(A) Colocalization of Spartan with PCNA requires both its PIP box and UBZ domain. U2OS cells stably expressing SFB-SpartanWT, SpartanΔPIP, or SpartanΔUBZ were irradiated with UV through 5-micron filters and subsequently analyzed by immunofluorescence after 1 hr as in .
(B) Effects of Rad18 and Usp1 knockdown on Spartan focus formation. Cells expressing SFB-SpartanWT were transfected with control, Rad18, or Usp1 siRNA. Three days after transfection, cells were irradiated with UV and analyzed as in (A). Quantification of the percentage of PCNA-positive cells with Spartan foci is plotted. **: a p value of < 0.005; error bars: SD, >750 cells per condition.
(C) Spartan’s PIP box and UBZ domain are important for normal UV resistance. Parental U2OS cells or derivatives stably expressing SFB-SpartanWT, SpartanΔPIP, or SpartanΔUBZ were transfected with control, Spartan-1 (targeting the coding sequence; CDS), or Spartan-2 (targeting the 3′UTR) siRNA. Cell survival was analyzed after UV irradiation. Error bars: SD, n ≥ 2.