BLOOM'S HELICASE - ''GUARDIAN" OF THE GENOME

Bloom's helicase (BLM) belongs to the RecQ helicase family. BLMgene mutations are the cause of the syndrome known as Bloom's syndrome. BLM helicase exhibits ATPase activity and the ability to unwinding of DNA molecules in the direction of 3' -> 5' in a manner dependent on energy from ATP hydrolysis and presence of Mg2+ ions. This helicase also has a high efficiency in unwinding of DNA molecules consisting of more than two strands or containing spatial barriers. Another feature is the ability of BLM helicase to join short single-stranded DNA fragments into double-strand in a manner independent of ATP and full complementarity of linked fragments. RPA protein and the Fanconi's anemia proteins play an important role in the functioning of BLM helicase. RPA protein provides the correct location of BLM helicase and increases processivity of DNA unwinding. BLM helicase forms the BTB complex, whose elements are topoisomerase III alpha and BLAP75 protein (hRMI1). The main role of the BTB complex is a control of the process of homologous recombination and inhibition of sister chromatid exchange by blocking the unnecessary or incorrect recombination. BLM helicase is able to destabilize and remove Holliday's structure, double Holliday's structure and other intermediate products of recombination. It interacts with proteins involved in the process of replication and DNA repair. It provides correct process of replication by stabilizing and recreating of forks when there are stalled by DNA damage. It is involved in alternative lengthening of telomeres in cells with nonfunctional telomerase. BLM helicase is localized within PML nuclear bodies together with other proteins that are involved in DNA repair and moves with them into areas that contain DNA damage. Because of the involvement in the processes related to the maintenance of genomic stability, such as recombination, replication and DNA repair, BLM helicase belongs to the group of proteins known as "guardians" of the genome (caretakers of the genome).