Collapsed State Mediates the Low Fidelity of the DNA Polymerase β I260 Mutant

DNA polymerase β (Pol β) fills single nucleotide gaps during base excision repair. Deficiencies in Pol β can lead to increased mutagenesis and genomic instability in the cell, resulting in cancer. Our laboratory has previously shown that the I260 M somatic mutation of Pol β, which was first identified in prostate cancer, has reduced nucleotide discrimination in a sequence context-dependent manner. I260 M incorporates the incorrect G opposite A in this context more readily than WT. To identify the molecular mechanism of the reduced fidelity of I260M, we studied incorporation using single turnover kinetics and the nature and rates of conformational changes using steady-state fluorescence and Förster resonance energy transfer (FRET). Our data indicate that the I260 M mutation affects the fingers region of rat Pol β by creating a "collapsed" state in both the open (in the absence of nucleotide) and closed (prior to chemistry) states. I260 M is a temperature-sensitive mutator and binds nucleotides tighter than the WT protein, resulting in reduced fidelity compared to the WT. Additionally, we have generated a kinetic model of WT and I260 M using FRET and single turnover data, which demonstrates that I260 M precatalytic conformation changes differ compared to the WT as it is missing a precatalytic noncovalent step. Taken together, these results suggest that the collapsed state of I260 M may decrease its ability for nucleotide discrimination, illustrating the importance of the "fingers closing" conformational change for polymerase fidelity and accurate DNA synthesis.