The Bloom syndrome helicase is a substrate of the mitotic Cdc2 kinase

Bloom syndrome (BS) is a rare human autosomal recessive disorder characterized by marked genetic instability associated with greatly increased predisposition to a wide range of cancers affecting the general population. BS arises through mutations in both copies of the BLM gene which encodes a 3'-5' DNA helicase identified as a member of the RecQ family. Several studies support a major role for BLM in the cellular response to DNA damage and stalled replication forks. However, the specific function(s) of BLM remain(s) unclear. The BLM protein is strongly expressed and phosphorylated during mitosis, but very little information is available about the origin and the significance of this phosphorylation. We show here that ATM kinase provides only a limited contribution to the mitotic phosphorylation of BLM. We also demonstrate that BLM is directly phosphorylated at multiple sites in vitro by the mitotic cdc2 kinase, and identify two new sites of mitotic BLM phosphorylation: Ser-714 and Thr-766. Our results identify BLM helicase as a new substrate for cdc2, which may have potential physiological implications for the role of BLM in mitosis.