Ssd1 is a nucleo-cytoplasmic shuttling protein with multiple roles in the life of its target mRNAs, from transcription and maturation to polarized localization, translation and decay. Interaction between Ssd1 and its target mRNAs initiates during transcription in the nucleus and, depending on the extent of its phosphorylation by the RAM kinase Cbk1, results in subsequent localization of the mRNAs either to the bud for translation or to stress granules (SG) and P-bodies (PB) for mRNA storage or decay. Ssd1 lacking a nuclear localization signal (Ssd1(417-427)11A), and thus excluded from nuclear entry, forms large cytoplasmic aggregates that colocalize with Hsp104 and Sis1, markers of “Insoluble Protein Deposition” sites (IPOD). Overexpression of Ssd1(417-427)11A enhances the mild toxicity of wild type Ssd1. Activation of a separate cryptic NLS within Ssd1 or deletion of its prion-like domain suppresses IPOD formation of Ssd1(417-427)11A. Thus, nuclear shuttling of Ssd1 is required to prevent prion-like, non-productive aggregate formation.
The human RNA-binding protein Fused in Sarcoma (FUS) is implicated in ALS, a fatal neurodegenerative disease. While not a homolog of Ssd1, FUS shares domain structures with Ssd1 and, when expressed in yeast, localizes in the same pattern as Ssd1, including formation of IPODs upon overexpression. Accordingly, our studies on Ssd1 provide a framework for understanding the molecular basis of FUS structure. We use these observations to obtain insight into processes in which FUS may be involved in mammalian cells, particularly neurons. This could help to understand the underlying causes of neurodegenerative diseases, in which mutated RNA-binding proteins play a central role.