Identification and functional characterization of a TIA-1-related nucleolysin

We recently reported the molecular cloning of a cytotoxic granule-associated RNA-binding protein designated TIA-1. The ability of recombinant TIA-1 to induce DNA fragmentation in permeabilized cells suggested that this protein is the granule component responsible for inducing apoptosis in cytolytic lymphocyte (CTL) targets. Here we report the characterization of a cDNA encoding a TIA-1-related protein designated TIAR. The deduced amino acid sequence of TIAR reveals it to be a 42-kDa protein possessing three RNA-binding domains and a carboxyl-terminal auxiliary domain. Although the RNA-binding domains of TIA-1 and TIAR share greater than 85% amino acid homology, their carboxyl-terminal auxiliary domains are only 51% homologous. The carboxyl terminus of TIAR contains a lysosome-targeting motif, indicating that TIAR is probably a cytotoxic granule-associated protein. Like TIA-1, purified recombinant TIAR induced DNA fragmentation in permeabilized target cells. Although immunoblotting analysis of post-nuclear supernatants revealed TIA-1 protein to be restricted to CTLs, PCR analysis revealed the expression of TIA-1 and TIAR mRNA transcripts in a wide variety of cell types. Our data suggest that the granules of CTLs contain at least two candidate nucleolysins involved in CTL killing.