conforma-tion 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

maximum displacement of 0.7 A˚ (Fig 2C)

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 2 IC50 values

determined from inhibition assays of recombinant

RT(Glu138Lys) are also shown in Table 2 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

ste-reochemistry 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 mole-cules 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 deter-mination 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(Lys101-Glu) structures, they are shifted in position and hydro-gen 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 2 Potency of PETT-2 against HIV-1 RT(Glu138Lys).

a Data from reference [30].

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

chan-ges 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

side-chain 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

hydropho-bic core of the pocket which are more structurally

con-strained 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

wild-type 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

require-ments of nevirapine hydrogen bonding via water

mole-cules to side-chains at the outer edge of the pocket

(Lys101, Glu138) and aromatic ring stacking to

tyro-sines in the inner region This would also explain why

nevirapine, although a much bulkier molecule than

ef-avirenz, 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

nevi-rapine does not appear to optimally fit the NNRTI

pocket, there are possibilities for designing analogues

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, allow-ing 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 carb-oxyl 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 muta-tion 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 poss-ible 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 conse-quence 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, par-ticularly 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 limita-tion 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] Unfor-tunately 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 co-crystallization 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

feature, the significant movement of mutated

side-chains 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

nevi-rapine, 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

optimiza-tion 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

ef-avirenz 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

signi-ficant 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

analy-sis Combining such data with biochemical and

virolo-gical studies will be of value in contributing to the

development of further generations of NNRTIs with

improved resilience to existing drug resistance

muta-tions 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)

contain-ing either Lys101Glu or Glu138Lys mutations were made

using site-directed mutagenesis as previously reported [34]

Purification of mutant RTs from recombinant Escherichia

coliwas 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

determin-ation Crystals of complexes of RTs with NNRTIs were

grown and soaked in 50% PEG 3400 prior to data

collec-tion as described previously [40,41] All the crystals of

mutant RT inhibitor complexes used in this work were obtained by co-crystallization X-ray data were either col-lected at SRS, Daresbury, UK and APS, Chicago, IL, USA, using the oscillation method or at the Photon Fac-tory 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 sta-tistics are given in Table 1

Structure solution and refinement

The molecular orientation and position of HIV-1 RT het-erodimers in each unit cell were determined using rigid-body 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-fac-tor refinement with bulk solvent correction and anisotropic B-factor scaling Model rebuilding was carried out using ‘o’ [49] Table 1 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; 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-galac-tosidase reporter in HeLa CD4-LTR-b-gal cells as des-cribed previously [51] Quantitation of the b-galactosidase was carried out by measurement of the activation of the chemoluminescent substrate, tropix, after 3 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-ing the incorporation of radiolabelled dGTP [39] Curve

fit-ting of enzyme inhibition data and IC50determination were

carried out using origin (Microcal)

Acknowledgements

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

Support-ing grants from the Medical Research Council, the

Biotechnology and Biological Sciences Research

Coun-cil and the EC (QLKT-2000–00291, QLKT2-CT-2002–

01311) to DKS are acknowledged

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