Karow J K, Newman R H, Freemont P S, Hickson I D
Imperial Cancer Research Fund Laboratories, Institute of Molecular Medicine, University of Oxford, John Radcliffe Hospital, Oxford OX3 9DS, UK.
Curr Biol. 1999 Jun 3;9(11):597-600. doi: 10.1016/s0960-9822(99)80264-4.
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.
布卢姆综合征是一种隐性人类遗传病，与多种癌症的高发率相关。布卢姆综合征基因产物BLM属于DNA解旋酶的RecQ亚家族，是人类细胞维持基因组稳定性所必需的——特别是抑制姐妹染色单体之间的相互交换。我们结合尺寸排阻色谱法和电子显微镜以及无参考图像处理技术，研究了BLM的四级结构。我们发现BLM形成六聚体环状结构，其总直径约为13 nm，围绕着一个直径约为3.5 nm的中心孔。还检测到一种边长约为11 nm、孔直径约为4 nm的四重对称方形结构，它可能代表一种不同的寡聚体形式或六聚体形式的侧视图。色谱研究表明，大多数具有酶活性的BLM的表观分子量> 700 kDa，这与BLM的寡聚体结构一致。这为真核细胞来源的寡聚体环状解旋酶提供了首次结构分析。这些结果对BLM的作用机制具有启示意义，并表明其他RecQ家族解旋酶，包括与沃纳综合征相关的WRN蛋白，也可能采用环状结构。
