Regulation of the human T-cell leukemia virus gene expression depends on the localization of regulatory proteins Tax, Rex and p30 II in specific nuclear subdomains

Abstract

The human T-cell leukemia virus HTLV-1 encodes regulatory proteins, Tax, Rex and p30 II, which are involved in the control of viral gene expression at the transcriptional and post-transcriptional levels. Tax localizes in unique nuclear bodies that contain components of the transcription and splicing complexes. In this work, we studied the relative intracellular localizations of Tax, Rex and p30 II. Run-on transcription assays and immunocytochemistry at light and electron microscopy levels indicated that the Tax nuclear bodies included both de novo transcribed RNA and the RNA polymerase II form that is phosphorylated on its carboxy-terminal domain whereas contacts with chromatin were observed at the periphery of these nuclear bodies. Rex first accumulated in nucleolar foci and then spread across the whole nucleus to display a diffuse and punctuate nucleoplasmic distribution. This distribution of Rex was observed in HTLV-1 transformed lymphocytes and in COS cells expressing the HTLV-1 provirus. Rex colocalized with the cellular export factor CRM-1 in the nucleolar foci as well as in the nucleoplasmic foci that did not overlap with Tax nuclear bodies but were found at the boundaries of the Tax bodies. In addition, we demonstrate that p30 II interacts with Rex and colocalizes with the Rex/CRM-1 complexes in the nucleoli leading to their clearance from the nucleoplasm. Our results suggest that transcripts originating from Tax-induced activation of gene expression at the boundaries of the Tax bodies are transported out of the nucleus by nucleoplasmic Rex/CRM-1 complexes that are first assembled in nucleolar foci. In addition, p30 II might exert its negative effect on viral RNA transport by preventing the release of the Rex/CRM-1 complexes from sequestration in nucleolar foci. These data support the idea that the transcriptional and post-transcriptional regulation of HTLV-1 gene expression depends on the concentration of select regulatory complexes at specific area of the nucleus.