Xeroderma Pigmentosa Group A (XPA), Nucleotide Excision Repair and Regulation by ATR in Response to Ultraviolet Irradiation

Abstract

The sensitivity of Xeroderma pigmentosa (XP) patients to sunlight has spurred the discovery and genetic and biochemical analysis of the eight XP gene products (XPA-XPG plus XPV) responsible for this disorder. These studies also have served to elucidate the nucleotide excision repair (NER) process, especially the critical role played by the XPA protein. More recent studies have shown that NER also involves numerous other proteins normally employed in DNA metabolism and cell cycle regulation. Central among these is ataxia telangiectasia and Rad3-related (ATR), a protein kinase involved in intracellular signaling in response to DNA damage, especially DNA damage-induced replicative stresses. This review summarizes recent findings on the interplay between ATR as a DNA damage signaling kinase and as a novel ligand for intrinsic cell death proteins to delay damage-induced apoptosis, and on ATR's regulation of XPA and the NER process for repair of UV-induced DNA adducts. ATR's regulatory role in the cytosolic-to-nuclear translocation of XPA will be discussed. In addition, recent findings elucidating a non-NER role for XPA in DNA metabolism and genome stabilization at ds-ssDNA junctions, as exemplified in prematurely aging progeroid cells, also will be reviewed.

Keywords: ATR-XPA interaction; Ataxia telangiectasia and Rad3-related (ATR); Non-NER functions of XPA; Nucleotide excision repair (NER); Regulation of XPA by ATR; UV irradiation; XPA cytoplasmic interactions and functions; XPA nuclear import; XPA phosphorylation and acetylation; Xeroderma pigmentosum Group A (XPA).

Figure 1. Possible alternative folding conformations of ATR-H vs. ATR-L.

There currently are no 3-demensional structures described for ATR. The diagrammatic representations presented here are based on the predictions of Hilton et al. for the N-terminal regions of ATR-H vs. ATR-L . The N-terminal region of ATR-H, which has the cis-Pro⁴²⁹ isomer and an unphosphorylated Ser⁴²⁸, is accessible to both tBid binding and to Flag antibody binding. Thus, ATR-H is presented in an open conformation. In ATR-L, which contains a phosphorylated Ser⁴²⁸ and a trans-Pro⁴²⁹, the BH3 domain is inaccessible to tBid binding as is the Flag tag . Thus, ATR-L is drawn with a folded N-terminal region. The N-terminus of ATR contains the ATRIP binding site; binding of ATRIP leads to activation of the ATR kinase via interaction with the C- terminal PIKK region ^–. Although speculative, the lower two diagrams of ATR-L illustrate this folding of the N-terminal region onto the C-terminal region, perhaps mediated by ATRIP binding.

Figure 2. Normal and UV-induced redistribution during progression through the cell cycle in p53-competent human cells.

This model is based on the studies of Li et al. ^– In non-damaged cells in the G1 phase XPA (X) is mostly located in the cytosol, likely bound to cXBP (C), a hypothetical cytosolic XPA sequestration protein. Exposure of G1 cells to UV does not change this distribution. Likewise, in S phase cells XPA is mostly cytosolic; however, UV exposure induces a release of XPA from cXBP and a translocation of XPA into the nucleus. This XPA nuclear translocation in S phase requires the importin α4 transport protein and is ATR kinase- and p53-dependent in p53-competent cells. XPA is primarily located in the nucleus in G2 phase cells, transported there via importin α7 in a process independent of UV exposure. The XPA redistributes to the cytosol during the M-G1 phase transition and reassociates with cXBP.