A SWI/SNF subunit regulates chromosomal dissociation of structural maintenance complex 5 during DNA repair in plant cells

Abstract

Significance DNA repair is essential for accurate maintenance of genetic information in all types of cells, but what regulatory mechanisms act on the complexes involved in this process has been unclear. The structural maintenance complex 5/6 (SMC5/6) associates with chromosomes under normal conditions and is recruited to DNA double-strand breaks for repair. However, the mechanism by which this conserved complex becomes dissociated from its original chromosomal location during DNA repair is completely unknown. Here, we describe a mechanism in which the chromosomal dissociation of SMC5 in plant cells is mediated via SWI3B, a subunit of the SWI/SNF complex. Given that these proteins are evolutionarily conserved, the current findings may provide hints for the study of DNA repair in other species. DNA damage decreases genome stability and alters genetic information in all organisms. Conserved protein complexes have been evolved for DNA repair in eukaryotes, such as the structural maintenance complex 5/6 (SMC5/6), a chromosomal ATPase involved in DNA double-strand break (DSB) repair. Several factors have been identified for recruitment of SMC5/6 to DSBs, but this complex is also associated with chromosomes under normal conditions; how SMC5/6 dissociates from its original location and moves to DSB sites is completely unknown. In this study, we determined that SWI3B, a subunit of the SWI/SNF complex, is an SMC5-interacting protein in Arabidopsis thialiana. Knockdown of SWI3B or SMC5 results in increased DNA damage accumulation. During DNA damage, SWI3B expression is induced, but the SWI3B protein is not localized at DSBs. Notably, either knockdown or overexpression of SWI3B disrupts the DSB recruitment of SMC5 in response to DNA damage. Overexpression of a cotranscriptional activator ADA2b rescues the DSB localization of SMC5 dramatically in the SWI3B-overexpressing cells but only weakly in the SWI3B knockdown cells. Biochemical data confirmed that ADA2b attenuates the interaction between SWI3B and SMC5 and that SWI3B promotes the dissociation of SMC5 from chromosomes. In addition, overexpression of SMC5 reduces DNA damage accumulation in the SWI3B knockdown plants. Collectively, these results indicate that the presence of an appropriate level of SWI3B enhances dissociation of SMC5 from chromosomes for its further recruitment at DSBs during DNA damage in plant cells.