Abstract
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, stabilizing 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 increased association of
HuR with pp32 and APRIL through protein–protein interactions
and that these ligands partially colocalize with HuR in cytoplasmic
foci. HuR associations with the hnRNP complex also increase,
but through RNA links. CRM1 coimmunoprecipitates with HuR only
after heat shock, and nuclear export of HuR becomes sensitive
to leptomycin B, an inhibitor of CRM1. Export after heat shock
requires the same domains of HuR (HNS and RRM3) that are essential
for binding pp32 and APRIL. In situ hybridization and
coimmunoprecipitation experiments show that LMB treatment blocks
both hsp70 mRNA nuclear export and its cytoplasmic interaction
with HuR after heat shock. Together, our results argue that
upon heat shock, HuR switches its export pathway to that of
its ligands pp32 and APRIL, which involves the nuclear export
factor CRM1. HuR and its ligands may be instrumental in the
nuclear export of heat-shock mRNAs.