African swine fever virus–encoded protein MGF 505–3R impairs innate immunity via ubiquitin–mediated degradation of MyD88

African swine fever (ASF) is a highly contagious viral disease caused by the African swine fever virus (ASFV), which primarily affects pigs. ASFV encodes a variety of proteins that contribute to immune evasion, with the mechanisms of immune escape being diverse, complex, and not yet fully understood. In this study, the MGF 505-3R protein of ASFV was identified as a potential inhibitor of the host's inflammatory response. We demonstrate that MGF 505-3R suppresses the host antiviral response by promoting the ubiquitin-mediated degradation of MyD88, with the amino acid region 89-277 being essential for this function. Notably, this region directly mediates the interaction with MyD88 and induces its ubiquitination. Furthermore, MGF 505-3R and its derived peptide significantly inhibit the production of type I (IFN-α/β) and type III (IFN-λ) interferons, in addition to impairing NF-κB activation by blocking p65 phosphorylation and nuclear translocation. The MGF 505-3R peptide effectively attenuates the host inflammatory storm, decreasing the expression of cytokines such as TNF-α and IL-1β, and alleviating DSS-induced colitis in male C57BL/6 mice. These findings highlight the dual role of MGF 505-3R in suppressing both inflammatory and interferon pathways, underscoring its potential as a therapeutic candidate for inflammatory diseases and a target for antiviral strategies.