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**9** 

*USA* 

**Role of RPA Proteins in** 

*Idaho State University,* 

**Radiation Repair and Recovery** 

Patrick E. Gygli, J. Scott Lockhart and Linda C. DeVeaux

Repair of radiation-induced DNA damage requires a complex series of protein interactions. Single-stranded DNA (ssDNA) binding proteins (RPA/SSB) have long been known to play a passive, protective role in DNA replication and repair, by coating ssDNA. Recent evidence, however, suggests a much more active function for these ubiquitous proteins. In this review, we provide a summary of the background of ssDNA binding proteins, and incorporate recent experimental observations into current models of dynamic interactions between these proteins and cellular DNA repair enzymes. These results point to a highly choreographed, interactive mechanism, with RPA/SSB at the center of the coordination.

There are two primary classes of ssDNA binding proteins, which share secondary and tertiary structural features, but are distinct in their quaternary structures. The eukaryotic Replication Protein A class (RPAs) consists of heterotrimeric proteins while the bacterial class (SSBs) consists of a range of homo-multimers. Members of the archaeal domain may possess either the RPA or SSB types, or combinations unique to this domain, illustrated in Figure 1 (Kerr, et

SSBs/RPAs are primarily identified by the presence of a structurally conserved oligonucleotide/oligosaccharide binding motif (OB-fold) (Kerr, et al., 2003; Richard, et al., 2009; Shereda, et al., 2008; Theobald, et al., 2003; Wold, 1997). The canonical OB-fold consists of five structurally conserved β-strands, forming two β-sheets, and their inter-spaced variable loops, which form a tertiary flattened β-barrel. The non-specific binding of ssDNA occurs on the surface of the β-barrel in a cleft between the variable loops. The binding of nucleotides is mediated through stacking interactions with aromatic residues and packing interactions with hydrophobic residues. Binding to the phosphodiester backbone also occurs through electrostatic interactions. The OB-folds have a binding polarity that specifies the

The majority of bacterial and mitochondrial SSBs function as homotetramers, with a single OB-fold per monomer and thus four OB-folds per complex. For members of the *Deinococcus/Thermus* branch, the SSB functions as a homodimer and maintains the theme of four OB-folds per complex by having two non-identical OB-folds per monomer (Bernstein, et al., 2004; Eggington, et al., 2004; Filipkowski, et al., 2006; Filipkowski & Kur, 2007;

al., 2003; Lin, et al., 2008; Richard, et al., 2009; Shereda, et al., 2008; Wold, 1997).

orientation on the bound ssDNA (Shereda, et al., 2008; Theobald, et al., 2003).

**1. Introduction** 

**2. Structure** 
