Robert E London
National Institute of Environmental Health Sciences, USA
Posters & Accepted Abstracts: J Material Sci Eng
The genome of RNA viruses such as HIV is subject to severe size limitations that result from the chemical instability of the RNA polymer and from biological instability attributed to reliance on low fidelity polymerases. The average size of the RNA virus is only about 9 kB. Genome compression is achieved by reliance on strategies that minimize the size of both the genome and the viral proteome. HIV-1 reverse transcriptase (RT), a critical enzyme of the viral life cycle, undergoes a complex maturation process, required so that a pair of p66 precursor proteins can develop conformationally along different pathways, one evolving to form active polymerase and ribonuclease H (RH) domains, while the second forms a non-functional polymerase and a proteolyzed RH domain. These parallel maturation pathways rely on the structural ambiguity of a metamorphic polymerase domain, for which the sequence-structure relationship is not unique. This strategy thus allows formation of two alternate structures, each fulfilling a different function, from a single gene sequence. Alternate strategies more commonly encountered involve utilization of symmetric multimers, as in the case of HIV-1 protease, and reliance on multi-functional proteins, also true of RT. Recent progress toward unraveling the general features of the maturation pathway has been made using a combination of crystallographic, nuclear magnetic resonance, and molecular modeling approaches. The structure of the monomeric p66 precursor was obtained by deletion of a loop required for the metamorphic conversion, in effect introducing an isomerization-restriction mutation. Structural maturation involves three major steps: Domain rearrangement; dimerization and; subunit-selective RH domain proteolysis. I will summarize the major structural changes that occur during the maturation process, and describe how mutations, often viewed within the context of the mature RT heterodimer, can exert a major influence on maturation and dimerization. Several steps in the RT maturation pathway may provide attractive targets for future drug development.