Equine infectious anemia virus Mat protein serves as a Tat cofactor to facilitate viral transcription

Equine infectious anemia virus (EIAV) is a member of the Lentivirus genus of the Retroviridae family and serves as an important model for studying lentiviral pathogenesis. A novel EIAV-encoded protein, designated Mat, was previously identified in our laboratory, yet its biological functions remained unclear. Here, we demonstrate that disruption of Mat moderately reduced EIAV replication in equine monocyte-derived macrophages, and this defect was restored when Mat was supplied in trans. Further analyses revealed that Mat exhibits nuclear localization and enhances Tat-mediated transcriptional activation of the long terminal repeat (LTR), an essential and conserved step in lentiviral replication, by interacting with viral Tat and the host factor Cyclin T1 (CycT1), but not with the trans-acting responsive RNA element (TAR) within the LTR. Moreover, we found that Mat stabilizes EIAV Tat (eTat) and promotes the association of eTat with both TAR and equine CycT1. These findings indicate that Mat serves as a cofactor of Tat to regulate EIAV replication.IMPORTANCEIn lentiviruses, Tat-mediated activation of the LTR promoter is essential for viral replication. Mechanistically, Tat binds to TAR (a structured RNA at the 5' end of nascent viral transcripts, corresponding to the R region of the LTR), recruits host factors, including CycT1, and facilitates RNA polymerase II elongation, thereby promoting efficient production of full-length viral transcripts required for lentiviral replication. We recently identified an uncharacterized EIAV protein, Mat. Here, we show that Mat inactivation moderately attenuates EIAV replication in vitro, and this defect is rescued by ectopic Mat expression. Mat enhances Tat-mediated activation of EIAV LTR activity, and we further confirm that Mat interacts with EIAV Tat (eTat) and equine CycT1 (eqCycT1), stabilizes eTat, and promotes the association of eTat with both TAR and eqCycT1. These results suggest that Mat acts as an eTat cofactor in EIAV transcriptional regulation, providing insights into the complex transcriptional regulatory mechanisms of lentiviruses.