hMMS2 serves a redundant role in human PCNA polyubiquitination

In this report we show that mammalian cells in which MMS2 was reduced by siRNA-mediated knockdown maintains PCNA polyubiquitination while a knockdown of RAD18 or UBC13 abrogates PCNA ubiquitination. Moreover, the additional knockdown of a UEV1A (MMS2 homolog) does not deplete PCNA polyubiquitination. Finally, mouse embryonic stem cells null for MMS2 with or without the additional depletion of mUEV1A continue to polyubiquitinated PCNA with normal kinetics.Our results point to a high level of redundancy in the DDT pathway and suggest the existence of another hMMS2 variant (hMMSv) or complex that can compensate for its loss.Protecting the integrity of the DNA genome is important for the long-term survival of higher eukaryotes [1]. Given the importance of genomic integrity, it is not surprising that an elaborate system of cell cycle checkpoints and DNA repair systems has evolved in higher vertebrates. However, the failure to remove DNA lesions in a timely and efficient manner often forces a cell to bypass the damage in order to avoid replication stalling. This is accomplished through DNA damage tolerance (DDT), an important component of the DNA damage response. DDT allows replication machinery stalled at sites of DNA damage to continue with DNA synthesis by allowing bypass of such sites in either an error-prone or error-free manner [2]. The control of such a pathway is dependent on the modification status of the sliding clamp PCNA.In the absence of DNA damage, yeast PCNA is typically sumoylated on K164 which promotes normal S phase progression by preventing unwanted recombination, while replication stress results in PCNA ubiquitination on the same lysine residue [3,4]. There is a central role for Rad6 (ubiquitin conjugating E2 enzyme) and Rad18 ubiquitin ligase (E3 ligase) in PCNA ubiquitination. During genotoxic stress, Rad6 is recruited by Rad18 to sites of DNA damage where it monoubiquitinates PCNA [4]. Monoubiquitinated PCNA facilitates the recruitment of error-pron