Oligomeric ring structure of the Bloom's syndrome helicase

Bloom's syndrome is a recessive human genetic disorder associated with an elevated incidence of many types of cancer. The Bloom's syndrome gene product, BLM, belongs to the RecQ subfamily of DNA helicases and is required for the maintenance of genomic stability in human cells - in particular, the suppression of reciprocal exchanges between sister chromatids. We have investigated the quaternary structure of BLM using a combination of size-exclusion chromatography and electron microscopy with reference-free image processing. We found that BLM forms hexameric ring structures with an overall diameter of approximately 13 nm surrounding a central hole of approximately 3.5 nm diameter. A fourfold symmetric square form with approximately 11 nm sides and a hole of approximately 4 nm diameter was also detected, which might represent a distinct oligomeric species or a side view of the hexameric form. Chromatography studies indicated that the majority of enzymatically active BLM has an apparent molecular mass of > 700 kDa, which is consistent with an oligomeric structure for BLM. This provides the first structural analysis of an oligomeric ring helicase of eukaryotic cellular origin. These results have implications for the mechanism of action of BLM and suggest that other RecQ family helicases, including the WRN protein associated with Werner's syndrome, might also adopt ring structures.