TIA-1 is an RNA-binding protein that promotes the assembly of stress granules (SGs), which are cytoplasmic inclusions that contain stalled translation initiation complexes under stress. The RNA recognition motifs of TIA-1 are linked to a glutamine-rich prion-related domain (PRD). Truncation mutants lacking the PRD domain do not induce spontaneous SGs and are not recruited to arsenite-induced SGs, whereas the PRD forms aggregates that are recruited to SGs in low-level-expressing cells but prevent SG assembly in high-level-expressing cells. The PRD of TIA-1 exhibits prion-like characteristics, including concentration-dependent aggregation, resistance to protease digestion, sequestration of HSP27, HSP40, and HSP70, and induction of HSP70. Substitution of the PRD with the aggregation domain of a yeast prion, SUP35-NM, reconstitutes SG assembly, confirming that a prion domain can mediate SG assembly. Mouse embryonic fibroblasts (MEFs) lacking TIA-1 exhibit impaired ability to form SGs, although they exhibit normal phosphorylation of eukaryotic initiation factor (eIF2α) in response to arsenite. These results reveal that prion-like aggregation of TIA-1 regulates SG formation downstream of eIF2α phosphorylation in response to stress. TIA-1 and TIAR are modular proteins composed of three amino-terminal RNA recognition motifs and a carboxy-terminal glutamine-rich motif that is structurally related to prion protein. Overexpressed TIA-1 induces SG formation and represses reporter gene expression, whereas the isolated PRD of TIA-1 forms cytoplasmic microaggregates that sequester endogenous TIA-1 and TIAR and promote expression of cotransfected reporter genes. These data suggest that the PRD is capable of self-oligomerization and that sequestration of full-length TIA proteins prevents their repressive effects on reporter gene expression. Given that the aggregation of the TIA proteins into SGs is reversible in cells exposed to a sublethal stress, but irreversible in cells exposed to a lethal stress, factors that regulate TIA aggregation may thus influence cell survival. Prions are infectious proteins that can exist in at least two interchangeable forms that exhibit different physical properties, especially solubility. In animals, prion protein can assume two conformations: the soluble conformer PrP^C is rich in α-helices, whereas the insoluble scrapie conformer PrP^Sc is rich in β-pleated sheets. PrP^Sc has the capacity to convert PrP^C into PrP^Sc, a requisite feature of infectious propagation. In yeast, several proteins possessing prion-related domains serve as protein-based epigenetic elements that confer distinct, herTIA-1 is an RNA-binding protein that promotes the assembly of stress granules (SGs), which are cytoplasmic inclusions that contain stalled translation initiation complexes under stress. The RNA recognition motifs of TIA-1 are linked to a glutamine-rich prion-related domain (PRD). Truncation mutants lacking the PRD domain do not induce spontaneous SGs and are not recruited to arsenite-induced SGs, whereas the PRD forms aggregates that are recruited to SGs in low-level-expressing cells but prevent SG assembly in high-level-expressing cells. The PRD of TIA-1 exhibits prion-like characteristics, including concentration-dependent aggregation, resistance to protease digestion, sequestration of HSP27, HSP40, and HSP70, and induction of HSP70. Substitution of the PRD with the aggregation domain of a yeast prion, SUP35-NM, reconstitutes SG assembly, confirming that a prion domain can mediate SG assembly. Mouse embryonic fibroblasts (MEFs) lacking TIA-1 exhibit impaired ability to form SGs, although they exhibit normal phosphorylation of eukaryotic initiation factor (eIF2α) in response to arsenite. These results reveal that prion-like aggregation of TIA-1 regulates SG formation downstream of eIF2α phosphorylation in response to stress. TIA-1 and TIAR are modular proteins composed of three amino-terminal RNA recognition motifs and a carboxy-terminal glutamine-rich motif that is structurally related to prion protein. Overexpressed TIA-1 induces SG formation and represses reporter gene expression, whereas the isolated PRD of TIA-1 forms cytoplasmic microaggregates that sequester endogenous TIA-1 and TIAR and promote expression of cotransfected reporter genes. These data suggest that the PRD is capable of self-oligomerization and that sequestration of full-length TIA proteins prevents their repressive effects on reporter gene expression. Given that the aggregation of the TIA proteins into SGs is reversible in cells exposed to a sublethal stress, but irreversible in cells exposed to a lethal stress, factors that regulate TIA aggregation may thus influence cell survival. Prions are infectious proteins that can exist in at least two interchangeable forms that exhibit different physical properties, especially solubility. In animals, prion protein can assume two conformations: the soluble conformer PrP^C is rich in α-helices, whereas the insoluble scrapie conformer PrP^Sc is rich in β-pleated sheets. PrP^Sc has the capacity to convert PrP^C into PrP^Sc, a requisite feature of infectious propagation. In yeast, several proteins possessing prion-related domains serve as protein-based epigenetic elements that confer distinct, her