Copyright © 1991, American Society for Microbiology The Rex Proteins of Human T-Cell Leukemia Virus Type II Differ by

The Rex proteins of human T-cell leukemia virus types I and II (HTLV-I and HTLV-II) induce cytoplasmic expression of unspliced gag-pol mRNA and singly spliced env mRNA and are critical for virus replication. Two rex gene products, p27re ' and p2l'ex of HTLV-I and p26X and p24'ex of HTLV-ll, have been detected in HTLV-infected cells; however, the structural and biological relationship of the proteins has not been clearly elucidated. Endoproteinase digestion and phosphoamino acid analysis of HTLV-ll Rex indicated that p24'x has the same amino acid backbone as p26rx and that the larger apparent molecular size of p26'X is attributable to serine phosphorylation. The human T-cell leukemia virus type II (HTLV-II) rex gene encodes two proteins of 26 and 24 kDa (10, 11). Analogous rex gene products of 27 and 21 kDa are also encoded by HTLV-I (7, 8). The structural and biological relationship of the two rex gene products has not been clearly defined, although it has been established that both proteins share the same carboxy terminus (10, 11). Muta-tions that remove the first methionine (Met) initiation codo