MxB

MxB, also known as MX2, is a member of the myxovirus resistance (Mx) family of interferon-induced GTPases found in mammals. The protein was identified as part of the cellular antiviral response and is best known for restricting replication of certain viruses, most notably HIV-1, in human cells. The MX2 gene encodes MxB, which is upregulated by type I and III interferons during the innate immune response.

Structure and localization: MxB contains a large GTPase domain and an N-terminal region essential for its antiviral activity. In human cells, MxB localizes primarily to the nuclear envelope, and this localization is linked to its ability to block HIV-1 nuclear import, a key step before integration of the viral genome.

Mechanism: The antiviral action of MxB appears to involve interference with the nuclear import of the viral genome and can depend on the integrity of the viral capsid and on host cell factors at the nuclear pore. The precise molecular mechanism remains an active area of research and is considered distinct from the cytoplasmic antiviral activity of the related MxA protein.

Scope and species variation: Orthologs of MxB exist in other mammals, with varying levels of antiviral potency. HIV-1 restriction by MxB is most robust in human cells, and while some studies report activity against other lentiviruses under certain conditions, the spectrum is not universal across all viruses.

Clinical relevance: Research on MxB contributes to understanding innate immune defenses against viral infection. While MX2/MxB is a significant model for antiviral restriction, there are no approved therapies that directly harness its activity at this time.