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conformational transition to occur, presumably near the GH loop, that would be required for viral<br />

uncoating. The conformational transition would lead to productive uncoating, even in the presence of<br />

bound drug.<br />

Page 516<br />

This hypothesis suggests that drug-resistant mutations would be of the type that destabilize protein<br />

structures. Such mutations are typically from larger to smaller side chains or from highly branched to<br />

less branched side chains [43–45]. Four of five observed compensation mutants are of this type (Table<br />

1). The remaining mutant, Val1153 rarrow.gif Ile, while a priori might not be predicted to destabilize<br />

the capsid, has been shown experimentally to decrease viral thermostability when compared with native<br />

HRV14 [28].<br />

The advantage of the stabilization-destabilization hypothesis is that it allows for a single mechanism of<br />

resistance for both compensation mutants occurring in the hydrophobic pocket as well as those that are<br />

outside the pocket near the receptor binding site. Both of these sites are close to the GH loop of VP1 that<br />

becomes disordered under acidic conditions. This stabilization—destabilization hypothesis also explains<br />

the behavior of acid-resistant mutants, which are predominately of the type which should stabilize a<br />

protein to conformational changes. Wild-type viruses have a preferred stability profile, which allows<br />

uncoating under the proper conditions. Mutations or addition of drug can perturb this profile, resulting in<br />

decreased replication. A second perturbation might then compensate for the first, restoring virulence<br />

(Figure 11). This behavior has been observed in HRV1A, in which mutants have been isolated that<br />

require presence of drug to replicate. These mutations are more acid-labile<br />

Figure 11<br />

The effects of various rhinovirus manipulations. The solid line depicts<br />

the native rhinovirus uncoating profile. Mutations and drugs can<br />

effect this profile to make the virus more or less stable to<br />

pH- or temperature-induced changes.<br />

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