hnRNP A1 relocalization to the stress granules reflects a role in the stress response.

hnRNP A1 is a nucleocytoplasmic shuttling protein that is involved in many aspects of mRNA metabolism. We have previously shown that activation of the p38 stress-signaling pathway in mammalian cells results in both hyperphosphorylation and cytoplasmic accumulation of hnRNP A1, affecting alternative splicing regulation in vivo. Here we show that the stress-induced cytoplasmic accumulation of hnRNP A1 occurs in discrete phase-dense particles, the cytoplasmic stress granules (SGs). Interestingly, mRNA-binding activity is required for both phosphorylation of hnRNP A1 and localization to SGs. We also show that these effects are mediated by the Mnk1/2 protein kinases that act downstream of p38. Finally, depletion of hnRNP A1 affects the recovery of cells from stress, suggesting a physiologically significant role for hnRNP A1 in the stress response. Our data are consistent with a model whereby hnRNP A1 recruitment to SGs involves Mnk1/2-dependent phosphorylation of mRNA-bound hnRNP A1.