Investigation of the nucleotide selection mechanism of poliovirus Rna-dependent Rna polymerase

Open Access

Author:

Liu, Xinran

Graduate Program:

Chemistry

Degree:

Doctor of Philosophy

Document Type:

Dissertation

Date of Defense:

October 01, 2015

Committee Members:

• David D Boehr, Dissertation Advisor
• Craig Eugene Cameron, Committee Member
• Scott A Showalter, Committee Member
• John H Golbeck, Committee Member

Keywords:

• Poliovirus
• nucleotide
• selection
• fidelity
• polymerase

Abstract:

RNA viruses cause many diseases including severe acute respiratory syndrome (SARS), the common cold, hepatitis C, poliomyelitis, and so on. However, antiviral strategies against RNA virus infections are very limited. For example, there are no FDA approved (Food and Drug Administration) antiviral compounds for the treatment of picornavirus infection. Only three vaccines are available to prevent the transmission of picornaviruses. The severity of public health issues associated with these viruses and the scarcity of treatment options make the development of antiviral drugs and vaccines high priorities. One very promising antiviral target is the virally encoded RNA-dependent RNA polymerase (RdRp). The RdRp is the most conserved protein among RNA viruses. One antiviral strategy is to modulate the RdRp’s error rate of nucleotide incorporation. The working principle of this antiviral strategy derives from the ‘quasispecies’ nature of RNA viruses. RNA viruses utilize the error-prone RdRp to replicate a genetically diverse population where viral genomes do not contain a unique sequence but a pool of genetically variable sequences. It has been shown that mere two-fold changes in the RdRp error rate (either higher or lower error) lead to viral attenuation. Increasing the viral mutation rate through the action of nucleoside analogs leads to lethal mutagenesis due to the accumulation of an excess amount of mutations and loss of viable genetic information. Decreasing the viral mutation rate is also detrimental to the virus due to a constrained viral ability to adapt to the host environment. Understanding the nucleotide selection mechanisms of RdRps would therefore opens up new treatment strategies including the development of live, attenuated vaccines and/or antiviral drugs. Poliovirus (PV) RdRp is a great model system to study the nucleotide selection mechanism. This dissertation mainly focuses on investigations into the fidelity mechanism of PV RdRp via a combination of kinetic and NMR experiments. Among all the DNA and RNA polymerases, the prechemistry conformational change is a critical fidelity checkpoint. In RdRps and other polymerases, it has been proposed that this step involves the rearrangement of the triphosphate group and nucleobase of the incoming nucleotide into productive conformation, and the repositioning of a general acid to help catalyze phosphodiester bond formation. In PV RdRp, conserved structural motif D is responsible for the repositioning of the general acid via an “open” to a “closed” state transition during the prechemistry conformational change. In this dissertation, I show that the T362I substitution, which originates from the Sabin 1 vaccine strain, lowers enzyme fidelity by shifting the motif D equilibrium more to the “closed” state. PV encoding this low fidelity variant is more sensitive to ribavirin and is moderately attenuated in a mouse model. Other Sabin substitutions in the RdRp also change catalysis and fidelity. I show that the Sabin RdRp, which has all four substitutions (i.e. D53N, Y73H, K250E and T362I), discriminates against nucleotides with noncognate nucleobase to the same extent as wild-type (WT) enzyme, but more efficiently catalyzes incorporation of 2’-deoxy nucleotides. My studies suggest that there is more selective pressure to maintain nucleobase discrimination than sugar discrimination in the Sabin vaccine strain. Besides motif D, motif F is another structure that we propose is involved in nucleotide selection. I propose that substitutions on motif F lead to changes in RdRp fidelity by perturbing triphosphate conformations necessary for efficient nucleotide addition. My studies on the nucleotide selection mechanism of PV RdRp likely extend to RdRps of other viruses due to the strong structural and functional similarities among the RdRps. As such, my studies open up new possibilities for engineering attenuated vaccine candidates through modifications of motifs D and F in these RNA viruses. Such developments will be critical in the treatment of current and future virus outbreaks.