STRUCTURE AND FUNCTION OF THE PROTEINS EN CODED BY THE PX REGION OF HTLV-1

HTLV-I, like other retroviruses, contains the genes gag, pol and env. These genes encode the core proteins, the protease, the reverse transc riptase and the surface and transmembrane glycoproteins, respectively. In addition, this virus harbors a unique 2 kb region located at the 3 ' end of the env gene. This region, initially called pX, was found to play a major role in leukemogenesis and viral expression. Until 1992, this part of the viral genome was considered as encoding a unique doub ly spliced mRNA of 2.1 kb, allowing the expression of three proteins: p40Tax, p27Rex and p21Rex. p40Tax is a nuclear phosphoprotein which up regulates viral replication at the level of the LTR and transactivates numerous cellular genes. p27Rex, a nucleolar phosphoprotein, is a pos ttranscriptional regulator of viral expression. The function of the p2 1Rex protein is currently unknown. Recently, novel mRNAs encoded by th e pX region have been identified by reverse transcriptase-PCR in HTLV- I infected cell-lines as well as in lymphocytes from HTLV-I seropositi ve patients. These mRNAs encode proteins of 12, 13 and 30 kd, localise d in the cytoplasm, the nucleus and the nucleolus, respectively. Thus HTLV-I, like HIV, has developped fine posttranscriptional alternative splicing mechanisms to increase the complexity of its genome. The disc overy of these proteins might provide new insights in the study of HTL V-I associated diseases.