Protein ligands mediate the CRM1-dependent export of HuR in response to heat shock

AU-rich elements (AREs) located in the 3' UTRs of the messenger RNAs (mRNAs ) of many mammalian early response genes promote rapid mRNA turnover. HuR, an RRM-containing RNA-binding protein, specifically interacts with AREs, st abilizing these mRNAs. HuR is primarily nucleoplasmic, but shuttles between the nucleus and the cytoplasm via a domain called HNS located between RRM2 and RRM3. We recently showed that HuR interacts with two protein ligands, pp32 and APRIL, which are also shuttling proteins, but rely on NES domains recognized by CRM1 for export. Here we show that heat shock induces increas ed association of HuR with pp32 and APRIL through protein-protein interacti ons and that these ligands partially colocalize with HuR in cytoplasmic foc i. HuR associations with the hnRNP complex also increase, but through RNA l inks. CRM1 coimmunoprecipitates with HuR only after heat shock, and nuclear export of HuR becomes sensitive to leptomycin B, an inhibitor of CRM1. Exp ort after heat shock requires the same domains of HuR (HNS and RRM3) that a re essential for binding pp32 and APRIL. In situ hybridization and coimmuno precipitation experiments show that LMB treatment blocks both hsp70 mRNA nu clear export and its cytoplasmic interaction with HuR after heat shock. Tog ether, our results argue that upon heat shock, HuR switches its export path way to that of its ligands pp32 and APRIL, which involves the nuclear expor t factor CRM1. HuR and its ligands may be instrumental in the nuclear expor t of heat-shock mRNAs.