Physical and Functional Interaction between the Human T-Cell Lymphotropic Virus Type 1 Tax₁ Protein and the CCAAT Binding Protein NF-Y

Abstract

Tax₁, a potent activator of human T-cell lymphotropic virus type 1 (HTLV-1) transcription, has been shown to modulate expression of many cellular genes. Tax₁ does not bind DNA directly but regulates transcription through protein-protein interactions with sequence-specific transcription factors. Using the yeast two-hybrid system to screen for proteins which interact with Tax₁, we isolated the B subunit of the CCAAT binding protein NF-Y from a HeLa cDNA library. The interaction of Tax₁ with NF-YB was specific in that NF-YB did not interact with a variety of other transcription factors, including human immunodeficiency virus Tat, human papillomavirus E6, and Bicoid, or with the M7 (amino acids ²⁹CP-AS) Tax1 mutant. However, NF-YB did interact with the C-terminal Tax1 mutants M22 (¹³⁰TL-AS) and M47 (³¹⁹LL-RS). We also show that in vitro-translated NF-YB specifically bound to a glutathione S-transferase–Tax₁ fusion protein. Further, Tax₁ coimmunoprecipitated with NF-Y from nuclear extracts of HTLV-1-transformed cells, providing evidence for in vivo interaction of Tax₁ and NF-YB. We further demonstrate that Tax₁ specifically activated the NF-Y-responsive DQβ promoter, as well as a minimal promoter which contains only the Y-box element. In addition, mutation of the Y-box element alone abrogated Tax₁-mediated activation. Taken together, these data indicate that Tax₁ interacts with NF-Y through the B subunit and that this interaction results in activation of the major histocompatibility complex class II promoter. Through activation of this and other NF-Y driven promoters, the Tax₁–NF-Y interaction may play a critical role in causing cellular transformation and HTLV-1 pathogenesis.