The RNA-binding proteins TDP-43 and Fused in Sarcoma (FUS) play central roles in neurodegeneration associated with amyotrophic lateral sclerosis and frontotemporal lobar degeneration. Both proteins are components of messenger ribonucleoprotein (mRNP) granules and show cytoplasmic mislocalization in affected tissues. Recently, ataxin-2 was identified as a potent modifier of TDP-43 toxicity in an RNA-dependent manner. This study investigated to clarify how ataxin-2 modifies the TDP-43 and FUS pathological pathway. The expression of cytoplasmic TDP-43, the 35-kDa C-terminal fragment (TDP-p35f), and mutant FUS recruited ataxin-2 to mRNP granules, whereas increased ataxin-2 inhibited the mRNP granule formation of the 35-kDa C-terminal fragment and mutant FUS. A subcellular compartment analysis showed that the overexpressed ataxin-2 increased the cytoplasmic concentrations of both proteins, whereas it decreased their nuclear distributions. These data indicate that increased ataxin-2 impairs the assembly of TDP-43 and FUS into mRNP granules, leading to an aberrant distribution of RNA-binding proteins. Consequently, these sequences may exacerbate the impairment of the RNA-quality control system mediated by amyotrophic lateral sclerosis/frontotemporal lobar degeneration-associated RNA-binding proteins, which forms the core of the degenerative cascade.