MxA

MxA is myxovirus resistance protein A, a cytoplasmic, interferon-inducible GTPase encoded by the MX1 gene in humans. It belongs to the family of dynamin-like large GTPases and is one of the principal mediators of the type I interferon antiviral response. MxA shows broad activity against a range of negative-sense RNA viruses, notably influenza A virus and other orthomyxoviruses, paramyxoviruses, and bunyaviruses; activity can vary by virus strain and experimental system. Expression of MxA is low in unstimulated cells but is strongly upregulated by IFN-α and IFN-β.

MxA functions as a cytoplasmic multimer and is thought to inhibit viral replication by interacting with viral ribonucleoprotein complexes or block steps such as transcription or nucleocapsid trafficking, in a virus-specific manner. The exact mechanisms remain an area of active research, with different modes described for different viruses. In humans, MxA is considered a major effector of the interferon-induced antiviral state; the related protein MxB (MX2) has distinct antiviral properties.

MxA expression is induced by type I interferons in many cell types, including epithelial and immune cells. Because its presence reflects an activated antiviral state, MxA levels are used as a biomarker of interferon signaling in research and clinical contexts. MxA is also studied for insights into innate immune defense and viral pathogenesis.

In vertebrates, Mx proteins are conserved, with mouse Mx1 providing potent influenza restriction and human MxA serving as a broad-acting antiviral factor.