Transferrin receptor 1 (TfR1) functions as an entry receptor for scale drop disease virus to invade the host cell via clathrin-mediated endocytosis.

Scale drop disease virus (SDDV), a distinct member of the genus within the family, has emerged as a novel threat to global teleost aquaculture. Despite its importance, the pathogenic mechanism of SDDV remains largely elusive. In this study, we identified mandarin fish transferrin receptor 1 (TfR1) as an entry receptor for SDDV to invade host cells. First, TfR1 was detected in high abundance in purified SDDV virions and exhibited dynamic responses to SDDV infection, showing distinct regulatory patterns both and . Overexpression of TfR1 in low-permissive fathead minnow (FHM) cells significantly enhanced SDDV replication, particularly during the early stages of viral binding and entry. Conversely, antibody-blocking experiments and treatment with the TfR1 inhibitor ferristatin II significantly suppressed SDDV entry. Further investigation revealed that TfR1 directly interacted with the major capsid protein (MCP) of SDDV, and the helical domain of TfR1 was identified as the crucial docking site. The binding site within the helical domain was determined, and disrupting this interaction significantly reduced viral entry and host mortality. Finally, we demonstrated that SDDV could activate Src kinase-mediated tyrosine phosphorylation of TfR1. This phosphorylation event enhanced the internalization of TfR1 and facilitated clathrin-mediated endocytosis. Collectively, our study provides compelling evidence to confirm that TfR1 functions as an entry receptor that mediates SDDV entry into host cells via clathrin-mediated endocytosis, leading to a lethal infection outcome. Our work lays the groundwork for the development of targeted therapeutic strategies to mitigate the impact of SDDV in aquaculture.IMPORTANCETfR1, a dimeric glycoprotein classified as a type II transmembrane receptor, facilitates the cellular internalization of holo-transferrin. In several mammalian and avian RNA viruses and small DNA viruses, TfR1 serves as a crucial receptor to mediate the entry of viruses into host cells. As an emerging large DNA virus, SDDV poses an emerging threat to teleosts globally; however, its underlying pathogenic mechanisms remain poorly understood. In this study, we are the first to identify TfR1 as a crucial receptor for SDDV entry. We demonstrated a specific interaction between TfR1 and the major capsid protein (MCP) of SDDV, with the helical domain of TfR1 acting as the binding site. Moreover, we confirmed that SDDV enters cells through TfR1-mediated clathrin-dependent endocytosis. This work highlights the essential role of TfR1 in aquatic DNA viral infections and establishes the theoretical foundation for developing targeted therapeutic strategies against SDDV.