Nucleolar targeting signal of human T-cell leukemia virus type I rex-encoded protein is essential for cytoplasmic accumulation of unspliced viral mRNA

The posttranscriptional regulator (rex) of human T-cell leukemia virus type I is known to be located predominantly in the cell nucleolus and to induce the accumulation of gag and env viral mRNAs. The N-terminal 19 amino acids of rex-encoded protein (Rex) has been shown to be sufficient to direct hybrid proteins to the cell nucleolus. We have studied the function of the nucleolar targeting signal (NOS) of rex by using full-length proviral DNA and mutant rex expression plasmids. Partial deletions of the NOS sequence abolished the accumulation of unspliced cytoplasmic mRNA, although the gene products of rex mutants were found in the nucleoplasm. These results indicate that NOS sequence, or nucleolar localization of Rex, is essential for Rex function.