ATR-dependent checkpoint modulates XPA nuclear import in response to UV irradiation

XPA localization to the nucleus depends on DNA damage and cell cycle stage.

“XPA has an N-terminal NLS (Figure 1), but its localization to the nucleus where it functions also depends on DNA damage and cell cycle stage [24], [32], [33].”

In unstressed cells, XPA is primarily restricted to the cytoplasm in G1 and S phases, but is present predominantly in the nucleus during G2 phase.

“However, more recent biochemical fractionation and immunofluorescence studies demonstrate that in unstressed cells XPA is primarily restricted to the cytoplasm of cells in G1 and S phases, though it is present predominately in the nucleus during the G2 phase [24], [25], [31], [32].”

There may be two stages of XPA nuclear import: one occurs naturally in the absence of DNA damage, and another massive transport (particularly in S-phase) is initiated by specific DNA damage signaling.

“These observations indicate that there maybe two stages of XPA nuclear import: one occurs naturally in the absence of DNA damage, and another, the massive transport (particularly in S-phase), is initiated by specific DNA damage signaling [24], [32], [33].”

The DNA damage-induced nuclear import of cytoplasmic XPA for NER during S-phase is a DNA damage checkpoint-dependent process mediated by ATR and p53.

“It was shown previously that the DNA damage-induced nuclear import of cytoplasmic XPA for NER during S-phase is a DNA damage checkpoint-dependent process mediated by ataxia telangiectasia and Rad3-related (ATR) protein and p53 tumor suppressor protein [24], [25], [33].”

Efficient nuclear import of XPA may occur as early as 1⁄2 hour post UV-irradiation.

“This suggests that the efficient nuclear import of XPA may occur as early as ½ hour post UV-irradiation, consistent with our previous report [24].”

The dependence of XPA-importin-α4 binding on ATR may occur via ATR regulation of other cytoplasmic factors involved in regulating the XPA-importin-α4 binding.

“This suggests that the dependence of the XPA-importin-α4 binding on ATR may occur via ATR regulation of other cytoplasmic factors involved in regulating the XPA-importin-α4 binding [24], [33].”

Immunofluorescence images may accurately depict XPA distribution within an individual cell but do not reflect the subcellular localization of XPA within a population of cells, whereas biochemical fractionation studies represent the average XPA distribution within millions of cells.

“Though immunofluorescence images may accurately depict XPA distribution within an individual cell they do not reflect the subcellular localization of XPA within a population of cells as do our biochemical fractionation studies which represent the average XPA distribution within millions of cells [24], [32], [33].”

Cisplatin, like UV irradiation, induces XPA nuclear import.

“we previously showed that cisplatin, like UV irradiation, induced XPA nuclear import [24], [32].”

ATR is responsible for phosphorylation of XPA after UV irradiation.

“ATR also seems to be responsible for XPA phosphorylation after UV radiations, although this modification is not required for XPA foci formation [100–102].”

XPA nuclear foci formation after UV irradiation depends on ATR.

“at nuclear foci after UV irradiation is dependent upon ATR.”

Phosphorylation of XPA after UV irradiation is not needed for XPA foci formation.

“ATR also seems to be responsible for XPA phosphorylation after UV radiations, although this modification is not required for XPA foci formation [100–102].”

Following UVB, XPA moves from cytoplasm to nucleus by binding XAB1.

“After UVB irradiation, XPA undergoes a dramatic cytoplasm-to-nucleus translocation via association with cytoplasmic GTPase XPA binding protein 1 (XAB1) (21, 22).”

The ATR checkpoint regulates the cellular activity of the nucleotide excision repair pathway.

“suggesting that ATR checkpoint modulates the cellular activity of NER pathway [187, 188]”

Upon UV exposure, ATR phosphorylates XPA at serine 196 and controls XPA entry into the nucleus.

“In response to UV damage, ATR phosphorylates XPA at Serine 196 and regulates the nuclear import of XPA”