Interferon-Stimulated Gene 15 in the Control of Cellular Responses to Genotoxic Stress

Jeon, Young Joo; Park, Jong Ho; Chung, Chin Ha

The sliding clamp proliferating cell nuclear antigen (PCNA) serves as a processivity factor as well as a platform for recruiting necessary components for DNA replication. Moreover, PCNA is critically involved in DNA lesion bypass by acting as a scaffold that recruits essential components for DDT (Moldovan et al., 2007) , indicating that PCNA plays an additional key role in the maintenance of genome stability and cell survival under DNA damage conditions. When replicating cells encounter DNA damage, PCNA undergoes numerous PTMs, such as ubiquitination and sumoylation (Bergink and Jentsch, 2009; Jackson and Durocher, 2013; Mailand et al., 2013; Ulrich and Walden, 2010) . UV induces mono-ubiquitination of a highly conserved Lys164 residue in PCNA by the ubiquitin E3 ligase RAD6-RAD18 complex (Hoege et al., 2002) . This PCNA ubiquitination triggers the replacement of replicative DNA polymerases, such as Pol, by damage-tolerant Y family of DNA polymerases, including Pol, for translesion DNA synthesis (TLS) (Bienko et al., 2005; Lehmann et al., 2007; Stelter and Ulrich, 2003) . TLS polymerases bypass DNA lesion and therefore DNA replication can proceed without the need of removal of the damage and the risk of fork collapse (Sale, 2012; Sale et al., 2012) . However, TLS polymerases lack proofreading activity and frequently incorporate incorrect nucleotides, therefore are potentially mutagenic (Loeb and Monnat, 2008; Matsuda et al., 2000; Sale, 2012; Sale et al., 2012) . Thus, error-prone TLS polymerases have to be released from PCNA after DNA lesion bypass for preventing Fig. 3 (Bienko et al., 2005; Lehmann et al., 2007; Stelter and Ulrich, 2003) . After DNA lesion bypass, the ISG15 E3 ligase EFP, but not HERC5, is tethered to mono-ubiquitinated PCNA, and generates mono-ISGylated PCNA (at Lys168), leading to formation of PCNA conjugated with both ubiquitin and ISG15 in different subunits (Park et al., 2014) . ISGylated PCNA then recruits PCNA-interacting peptide (PIP) motif-containing USP10 for deubiquitination of PCNA, which causes the release of Pol from PCNA for termination of TLS. EFP conjugates an additional ISG15 to PCNA at Lys164, thus forming PCNA ligated with two ISG15 molecules in the same subunit, likely for preventing monoubiquitination at Lys164 and re-recruitment of Pol. Finally, UBP43 is induced at a later period and cleaves off ISG15 molecules from PCNA for reloading of replicative DNA polymerases and resuming of DNA replication. For this sequential modification of PCNA (i.e., mono-ubiquitination, ISGylation, deubiquitination, ISGylation, and deISGyation), expression of the proteins responsible for the processes (i.e., RAD6-RAD18, ISG15 and EFP, USP10, and UBP43) is induced in order after UV irradiation, although it remains mysterious how the timely expression of each component of PCNA modifications is regulated. Moreover, knockdown of any of ISG15, EFP, and USP10 as well as Lys-to-Arg mutations of the ISGylation sites in PCNA lead to a dramatic increase in UV-mediated mutation frequency with a decrease in cell survival (Park et al., 2014) . Thus, PCNA ISGylation appears to play a crucial role in termination of error-prone TLS after DNA lesion bypass for escaping from excessive mutagenesis and thereby maintaining genome stability.