Structural basis of bloom syndrome (BS) causing mutations in the BLM helicase domain

Background: Bloom syndrome (BS) is characterized by mutations within the BL M gene. The Bloom syndrome protein (BLM) has similarity to the RecQ subfami ly of DNA helicases, which contain seven conserved helicase domains and sha re significant sequence and structural similarity with the Rep and PcrA DNA helicases. We modeled the three-dimensional structure of the BLM helicase domain to analyze the structural basis of BS-causing mutations. Materials and Methods: The sequence alignment was performed for RecQ DNA he licases and Rep and PcrA helicases. The crystal structure of PcrA helicase (PDB entry 3PJR) was used as the template for modeling the BLM helicase dom ain. The model was used to infer the function of BLM and to analyze the eff ect of the mutations. Results: The structural model with good stereo chemistry of the BLM helicas e domain contains two subdomains, 1A and 2A. The electrostatic potential of the model is highly negative over most of the surface, except for the clef t between subdomains 1A and 2A which is similar to the template protein. Th e ATP-binding site is located inside the model between subdomains 1A and 2A ; whereas, the DNA-binding region is situated at the surface cleft, with po sitive potential between 1A and ZA. Conclusions: The three-dimensional structure of the BLM helicase domain was modeled and applied to interpret Bs-causing mutations. The mutation I841T is likely to weaken DNA binding, while the mutations C891R, C901Y, and Q672 R presumably disturb the ATP binding. In addition, other critical positions are discussed.