Localization of the Bloom syndrome helicase to punctate nuclear structuresand the nuclear matrix and regulation during the cell cycle: Comparison with the Werner's syndrome helicase

The Bloom (BLM) and Werner's (WRN) syndrome proteins may regulate recombina tion and DNA repair. Using a novel polyclonal antibody to human BLM, we det ected the 170-kda BLM antigen in wild-type but not Bloom syndrome cells. BL M was localized to punctate nuclear structures. The level of BLM but not WR N was 3.6 fold-higher in G(1)/S-synchronized fibroblasts than in G(0)-synch ronized fibroblasts. BLM-positive cells invariably expressed topoisomerase II alpha, whereas topoisomerase rip was expressed constitutively. Transfect ions of BLM deletion mutants demonstrated that the C-terminal domain of BLM mediated nuclear entry and the central helicase domain was necessary for p roducing the punctate pattern. By subcellular fractionation, BLM was found primarily in high-salt extracts of the nucleoplasm and the nuclear matrix a nd was enriched in G(1)/S-synchronized cells compared with G(0)-synchronize d cells. There was no interaction between BLM and WRN or topoisomerases II alpha and II beta in fibroblasts. These results demonstrate that BLM is tar geted to specific nuclear structures and that its expression is enhanced du ring cell growth. The known nucleolar localization of WRN, its invariant ex pression during the cell cycle, and the lack of interaction between BLM and WRN suggest distinct roles for BLM and WRN in processes such as DNA repair and recombination. Mol. Carcinog. 26:261-273, 1999. (C) 1999 Wiley-Liss. I nc.