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.

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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.

The K103N substitution is a frequently observed HIV-1 RT mutation in patients who do not respond to combinationtherapy. The drugs Efavirenz, MSC194 and PNU142721 belong to the recent generation of NNRTIs characterized by an improved resistance profile to the most common single point mutations within HIV-1 RT, including the K103N mutation. In the present study we present structural observations from Efavirenz in complex with wild-type protein and the K103N mutant and PNU142721 and MSC194 in complex with the K103N mutant. The structures unanimously indicate that the K103N substitution induces only minor positional adjustments of the three inhibitors and the residues lining the binding pocket. Thus, compared to the corresponding wild-type structures, these inhibitors bind to the mutant in a conservative mode rather than through major rearrangements. The structures implicate that the reduced inhibitory efficacy should be attributed to the changes in the chemical environment in the vicinity of the substituted N103 residue. This is supported by changes in hydrophobic and electrostatic interactions to the inhibitors between wild-type and K103N mutant complexes. These potent inhibitors accommodate to the K103N mutation by forming new interactions to the N103 side chain. Our results are consistent with the proposal by Hsiou et al. [Hsiou, Y., Ding, J., Das, K., Clark, A.D. Jr, Boyer, P.L., Lewi, P., Janssen, P.A., Kleim, J.P., Rosner, M., Hughes, S.H. & Arnold, E. (2001) J. Mol. Biol. 309, 437-445] that inhibitors with good activity against the K103N mutant would be expected to have favorable interactions with the mutant asparagines side chain, thereby compensating for resistance caused by stabilization of the mutant enzyme due to a hydrogen-bond network involving the N103 and Y188 side chains.Keywords: drug-resistance; HIV; NNRTI; reverse transcriptase.The use of highly active antiretroviral therapy (HAART) involving multidrug combinations has significantly reduced the death rates of HIV-1 infected individuals receiving such treatment [1]. Inhibitors of the HIV-1 reverse transcriptase (RT) constitute a cornerstone in this therapy and are commonly used in combination with inhibitors of the HIV-1 protease. The RT inhibitors belong to two classes, the nucleoside inhibitors and the non-nucleoside inhibitors (NNRTI). Whereas the NRTIs are nucleoside analogues with chain-terminating properties and affinity to active site residues, the NNRTIs include a wide range of series of chemical compounds characterized by noncompetitive binding to an allosteric site some 10 Å away from the active site. Structural comparison of RT in complex with template/primer and NNRTIs together with native RT complexes have shown that the NNRTIs inhibit the polymerase activity through long-range and short-range structural distortions in several of the RT subdomains. The distortions involve repositioning of residues in the nonnucleoside binding pocket (NNIBP) that impose steric impediments on the thumb subdomain flexibility forcing it to remain in the ...