Translational control by the La antigen - Structure requirements for rescue of the double-stranded RNA-mediated inhibition of protein synthesis

The La antigen is a protein which can bind both single-stranded and double- stranded forms of RNA and has regulatory effects on gene expression at the levels of transcription and translation. It was previously shown to inhibit the activation of the dsRNA-dependent protein kinase PKR by sequestering a nd/or unwinding double-stranded RNA. Here, we demonstrate that, as predicte d by these properties, the La antigen can rescue protein synthesis in the r eticulocyte lysate system from inhibition by low concentrations of dsRNA. T his effect is reversed by higher concentrations of dsRNA. Using a series of deletion mutants we have investigated the structural features of the La an tigen that are required for these effects. The ability to bind dsRNA is inf luenced by regions within both the previously characterized N-terminal RNP motif and the C-terminal half of the protein. La mutants with either N-term inal or C-terminal deletions retain the ability to inhibit the protein kina se activity of PKR and to rescue protein synthesis from inhibition by dsRNA . It is notable that sequences in the C-terminal half of the La antigen, in cluding a phosphorylation site at Ser366, which are needed for other regula tory effects of the protein on gene expression are dispensable for the effe cts of La on PKR. We suggest that La regulates PKR activity solely as a res ult of its ability to act as an RNA-binding protein that can compete with P KR for limiting amounts of dsRNA.