Rad54B Targeting to DNA Double-Strand Break Repair Sites Requires Complex Formation with S100A11

Abstract

S100A11 is involved in a variety of intracellular activities such as growth regulation and differentiation. To gain more insight into the physiological role of endogenously expressed S100A11, we used a proteomic approach to detect and identify interacting proteins in vivo. Hereby, we were able to detect a specific interaction between S100A11 and Rad54B, which could be confirmed under in vivo conditions. Rad54B, a DNA-dependent ATPase, is described to be involved in recombinational repair of DNA damage, including DNA double-strand breaks (DSBs). Treatment with bleomycin, which induces DSBs, revealed an increase in the degree of colocalization between S100A11 and Rad54B. Furthermore, S100A11/ Rad54B foci are spatially associated with sites of DNA DSB repair. Furthermore, while the expression of p21 WAF1/CIP1 was increased in parallel with DNA damage, its protein level was drastically down-regulated in damaged cells after S100A11 knockdown. Down-regulation of S100A11 by RNA interference also abolished Rad54B targeting to DSBs. Additionally, S100A11 down-regulated HaCaT cells showed a restricted proliferation capacity and an increase of the apoptotic cell fraction. These observations suggest that S100A11 targets Rad54B to sites of DNA DSB repair sites and identify a novel function for S100A11 in p21-based regulation of cell cycle. INTRODUCTIONProteins may exist in several complexes in a spatial and temporal manner to accomplish distinct functions. Analyses of the interacting partners will provide a strong insight into the physiological role of a particular factor. Therefore, it is essential to identify ideally all interacting partners of proteins in vivo to precisely be able to define their biological function. The investigation of protein complexes of solely endogenously expressed proteins avoid the tendency to detect false positive protein-protein interactions of examinations performed in vitro. A multitude of proteins are involved in both the detection and the repair of DNA damages. It is conceivable that some protein complexes involved in these processes are not yet discovered. A severe form of DNA damage that threaten the integrity of the genome are DNA double-strand breaks (DSBs). DSBs can lead to cell cycle arrest or illegitimate DNA rearrangements that can contribute to cell dysfunction, cell death, or carcinogenesis (Hoeijmakers, 2001). Homologous recombination is a major DNA repair pathway by which DSBs are repaired (Lettier et al., 2006).For the identification of specific interacting proteins of S100A11 (S100C, calgizzarin) we used in the present study a proteomic approach comprising mass spectrometry and immunological techniques. S100A11 belongs to the group of S100 proteins that are considered as multitasking proteins involved in several biological processes such as the Ca 2ϩ signaling network, cell growth and motility, cell cycle progression, transcription, and cell differentiation (Schafer and Heizmann, 1996;Donato, 2001;Eckert et al., 2004). It has been proposed that the S100 proteins are involved in...

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