Serves to protect the virus against complement attack by inhibiting both classical and alternative pathways of complement activation. Binds C3b and C4b. Heterodimer with A56 protein; disulfide-linked. 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 (By similarity). Belongs to the receptors of complement activation (RCA) family.