Fig. 6: Model for the role of USP1 activity and/or autocleavage during normal DNA replication progression.
From: USP1-trapping lesions as a source of DNA replication stress and genomic instability
During unperturbed replication, low levels of PCNA ubiquitylation allow for efficient TLS to overcome various replicative lesions, and autocleavage allows USP1 to be dynamically recruited to the fork. By contrast, the complete absence of USP1 activity leads to increased aberrant TLS Pol usage due to hyperubiquitylation of PCNA, resulting in reduced replication fork speed and low-level genomic instability. Impaired USP1 autocleavage caused by mutations disrupting the catalytic activity (C90S) or autocleavage site (GG/AA) additionally causes increased fork stalling and replication stress due to increased retention (i.e., “trapping”) of USP1 on DNA. The DNA-metalloprotease SPRTN is efficiently recruited to remove the trapped USP1 catalytic-mutant to mitigate these effects, owing to high Ub-PCNA levels in cells expressing this form of USP1. USP1 hyperactivity and deubiquitylation of PCNA in cells harboring the autocleavage-deficient GG/AA mutant, on the other hand, does not allow for effective SPRTN-mediated removal, resulting in high-level USP1 trapping. Thus, we propose that proper regulation of USP1 activity and autocleavage are both important for ensuring normal USP1 dynamics at the fork and genome stability.
