A Mosquito Salivary Protein Inhibits Activation of the Plasma Contact System by Binding to Factor XII and High Molecular Weight Kininogen*

The salivary glands of female mosquitoes contain a variety of bioactive substances that assist their blood-feeding behavior. Here, we report a salivary protein of the malarial vector mosquito, Anopheles stephensi, that inhibits activation of the plasma contact system. This factor, named hamadarin, is a 16-kDa protein and a major component of the saliva of this mosquito. Assays using human plasma showed that hamadarin dose-dependently inhibits activation of the plasma contact system and subsequent release of bradykinin, a primary mediator of inflammatory reactions. Reconstitution experiments showed that hamadarin inhibits activation of the plasma contact system by inhibition of the reciprocal activation of factor XII and kallikrein. Direct binding assays demonstrated that this inhibitory effect is due to hamadarin binding to both factor XII and high molecular weight kininogen and interference in their association with the activating surface. The assays also showed that hamadarin binding to these proteins depends on Zn2+ ions, suggesting that hamadarin binds to these contact factors by recognizing their conformational change induced by Zn2+ binding. We propose that hamadarin may attenuate the host's acute inflammatory responses to the mosquito's bites by inhibition of bradykinin release and thus enable mosquitoes to take a blood meal efficiently and safely.