VIRUS HTLV-I: STRUCTURE ET FONCTION DES PROTEINES DE LA REGION PX

HTLV-I, like other retroviruses, contains the genes gag, pol and env. These genes encode the core proteins, the protease, the reverse transcriptase 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 doubly spliced mRNA of 2.1 kb, allowing the expression of three proteins: p40(Tax), p27(Rex) and P21(Rex). p40(Tax) is a nuclear phosphoprotein which upregulates viral replication at the level of the LTR and transactivates numerous cellular genes. p27(Rex), a nucleolar phosphoprotein, is a posttranscriptional regulator of viral expression. The function of the p21(Rex) protein is currently unknown. Recently, novel mRNAs encoded by the pX region have been identified by reverse transcriptase-PCR in HTLV-I infected cell-lines as well as in lymphocytes from HTLV-I seropositive patients. These mRNAs encode proteins of 12, 13 and 30 kd, localised 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 discovery of these proteins might provide new insights in the study of HTLV-I associated diseases.