Association of HIV-1 Tat with the cellular protein, Puralpha, is mediated by RNA

Puralpha: a multifunctional single-stranded DNA- and RNA-binding protein

Puralpha is a ubiquitous, sequence-specific DNA- and RNA-binding protein which is highly conserved in eukaryotic cells. Puralpha has been implicated in diverse cellular functions, including transcriptional activation and repression, translation and cell growth. Moreover, this protein has been shown to be involved in regulating several human viruses which replicate in the central nervous system (CNS), including human immunodeficiency virus type I (HIV-1) and JC virus (JCV). Puralpha exerts part of its activity by interacting with cellular proteins, including pRb, E2F, cyclin A, Sp1 and members of the Y-box family of proteins, including YB-1 and MSY1, as well as viral proteins such as polyomavirus large T-antigen and HIV-1 Tat. The ability of Puralpha to interact with its target DNA sequence and to associate with several viral and cellular proteins is modulated by RNA. Puralpha has also been shown to be involved in cell growth and proliferation. Its association with pRb, E2F and cyclin A coupled with its fluctuating levels throughout the cell cycle, position Puralpha as a crucial factor in the cell cycle. Moreover, microinjection studies demonstrate that Puralpha causes either a G(1) or G(2) arrest depending on the cell cycle time of injection. The gene encoding Puralpha has been localized to a human locus which is frequently deleted in myelogenous leukemias and other cancers and Puralpha gene deletions have been detected in many cases of lymphoid cancers. The following review details the structural characteristics of Puralpha, its family members and the involvement of this protein in regulating various cellular and viral genes, viral replication and cell growth.

Interaction of HIV-1 Tat with Puralpha in nuclei of human glial cells: characterization of RNA-mediated protein-protein binding

A complex between the Tat protein, encoded by human immunodeficiency virus type 1 (HIV-1), and the cellular protein, Puralpha, has been implicated in activation of the late promoter of JC virus (JCV) and in enhancement of JCV DNA replication. JCV is the causative agent of progressive multifocal leukoencephalopathy (PML), an acquired immunodeficiency syndrome (AIDS) opportunistic infection of the brain. Puralpha also binds the HIV-1 TAR RNA element and activates HIV-1 transcription, suggesting a role for RNA binding in the action of this protein. Using immunoelectron microscopy, we find that in human glial cells expressing both proteins, Tat and Puralpha are colocalized in extranucleolar chromatin structural elements. The colocalized Puralpha and Tat are nearly exclusively nuclear, although individual proteins can be seen in both nucleus and cytoplasm, suggesting a preferential tropism of the complex for the nucleus. Analysis of the interaction between purified proteins indicates that the Tat-Puralpha interaction is strongly enhanced by the presence of RNA. Tat amino acids from 37-48 are essential for Tat binding. Residues 49-72, including the TAR RNA-binding domain, are critical for binding to Puralpha, while Cys(22), in the Tat transactivation domain, is responsible for an important global effect. Puralpha repeat II domains are involved in the interaction, and a polypeptide based on one such sequence inhibits binding. After RNase treatment of Puralpha enhancement of Tat binding can be partially restored by addition of a single-stranded JCV DNA PUR element, to which Tat does not bind. The results indicate that the Tat-Puralpha interaction is direct, rather than through an RNA link, and that RNA binding configures Puralpha for optimal interaction with Tat.