Epstein-barr virus nuclear antigen-1 binds to nuclear transporter karyopherin alpha1/NPI-1 in addition to karyopherin alpha2/Rch1

We searched for cellular proteins that interact with Epstein-Barr (EBV) virus nuclear antigen-1, which is a latent EBV origin-binding protein detected in all EBV latently infected cells and essential for maintenance of the latent EBV genome, by a yeast two-hybrid screening of a B lymphocyte cDNA library in this study. Interaction of polypeptides synthesized from three selected cDNA clones with EBNA-1 proteins was confirmed in vitro using their glutathione-S-transferase-fusion polypeptides and by coimmunoprecipitation analyses of B cell extracts with anti-EBNA-1 monoclonal antibodies and monospecific antibodies against cellular proteins of interest. We report the following: (i) Karyopherin alpha (karyopherin alpha1, hSRP1, and NPI-1), an adaptor subunit of nuclear localization signal receptors, which direct proteins to the nuclear pore, interacted with EBNA-1. (ii) EBNA-1 proteins endogenous in the B cell line Raji of Burkitt lymphoma origin bound to another adaptor protein, karyopherin alpha2 (hSRP1alpha, hRch1), interactions of which to recombinant EBNA-1 polypeptides were previously reported. (iii) Nearly 90% of all the cDNA clones examined was p32 (SF2-associated P32, p32/TAP, and gC1q-R), and endogenous EBNA-1 proteins in the Raji cells bound to p32, a potential of which to affect localization of EBNA-1 in transfected Vero cells has been recently suggested. These results suggest that EBNA-1, which has the unique NLS containing Lys-Arg and overlapping with one of the phosphorylation domains, is recognized and transported to the nuclei by these two distinct karyopherin alpha proteins, which are differentially expressed in different cell types, implying a regulatory localization system for EBNA-1.