The post-transcriptional regulator Rev of HIV: implications for its interaction with the nucleolar protein B23

Human T cell leukemia virus type 1 (HTLV-1) and human immunodeficiency virus type 1 (HIV-1) belong to the complex retrovirus whose replication is controlled by trans-acting proteins. HIV-1 encodes several regulatory proteins, including two essential trans-activations for viral replication, Rev and Tat. Both Rev and Tat have a nucleolar targeting signal and are actually located predominantly in the nucleoli. Within the nucleoli, Rev is localized to the combined regions of the dense fibrillar (DFC) and the granular (GC) components. Tat does not colocalize precisely with any nucleolar component tested, but partly overlaps regions of the DFC and the GC. Regions of both Rev and Tat are overlapped by the distribution of the major nucleolar protein B23. Overexpression of Rev causes nucleolar ballooning and general structural deformity with aberrant accumulation of rRNAs, whereas Tat does not have that effect. B23 is markedly accumulated in those nucleoli deformed by Rev. Components of the nucleolar DFC, GC, and fibrillar center domains are not accumulated but dispersed in a few small spots or larger patches within the enlarged nucleoli. Cytophotometric DNA determinations revealed that transient expression of Rev results in accumulation of G2, prophase, and mitotic cells which have failed cytokinesis, suggesting that Rev is capable of preventing or slowing the progression through mitosis. Tat, in contrast, does not affect the cell cycle. We speculate, based on these results, that Rev represses cell growth inhibiting the transport of ribosomal proteins and preribosomal particles across the nuclear envelope and affecting the cell cycle, both of which may be related to the proposed functions of B23.