A single point mutation in the nuclear localization domain of Sam68 blocksthe Rev/RRE-mediated transactivation

We have previously demonstrated that overexpression of Sam68 functionally s ubstitutes for, as well as synergizes with, HIV-1 Rev in RRE-mediated gene expression and virus replication. In addition, we have shown that the C-ter minal deletion mutants of Sam68 act with a transdominant negative phenotype in HIV replication, Previously, an Arginine429 mutation within the C-termi nal domain of Sam68 has been reported to be critical for the localization o f Sam68 in the nucleus. However, these studies were done in the context of truncated protein in which C-terminal amino acids 420-443 of Sam68 were fus ed to GFP. In contrast, we now report that the full length Sam68 protein ha ving the same mutation (Arginine429 --> Alanine) is completely localized in the nucleus while another Sam68 (Proline439-->Arginine) mutant is found in the cytoplasm. The localization of these Sam68 mutant proteins also correl ates,vith their function in RRE-mediated reporter gene expression, i.e. Sam 68 mutant protein that is localized in the cytoplasm failed to enhance RRE- mediated transactivation. Furthermore, we demonstrate that Sam68 P439-->R i nhibited the transactivation of RRE-mediated gene expression by both wild t ype Sam68 and Rev. These results indicate that the proline residue at posit ion 439 unlike arginine at position 429, mag play a critical role in target ing Sam68 protein to nucleus. We propose that these negative dominant mutan ts of Sam68 may have potential as anti-viral agents to combat AIDS.