Purα: A multifunctional single-stranded DNA- and RNA-binding protein

Purα is a ubiquitous, sequence-specific DNA- and RNA-binding protein which is highly conserved in eukaryotic cells. Purα 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). Purα 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 Purα to interact with its target DNA sequence and to associate with several viral and cellular proteins is modulated by RNA. Purα 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 Purα as a crucial factor in the cell cycle. Moreover, microinjection studies demonstrate that Purα causes either a G₁ or G₂ arrest depending on the cell cycle time of injection. The gene encoding Purα has been localized to a human locus which is frequently deleted in myelogenous leukemias and other cancers and Purα gene deletions have been detected in many cases of lymphoid cancers. The following review details the structural characteristics of Purα, its family members and the involvement of this protein in regulating various cellular and viral genes, viral replication and cell growth.