New insights into the functions of PCNA during DNA repair and replication

PCNA is an essential protein complex comprised of three identical subunits
(called Pol30 in yeast). It forms a sliding ring that is crucial for DNA replication and
DNA repair processes in vivo. PCNA functions as the sliding clamp for two out of
three major DNA polymerases, δ and ε. In addition to PCNA`s importance for the
processivity of polymerases, it also holds a crucial role in the DNA damage tolerance
pathway (DDT), which allows to replicate DNA despite the presence of DNA lesions.
To better characterize PCNA`s role in these processes, we analyzed four
Pol30 mutants (V180D, E143K, S152P, D150E) that were found in a screen for
sensitivity to mutations in the exonuclease EXO1. All the mutants exhibit low levels
of chromatin-bound PCNA in comparison to the wild type, suggesting that they
spontaneously dissociate and form unstable rings. These mutants exhibit a slippage-
based mutagenesis of up to 540-fold higher than that of a wild type control.
Unexpectedly, deletion of the alternative polymerase Rev3 in the background of
some of those mutants exhibited an even higher mutation rate (1600-fold higher than
that of WT cells), hinting to a yet unknown function of Rev3.
Further experiments with a proteomic approach will help us shed light into the
highly conservative and important processes that in humans play a major role in
cancer development.