Importin-beta family members, which shuttle between the nucleus and the 
cytoplasm, are essential for nucleocytoplasmic transport of macromolecules. We 
attempted to explore whether importin-beta family proteins change their cellular 
localization in response to environmental change. In this report, we show that 
transportin (TRN) was minimally detected in cytoplasmic processing bodies 
(P-bodies) under normal cell conditions but largely translocated to stress 
granules (SGs) in stressed cells. Fluorescence recovery after photobleaching 
analysis indicated that TRN moves rapidly in and out of cytoplasmic granules. 
Depletion of TRN greatly enhanced P-body formation but did not affect the number 
or size of SGs, suggesting that TRN or its cargo(es) participates in cellular 
function of P-bodies. Accordingly, TRN associated with tristetraprolin (TTP) and 
its AU-rich element (ARE)-containing mRNA substrates. Depletion of TRN increased 
the number of P-bodies and stabilized ARE-containing mRNAs, as observed with 
knockdown of the 5'-3' exonuclease Xrn1. Moreover, depletion of TRN retained TTP 
in P-bodies and meanwhile reduced the fraction of mobile TTP to SGs. Therefore, 
our data together suggest that TRN plays a role in trafficking of TTP between 
the cytoplasmic granules and whereby modulates the stability of ARE-containing 
mRNAs.