Inhibition of human T-cell leukemia virus type 2 Rex function by truncated forms of Rex encoded in alternatively spliced mRNAs.

Three mRNA species encoding the x-III open reading frame are expressed in human T-cell leukemia virus type 2 (HTLV-2)-infected cells. An mRNA composed of exons 1, 2, and 3 produces the essential posttranscriptional regulator Rex; shorter 1-3 and 1-B mRNAs encode a family of x-III proteins of unknown function that represent truncated forms of Rex. This report presents an analysis of the functional interactions between Rex and the x-III proteins, results of which suggest a role for the x-III proteins as negative regulators of Rex function. Cotransfection assays demonstrated that the x-III proteins were able to inhibit the ability of Rex to activate the expression of a Rex-dependent mRNA. Analysis of intracellular compartmentalization in actinomycin D-treated cells showed that coexpression of the x-III proteins resulted in the sequestration of Rex into the nuclear compartment. Subcellular fractionation studies showed that Rex was preferentially localized in the cytoplasmic or nuclear fraction depending on its phosphorylation status and that coexpression of Rex with the x-III proteins changed the phosphorylation pattern of Rex and the intracellular distribution of the x-III proteins. In vitro protein binding assays demonstrated the formation of Rex-Rex homomultimeric complexes; however, mixed Rex/x-III multimers were not detected. These findings indicated a correlation between phosphorylation and intracellular trafficking of Rex and suggested that the mechanism underlying the inhibitory effects of the x-III proteins might result from an interference with these processes.