Figure 3

Signaling network at Ionizing Radiation Induced Nuclear Foci (IRIF). A. Early after IR, damaged DNA triggers ATM activation, which consequently phosphorylates H2AX near the break sites and MDC1. RNF8 is recruited to IRIF through binding to phosphorylated MDC1. RNF8 locally ubiquitinates H2A, H2AX and possibly other yet unidentified proteins ('X'). B. RNF8-dependent ubiquitin conjugation is required for recruitment of 53BP1 and BRCA1. BRCA1 is present in a protein complex containing BARD1, Abraxas/CCDC98, BRCC36 and RAP80. Ubiquitin binding by RAP80 is required for BRCA1 recruitment. However, the nature of the ubiquitinated substrate RAP80 binds to (histones or other proteins), is still unclear. In addition to BRCA1, ubiquitination allows recruitment of 53BP1 (which binds to methylated H3/H4) through a yet unknown mechanism. C. IRIF disassembly may involve dephosphorylation of γH2AX by PP2A. The DUB USP3 is a candidate to remove conjugated ubiquitin at IRIF.