The E2 protein of classical swine fever virus (CSFV) is an envelope glycoprotein that is involved in virus attachment and entry. To date, the E2-interacting cellular proteins and their involvement in viral replication have been poorly documented. In this study, thioredoxin 2 (Trx2) was identified to be a novel E2-interacting partner using yeast two-hybrid screening from a porcine macrophage cDNA library. Trx2 is a mitochondrion-associated protein that participates in diverse cellular events. The Trx2-E2 interaction was further confirmed by glutathione S-transferase (GST) pulldown, in situ proximity ligation, and laser confocal assays. The thioredoxin domain of Trx2 and the asparagine at position 37 (N37) in the E2 protein were shown to be critical for the interaction. C lassical swine fever (CSF), caused by classical swine fever virus (CSFV), is a highly contagious, often fatal porcine disease with significant economic losses. CSFV belongs to the Pestivirus genus within the Flaviviridae family (1). It is an enveloped virus with a single-stranded, positive-sense RNA genome of approximately 12.3 kb in length. The RNA genome contains a single large open reading frame (ORF). This ORF is translated into a polyprotein, which is further processed into 12 mature proteins (N pro -C-E rns -E1-E2-p7-NS2-3-NS4A-NS4B-NS5A-NS5B) by viral and cellular proteases (2). The C, E rns , E1, and E2 proteins represent the structural components of the virion. The E1 and E2 glycoproteins are anchored to the envelope by their carboxyl termini, while E rns is loosely associated with the envelope. E rns presents as homodimers linked by disulfide bridges in virus-infected cells and in virions (3, 4). The C terminus of E2 functions as a membrane-spanning domain anchoring the E2-E1 or E2-E2 dimer to the viral envelope (5). E2 is also involved in virus attachment to and entry into the target cell (6). Furthermore, as a virulence determinant in pigs (7), the E2 protein can efficiently induce protective immune responses (8-13). Additionally, we recently showed that host -actin interacts with E2 and modulates the early life cycle of CSFV (14).Thioredoxins (Trxs) are a class of ubiquitously expressed redox proteins that contain a conserved consensus amino acid sequence (Cys-Gly-Pro-Cys) in the catalytic center. There are two distinct forms of Trxs (Trx1 and Trx2), which act as antioxidants facilitating the reduction of other proteins by cysteine thiol-disulfide exchange. Trx2 consists of 157 amino acid (aa) residues, and its thioredoxin domain is located in the C terminus of the protein.Trxs are involved in diverse biological processes, such as cell growth, proliferation, apoptosis, and gene regulation (15-18). Additionally, they interact with several transcription factors and