Recruitment of the RNA helicase RHAU to stress granules via a unique RNA-binding domain.

In response to environmental stress, the translation machinery of cells is reprogrammed. The majority of actively translated mRNAs are released from polysomes and driven to specific cytoplasmic foci called stress granules (SGs) where dynamic changes in protein-RNA interaction determine the subsequent fate of mRNAs. Here we show that the DEAH box RNA helicase RHAU is a novel SG-associated protein. Although RHAU protein was originally identified as an AU-rich element-associated protein involved in urokinase-type plasminogen activator mRNA decay, it was not clear whether RHAU could directly interact with RNA. We have demonstrated that RHAU physically interacts with RNA in vitro and in vivo through a newly identified N-terminal RNA-binding domain, which was found to be both essential and sufficient for RHAU localization in SGs. We have also shown that the ATPase activity of RHAU plays a role in the RNA interaction and in the regulation of protein retention in SGs. Thus, our results show that RHAU is the fourth RNA helicase detected in SGs, after rck/p54, DDX3, and eIF4A, and that its association with SGs is dynamic and mediated by an RHAU-specific RNA-binding domain.

The Bloom's syndrome helicase (BLM) interacts physically and functionally with p12, the smallest subunit of human DNA polymerase delta.

Bloom's syndrome (BS) is a cancer predisposition disorder caused by mutation of the BLM gene, encoding a member of the RecQ helicase family. Although the phenotype of BS cells is suggestive of a role for BLM in repair of stalled or damaged replication forks, thus far there has been no direct evidence that BLM associates with any of the three human replicative DNA polymerases. Here, we show that BLM interacts specifically in vitro and in vivo with p12, the smallest subunit of human POL delta (hPOL delta). The hPOL delta enzyme, as well as the isolated p12 subunit, stimulates the DNA helicase activity of BLM. Conversely, BLM stimulates hPOL delta strand displacement activity. Our results provide the first functional link between BLM and the replicative machinery in human cells, and suggest that BLM might be recruited to sites of disrupted replication through an interaction with hPOL delta. Finally, our data also define a novel role for the poorly characterized p12 subunit of hPOL delta.

The Bloom's syndrome helicase interacts directly with the human DNA mismatch repair protein hMSH6.

Bloom's syndrome (BS) is a rare genetic disorder characterised by genome instability and cancer susceptibility. BLM, the BS gene product, belongs to the highly-conserved RecQ family of DNA helicases. Although the exact function of BLM in human cells remains to be defined, it seems likely that BLM eliminates some form of homologous recombination (HR) intermediate that arises during DNA replication. Similarly, the mismatch repair (MMR) system also plays a crucial role in the maintenance of genomic stability, by correcting DNA errors generated during DNA replication. Recent evidence implicates components of the MMR system also in HR repair. We now show that hMSH6, a component of the heterodimeric mismatch recognition complex hMSH2/hMSH6 (hMutS(alpha)), interacts with the BLM protein both in vivo and in vitro. In agreement with these findings, BLM and hMSH6 co-localise to discrete nuclear foci following exposure of the cells to ionising radiation. However, the purified recombinant MutS(alpha) complex does not affect the helicase activity of BLM in vitro. As BLM has previously been shown to interact with the hMLH1 component of the hMLH1/hPMS2 (hMutL(alpha)) heterodimeric MMR complex, our present findings further strengthen the link between BLM and processes involving correction of DNA mismatches, such as in the regulation of the fidelity of homologous recombination events.