Walter A. Scott, Ph.D.
Professor of Biochemistry & Molecular Biology
Description of Research
Dr. Scott and his team are studying the replication machinery of human immunodeficiency virus (HIV). Replication is carried out by a virus-encoded protein, reverse transcriptase (RT). Since HIV RT is one of the simplest DNA polymerases, they hope to use this enzyme to investigate some of the most basic mechanisms of DNA synthesis such as nucleotide recognition, enzyme translocation, and internal conformational changes that occur during DNA synthesis.
RT is the target for many drugs that inhibit HIV replication. The Dr. Scott's laboratory investigators have focused on a class of drugs including 3'-azido-3'-deoxythymidine (AZT), 2',3'-dideoxyinosine (ddI), and 2',3'-didehydro-3'-deoxythymidine (d4T) that are incorporated into growing DNA chains by RT and result in chain termination. A novel activity of HIV RT gives this enzyme the ability to remove chain-terminating nucleotides from a blocked DNA chain after they have been incorporated. Removal involves the transfer of the inhibitory nucleotide to an intracellular acceptor molecule. The acceptor has not been identified unambiguously in infected cells; however, Dr. Scott's laboratory has shown that a common intracellular molecule such as ATP could serve as acceptor. The resulting product would be a dinucleoside polyphosphate, which has a unique structure and might be detectable in an infected cell extract. In the case of AZTMP-terminated DNA, the expected product would be AppppAZT. Therapy with AZT results in the selection of mutant viruses that are highly resistant to AZT. The AZT-resistance mutations occur in the RT coding region and the mutant enzyme has increased ability to carry out the unblocking reaction.
RT that is associated with a chain-terminated primer-template can either bind a nucleotide substrate for the unblocking reaction, in which case the chain-terminating nucleotide is removed; or it can bind the natural dNTP substrate that matches the next position on the template to form a dead-end complex. The dead-end complex cannot carry out either the DNA synthesis reaction or the unblocking reaction but it can be detected experimentally. The factors that determine which pathway the enzyme takes depend on translocation of the enzyme between two positions on the primer-template and they hope to explore the mechanism of translocation through the study of these reactions.
Dr. Scott and colleagues are very interested in what determines the sensitivity of HIV to various nucleoside inhibitors, the mechanisms of mutations in the enzyme that cause resistance to these drugs, and how the drugs might be modified to reduce the chance of selecting resistance mutations and to make them more effective antiretroviral drugs. The research team is also actively studying other steps in the HIV replication in hopes of finding new targets for drug development. Future development of anti-HIV drugs will depend on basic research that advances the researchers understanding of the HIV life cycle.
Selected Cancer-Related Publications
- Acosta-Hoyos AJ, Matsuura SE, Meyer PR, Scott WA. A role of template cleavage in reduced excision of chain-terminating nucleotides by Human Immunodeficiency Virus Type 1 reverse transcriptase containing the M184V mutation. J Virol 86:5122-33, 2012. Read more »
Collaborating in the Multidisciplinary Research Program(s):