Prevents cell to cell fusion via its interaction with A56 protein. The A56-K2 complex associates with components of the entry fusion complex (EFC) presumably to avoid superinfection and syncytium formation. Interacts with A56 protein. Component of extracellular enveloped virus (EEV) but not intracellular mature virus (IMV). Anchored to the surface of the outermost membrane of EEV via its interaction with A56 protein. Belongs to the serpin family. Poxviruses subfamily.