Intracellular redistribution of truncated La protein produced by poliovirus 3C(pro)-mediated cleavage

The La autoantigen (also known as SS-B), a cellular RNA binding protein, ma y shuttle between the nucleus and cytoplasm, but it is mainly located in th e nucleus. La protein is redistributed to the cytoplasm after poliovirus in fection. An in vitro translation study demonstrated that La protein stimula ted the internal initiation of poliovirus translation. In the present study , a part of the La protein was shown to be cleaved in poliovirus infected H eLa cells, and this cleavage appeared to be mediated by poliovirus-specific protease 3C (3C(pro)). Truncated La protein (dl-La) was produced in vitro from recombinant La protein by cleavage with purified 3C(pro) at only one G ln(358)-Gly(359) peptide bond in the 408-amino-acid (aa) sequence of La pro tein. The dl-La expressed in L cells was detected in the cytoplasm. However , green fluorescence protein linked to the C-terminal 50-aa sequence of La protein was localized in the nucleus, suggesting that this C-terminal regio n contributes to the steady-state nuclear localization of the intact La pro tein in uninfected cells. The dl-La retained the enhancing activity of tran slation initiation driven by poliovirus RNA in rabbit reticulocyte lysates. These results suggest that La protein is cleaved by 3C(pro) in the course of poliovirus infection and that the dl-La is redistributed to the cytoplas m. dl-La, as well as La protein, may play a role in stimulating the interna l initiation of poliovirus translation in the cytoplasm.