Diacylglycerol kinase (DGK) η translocates from the cytoplasm to punctate vehicles via osmotic shock. Apoptosis signal-regulating kinase (ASK) 3 (MAP kinase kinase kinase (MAPKKK) 15) is also reported to respond to osmotic shock. Therefore, in the present study, we examined the subcellular localization of DGKη and ASK3 expressed in COS-7 cells under osmotic stress. We found that DGKη was almost completely colocalized with ASK3 in punctate structures in response to osmotic shock. In contrast, DGKδ, which is closely related to DGKη structurally, was not colocalized with ASK3, and DGKη failed to colocalize with another MAPKKK, C-Raf, even under osmotic stress. The structures in which DGKη and ASK3 localized were not stained with stress granule makers. Notably, DGKη strongly interacted with ASK3 in an osmotic shock-dependent manner. These results indicate that DGKη and ASK3 undergo osmotic shock-dependent colocalization and associate with each other in specialized structures.
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