THE POSTTRANSCRIPTIONAL REGULATOR REX OF THE HUMAN T-CELL LEUKEMIA-VIRUS TYPE-I IS PRESENT AS NUCLEOLAR SPECKLES IN INFECTED-CELLS

The rex-encoded protein (Rex) of human T-cell leukemia virus type I (H TLV-I) is responsible for the cytoplasmic accumulation of incompletely spliced mRNAs that encode the virion structural proteins. Rex is know n to be located predominantly in the cell nucleoli in transient transf ections or in the isolated nuclei of HTLV-I-infected cells. However, p recise location of Rex under physiological conditions has not been det ermined unequivocally. Here we report that Rex is primarily located as intranucleolar speckles in HTLVI-infected cells, except for a few nuc leoplasmic speckles. This is in contrast to the more diffuse nucleolar distribution of the rev-encoded protein (Rev) of human immunodeficien cy virus type 1 (HIV-1), the functional homologue to Rex, in HTV-1-inf ected cells. Accumulation of Rev is associated with disruption of nucl eolar structure and cell death, whereas Rex does not have these effect s. The difference in distribution of Rex and Rev within the nucleoli m ay reflect the difference of toxicity toward the host cells. Involveme nt of the nucleolus in processing of certain mRNAs is also discussed. (C) 1995 Academic Press, Inc.