SOSS Complexes Participate in the Maintenance of Genomic Stability

Abstract

SUMMARY Proteins that bind to single-stranded DNA (ssDNA) are essential for DNA replication, recombinational repair and maintenance of genomic stability. Here we describe the characterization of a ssDNA-binding heterotrimeric complex, SOSS (Sensor Of Single-stranded DNA) in human, which consists of human SSB homologues hSSB1/2 (SOSS-B1/2), INTS3 (SOSS-A) and a previously uncharacterized protein C9orf80 (SOSS-C). We have shown that SOSS-A serves as a central adaptor required not only for SOSS complex assembly and stability, but also for facilitating the accumulation of SOSS complex to DNA ends. Moreover, SOSS-depleted cells display increased IR sensitivity, defective G2/M checkpoint and impaired HR repair. Thus, our study defines a pathway involving the sensing of ssDNA by SOSS complex and suggests that this SOSS complex is likely involved in the maintenance of genome stability.

Single strand DNA binding proteins 1 and 2 protect newly replicated telomeres

“…In mSSB1 ∆/∆ MEFs, mSSB1 protein was undetectable, confirming that we successfully generated a mSSB1-null allele ( Figure 1B and 1C). The localization of mSSB2 to IR-induced DNA damage foci was not affected upon mSSB1 deletion, in agreement with previous reports [17,18,20] (Figure 1C). Interestingly, mSSB2 protein level was upregulated in the absence of mSSB1 ( Figure 1B).…”

“…Although RPA recruitment to damaged DNA requires both CtIP and the MRN complex, localization of SOSS1 to damaged DNA in S/ G2 phase requires only MRN [17]. In addition, SOSS1 forms damage foci independent of RPA [16,17], and unlike RPA, hSSB1 does not localize to DNA replication foci [17]. These results reinforce the notion that RPA and SOSS1 localize to DSBs via distinct mechanisms, and likely play independent functions at DSBs.…”

“…Previous studies suggest that hSSB1 and hSSB2 both form complexes with INTS3 and possess overlapping functions in the DNA damage response (DDR) [17,18]. The elevated expression of mSSB2 observed when mSSB1 is deleted ( Figures 1C, 2B, 2C and 3F) suggests that upregulation of mSSB2 likely compensates for mSSB1 loss, thereby complicating the analysis of mSSB1 function.…”

“…Notably, long trains of end-to-end fused chromosomes, a hallmark of TRF2 loss, were rarely observed in the setting of mSSB1 deficiency. As activation of the ATM kinase is negatively impacted when hSSB1 is depleted [17,18,20], we monitored the Chk2 phosphorylation status in mSSB1 ∆/∆ MEFs treated with shTRF2. Chk2 phosphorylation in mSSB1 ∆/∆ MEFs appears similar to that observed in WT MEFs, although this analysis is complicated by the fact that mSSB2 levels increased following mSSB1 deletion and therefore any decline in Chk2 phosphorylation due to mSSB1 loss will not be evident ( Figure 2B).…”

“…Telomeres consist of TTAGGG repetitive sequences that terminate in a 3′ ss G-rich overhang, and invasion of the 3′ end into duplex regions of the telomere is postulated to be important to protect DNA ends from being recognized by RPA, which would INTS3 and C9orf80, termed sensor of ssDNA complex 1 and 2 (SOSS1/2) [17][18][19][20]. Although RPA recruitment to damaged DNA requires both CtIP and the MRN complex, localization of SOSS1 to damaged DNA in S/ G2 phase requires only MRN [17]. In addition, SOSS1 forms damage foci independent of RPA [16,17], and unlike RPA, hSSB1 does not localize to DNA replication foci [17].…”