Significance In the human organism, guanylate-binding proteins (GBPs) are involved in antimicrobial and antiviral activity, but the mechanism of GBPs’ action remains poorly understood. We have discovered that binding of a substrate molecule, GTP, to the enzyme triggers the release of an aforemasked lipid anchor, which results in GBP polymerization on the one hand and in the attachment of GBPs to lipid membranes on the other. Thus, membrane binding of GBPs competes with protein polymerization and, furthermore, leads to the membrane tethering, which could play a role in a clearance of engulfed pathogens from the cell. Altogether, our findings give deeper insights into GBPs’ molecular mechanism in the course of pathogen response.