Identification of Novel Stress Granule Components That Are Involved in Nuclear Transport

Background: Importin-alpha 1 belongs to a subfamily of nuclear transport adaptors and participates in diverse cellular functions. Best understood for its role in protein transport, importin-alpha 1 also contributes to other biological processes. For instance, arsenite treatment causes importin-alpha 1 to associate with cytoplasmic stress granules (SGs) in mammalian cells. These stress-induced compartments contain translationally arrested mRNAs, small ribosomal subunits and numerous proteins involved in mRNA transport and metabolism. At present, it is not known whether members of all three importin-alpha subfamilies locate to SGs in response to stress. Results: Here, we demonstrate that the oxidant diethyl maleate (DEM), arsenite and heat shock, promote the formation of cytoplasmic SGs that contain nuclear transport factors. Specifically, importin-alpha 1, alpha 4 and alpha 5, which belong to distinct subfamilies, and importin-beta 1 were targeted by all of these stressors to cytoplasmic SGs, but not to P-bodies. Importin-alpha family members have been implicated in transcriptional regulation, which prompted us to analyze their ability to interact with poly(A)-RNA in growing cells. Our studies show that importin-alpha 1, but not alpha 4, alpha 5, importin-beta 1 or CAS, associated with poly(A)-RNA under nonstress conditions. Notably, this interaction was significantly reduced when cells were treated with DEM. Additional studies suggest that importin-alpha 1 is likely connected to poly(A)-RNA through an indirect interaction, as the adaptor did not bind homopolymer RNA specifically in vitro. Significance: Our studies establish that members of three importin-alpha subfamilies are bona fide SG components under different stress conditions. Furthermore, importin-alpha 1 is unique in its ability to interact with poly(A)-RNA in a stress-dependent fashion, and in vitro experiments indicate that this association is indirect. Collectively, our data emphasize that nuclear transport factors participate in a growing number of cellular activities that are modulated by stress.